Focus Of Discoveries, A 9789812790354, 9789812790347

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Rudolf P. Huebener

Emeritus Professor, Eberhard Karls University of Tübingen, Germany

Heinz Luebbig

Retired, Physikalisch-Technische Bundesanstalt, Germany

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Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

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A FOCUS OF DISCOVERIES Copyright © 2008 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

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ISBN-13 978-981-279-034-7 ISBN-10 981-279-034-9

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Preamble

Up to now several articles and books have appeared already dealing with the foundation and the history of the Physikalisch–Technische Reichsanstalt (PTR) and its successor institution of today, the Physikalisch–Technische Bundesanstalt (PTB). Also this newly presented book uses the historical development of the Reichsanstalt as a framework. However, it concentrates much more on the scientific achievements of this research facility, which was clearly unique at the time. As an institution of the Government, the PTR/PTB represented a novelty in Europe, which until today is committed to the principle of Werner Siemens from 1883: “Research in the natural sciences always provides a solid foundation for technical progress, and the industry of a country will never reach and keep an internationally leading position, unless at the same time it is at the top of the advances in the natural sciences.” The book ranges from the investigations of the spectral distribution of the emission of black bodies by Ferdinand Kurlbaum, Otto Lummer, and Ernst Pringsheim, which provided the stimulus and the experimental basis of the radiation law proposed by Max Planck in 1900, over the discovery of the Meissner effect in superconductors, the discovery of the element rhenium by Ida Tacke and Walter Noddack, up to the contributions by Hans Geiger, Walther Bothe, and Werner Kolhörster in the field of radioactivity. A separate chapter deals with the intensive relation between Albert Einstein and the Reichsanstalt, in which the great theoretician performed his only physical experiment together with Wander Johannes de Haas: an experiment for proving Ampere’s hypothesis of the molecular currents. By using many original documents the authors generate a very lively picture of the scientific activities at the time, which takes the reader back to this exciting period. v

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I am particularly happy, that this book appears close to the 120th birthday of the PTR/PTB, I thank the authors Prof. Rudolf Huebener and Dr. Heinz Luebbig for this birthday present, and I wish for the book a large readership and for the latter a stimulating and exciting reading. Ernst O. Göbel Prof. Dr. Ernst O. Göbel is President of the Physikalisch–Technische Bundesanstalt, Braunschweig and Berlin.

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Preface

If today one gets off the U-2 subway in Berlin at Ernst–Reuter Platz and leaves the subway station heading north, after only a few hundred meters one encounters a large terrain at the corner of Marchstrasse/Fraunhoferstrasse, the boundary of which is marked by an impressive wall and on which there exists a number of beautiful and aweinspiring buildings. We are dealing with the Berlin Institute of the Physical–Technical Federal Institution (Physikalisch–Technische Bundesanstalt, PTB ), the successor of the Physical–Technical Imperial Institution (Physikalisch–Technische Reichsanstalt, PTR) founded 120 years ago. At the time of the foundation of the latter, the terrain had been donated to the German State by the scientist and industrialist Dr. Werner Siemens under the condition that at this location a research institute be established which was to be financed by the State. The Reichsanstalt then turned out to be the forerunner worldwide of all other national institutes of metrology. Around the next-to-the-last turn of the century and a few years thereafter, extremely important scientific discoveries were made at this Imperial Institution, which have decisively shaped the physics of the last century. At this point we only mention the fact that the origin of the quantum theory created by Max Planck was associated with experiments carried out at this Institution. Furthermore, it was here that Walther Meissner discovered the effect subsequently named after him, and which represented a turning point in the field of superconductivity. Today, the larger part of the Physical– Technical Federal Institution is located in Braunschweig, about 200 km west of Berlin, where it was newly established after the Second World War. When not too long ago one of the two authors (R. H.) visited the PTB in Berlin once again and reflected upon the many extremely important developments which already started there at its inception, he developed a vii

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keen interest in the early history of this so highly successful establishment. At the same time he felt that these unique developments might also deserve to be told to a wider audience. When the second author (H. L.) agreed to participate in the project, the chances of realizing such a plan increased considerably. As a result we present this book, and hope that it will be interesting to a readership extending beyond only the specialists. On the one hand, the present book deals with the causes and motivations that resulted in the foundation of the Imperial Institution. These driving arguments from its pre-history can hardly be better illustrated than by quotations from a number of memoranda which at the time different people had prepared for the German Government. Therefore, in a separate chapter we present selected quotations from such memoranda. In principle, much content in these memoranda still remains valid today, perhaps within a different context. On the other hand, in our book we emphasize in particular the physical background and illustrate the advances accomplished in the case of a few selected outstanding examples. Here we restrict ourselves predominantly to the period before 1933. It was not our intention to present a complete and comprehensive treatment of the developments of the PTR. These excellent monographs already exist and are listed in our compilation of the literature. In this context, David Cahan from the Department of History of the University of Nebraska in Lincoln, Nebraska, USA, deserves special mention. We are pleased to thank Dr. Wolfgang Buck from the Institut Berlin of the PTB for his active and enthusiastic support. We are grateful to Kordula Braun, PTB, for her handling of the figures and to Hanna Pöhler, Tuebingen, for her preparation of the camera ready copy. Tuebingen and Berlin, January 2008 Rudolf Huebener

Heinz Luebbig

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Contents

Preamble

v

Preface

vii

1. The Foundation and the Key Role of Werner Siemens

1

2. Some Memoranda at the Beginning

9

3. The Start under President Hermann von Helmholtz

35

4. The Institute as a Model

45

5. The Optical Laboratory and the Birth of Quantum Theory

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6. The Low-Temperature Laboratory and the Discovery of the Meissner Effect

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7. The Chemical Laboratory and the Discovery of New Elements

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8. The Laboratory for Radioactivity

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9. The Imperial Institute and Albert Einstein

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10. Counting and Measuring — Quantum Statistics and Quantum Standards

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11. Fundamental Constants — the Best Information on Nature Available

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12. The Meter Convention for the Global Consistency of Measurements

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13. The Presidents of the Institute until 1933

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14. The Institute under the Nazi Dictatorship and a New Beginning

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Literature

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Name Index

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About the Authors

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Chapter 1

The Foundation and the Key Role of Werner Siemens

During the last decades of the 19th century in Germany, industry experienced a marked expansion, to a large extent due to the rapid advances achieved in science and technology. In particular knowledge in the natural sciences saw an impressive growth, which resulted in the generation of a series of new German industries based on high technology. As examples we mention the production of metals and steel, railroad construction and ship-building, cryo-technology, vehicles driven by explosive motors, electrical engineering, optics, mechanical engineering, chemistry, and eventually aviation. During this period of industrial expansion toward high-technology products, at many German universities new physical institutes were built and operated. Famous examples were the institutes in Berlin, Leipzig, Heidelberg, and Strasbourg. However, these institutes mainly served teaching purposes, and only in a minor way did they provide support for research in the field of physics. Apparently, there was a large discrepancy between industrial developments and the opportunity for basic research in the area of physics. Werner Siemens was one of the first people in Germany to recognize and address this deficiency of physical research at the universities. He had obtained his scientific and technical education at the Engineering and Artillery College in Berlin. In addition to being an industrialist he was very much of a scientist as well. In a memorandum of April 1883 Siemens pointed out that the German universities generated many well-educated school teachers and people lecturing at universities. However, according to him, there was a severe shortage of well-equipped laboratories suitable for physical research. Primarily, the university institutes established so far operated only as teaching institutions and provided very little opportunity 1

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for physical research. There was a strong need for a new type of institute, the members of which would be totally free of any teaching duties and thus be able to concentrate completely on their research subjects. How did Werner Siemens succeed in realizing his idea of a new institution devoted to fundamental research in the area of physics?

Fig. 1.1

Werner Siemens, 1887

Already in 1872 a small group of scientists in Prussia had assembled with the aim to improve the level of precision technology in the country. The group consisted of the astronomer Wilhelm Foerster, the physiologist and physicist Hermann von Helmholtz, the high-school teacher for

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natural sciences and mathematics Karl Schellbach, and Werner Siemens. They were concerned in particular with the fabrication of telescopes, microscopes, comparators, photometers, thermometers, barometers, spectroscopic instruments, and devices of geodesy and nautical science. In the same year they issued the “Schellbach Memorandum” describing the level of precision technology in Prussia and requesting financial support by the government for the establishment of an institute promoting studies in the natural sciences and precision mechanics. The memorandum was submitted to the Prussian Ministry of Education and Cultural Affairs, which subsequently forwarded it to the Prussian Academy of Sciences. Although Emperor Wilhelm I. and Crown Prince Friedrich supported these endeavours, in March 1873 the Academy rejected the proposed establishment of such an institute. The Academy had been known for its highly conservative attitude and for its animosity toward technical developments. Foerster then turned to the Triangulation Office of the State of Prussia, asking for support. The latter was directly under the control of the Chief of the General Staff, Helmuth von Moltke, who was a friend of the natural sciences and who had recognized the military importance of the precision technique. Von Moltke ordered the formation of a committee for investigating the state of the precision technique in Prussia. Already in December of 1873 this committee reported to the Prussian Parliament that an improvement of the precision technique could only be achieved by means of an institute supported by the State. The new technical institute would not have any obligation to teaching and would only concentrate on research. In 1875 the Prussian Ministry of Education and Cultural Affairs accepted the recommendations of the report. However, the question of proper accommodation remained unresolved. During the following seven years the Prussian Parliament again and again delayed the decision about the finances required for the building. In the end, the Technical College in Charlottenburg was considered as a possible site. However, by November of 1882 the construction of a new building for the Technical College had advanced only very little. In the meantime the level of the precision technique had changed considerably. In addition, the rapidly growing electric industry needed a reliable base for the electric units and standards. Furthermore, the French predominance in the field of metrology was to be broken. All this presented new tasks for a possible new Institute of the German Reich. Therefore, at the end of 1882 the members of the Prussian initiative began to think about their plan again. During the subsequent eighteen months Moltke and the Prussian Minister of Education and Cultural Affairs, Gustav von Goßler,

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again set up a committee for discussing the issue of the new institute. The members of the committee were representatives of science as well as of technology. The most important member was perhaps Werner Siemens. Siemens was convinced that the natural sciences represent the necessary base of industrial growth and modern technology, and that these needed adequate support. Hence, the new institute should solve not only shortterm technical problems, but in addition it should be involved with longterm fundamental research. In this way it would take up what could not be carried out by the universities and what never had been the task of the latter. The scientists employed by the institute would be free of all teaching and administrative duties. Siemens succeeded in convincing the other members of the committee of his ideas, which then were expressed in a report to the German Upper House of the Federal Parliament. Furthermore, the report warned against industrial competition by foreign countries. This warning extended in particular to France, Great Britain, and Russia, where the government supported pure science as well as technology, and thereby helped the economy. The report called for a physical–mechanical institute performing scientific and technical research in the areas of electricity, optics, mechanics, and metallurgy. In addition, it would test and certify all kinds of physical instruments, materials, and products. Siemens summarized his ideas about the establishment of a physical– mechanical institute in two memoranda from April 1883 and March 1884, respectively, which will be presented in Chapter 2 together with three other documents. In April 1883 he stated his firm principle regarding the importance of fundamental research: “Research in the natural sciences always provides a solid foundation for technical progress, and the industry of a country will never reach and keep an internationally leading position, unless at the same time it is at the top of the advances in the natural sciences.” For Siemens the motivation for this enterprise clearly originated from his lifelong experience with the strong relations between science, technology, economy, and government. He had built his international company Siemens & Halske by means of contracts with the Prussian military and with different European governments. He had been fascinated by electricity and its great technical potential. In 1849 the Siemens telegraph transmitted important decisions by the Parliament in the Paulskirche in Frankfurt via cable to Berlin. In the late 1860s the Siemens & Halske Company installed a telegraph connection over a 11 000 - km distance between London and Calcutta. Werner Siemens invented and developed the electric generator transforming mechanical into electric energy. 1879 he introduced the first

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operating electric train. In 1888, one year before he retired from his active business life, he was ennobled by the German Emperor. By that time his company dominated the low-voltage section of the German electric industry, and together with the Allgemeine Elektricitäts–Gesellschaft (AEG) of Emil Rathenau, the high-voltage section as well. In addition to engineering and technology, Siemens always loved and supported basic science. In 1845 he had participated in the foundation of the Physical Society of Berlin, which later became the German Physical Society. He was an excellent experimental physicist. As outstanding examples of his contributions to pure physics we mention: his work on the definition of absolute electric units and the development of standards, his studies of electrostatic induction and of the optimum conditions of electric current flow and of the operation of electric magnets, his investigations of the electric conductance of metals and the influence of the temperature, his discovery of the self-activated dynamo, and his participation in the development of the galvanometer and of other scientific instruments. Werner Siemens was a highly patriotic person. When the project of the physical–mechanical institute ran into difficulties regarding its accommodation, during spring of 1883 he again took the initiative. In a letter of July 7, 1883 to the Prussian Minister of Education and Cultural Affairs, Gustav von Goßler, he expressed his anxiety that there would be no room for the institute within the Technical College in Charlottenburg, and he mentioned that he owned property near the College which would be well suited for scientific laboratories. Therefore, he offered an area of 12 000 m2 of his property as a building site under the condition that Prussia would “build, equip, and permanently sustain” the corresponding buildings. His letter put new momentum into the project. In January 1884 Siemens renewed his offer, and in addition he promised to also donate the money necessary for the laboratory building. In March 1884 he changed his offer again and shifted his donation from Prussia to the German Reich. He offered “half a million Marks in property or capital ” in order to “serve my country and to demonstrate my love of the science, to which exclusively I owe my rise in life”. The key role of Werner Siemens can be seen in addition from the memoranda presented in the following chapter. At the request of German Chancellor Otto von Bismarck, in 1884 Siemens together with representatives of the government and some members of the previous committees worked out the complete organizational structure including the composition of the board, the budget, and the tasks of the future institute.The institute was to consist of two sections: a phys-

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Fig. 1.2 Document of the donation signed by Werner Siemens on November 12, 1885 “Ich Endesunterzeichneter erkläre hierdurch, daß ich das in dem beiliegenden Situationsplane mit a b c d umzeichnete, von der March–Straße, den projektierten Straßen 4 und 5 und der mit der March–Straße parallelen Linie b c begrenzte Grundstück mit der Grundfläche von 19800 Quadratmeter dem Deutschen Reiche zum Bau einer Reichsanstalt für experimentelle Naturforschung schenkungsweise überlasse, unter der Bedingung, daß das Reich die Kosten für Bau und Einrichtung und die Dotation der geplanten physikalisch–technischen Reichsanstalt übernimmt.” (I the undersigned declare herewith, that I donate the plot, marked in the attached street plan by a b c d , with the boundary of the projected streets 4 and 5 and of the line b c parallel to the March–Straße, with the area of 19800 square meters, to the German Reich, in order to build a Reichsanstalt for experimental natural science, under the condition, that the Reich takes over the costs of the building and the equipment and the endowment of the planned physical–technical Reichsanstalt.) (Siemens Archives, Munich).

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ical and a technical section. With this plan one hoped to convince the Parliament of the necessity and usefulness of the new facility. However, in spite of this effort, particularly in the year 1886 there appeared a series of serious obstacles against the project. For some time Chancellor Bismarck refused to provide financial support. On the other hand, the “Association of German Engineers” (Verein deutscher Ingenieure) strongly criticized the institute, asserting that it would serve mainly the personal interests of Werner Siemens and that the technical section would concentrate on the wrong subjects. In a letter to Bismarck from November of 1886 the “German Society for Mechanics and Optics” expressed similar doubts about the institute. As a result of these critical comments, the plans for the Institute ran into heavy difficulties in the German Parliament. Eventually, the Budget Commission approved the amount of 160 000 Marks assigned for the Technical Section, but it dropped the Physical Section completely. These disastrous developments caused Siemens, Foerster, and Crown Prince Friedrich to increase their efforts in favor of the original concept. It was in particular the Crown Prince, who tried to win support for the physical section. Then in March 1887 a newly formed German Parliament approved the amount of 700 000 Marks for the new institute including the physical section. In Parliament an argument stressing the increasing international competition in the area of high technology had helped considerably. Among the German physicists working at universities and technical colleges, to a large extent only Hermann von Helmholtz strongly supported the establishment of the Institute. Furthermore, it was the technical and scientific vision and the patriotism of Werner Siemens that led to the key role he played in this endeavour.

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Chapter 2

Some Memoranda at the Beginning

Prior to the foundation of the Physikalisch–Technische Reichsanstalt in the year 1887, several memoranda had been written pointing out the urgent need of the establishment of a research institute financed by the state. The tasks of such a state-operated facility were mainly experimental studies of problems resulting from the rapid industrial development of the time. This became quite clear, for example, in connection with the physical properties of glasses, as well as those of metals and alloys. The precision instruments for surveying provided important motivating material. Good control of the properties of glasses played an important role, for example, in the case of thermometric glass tubes, water-levels made of glass, and the instruments for astronomy continuously demanding higher quality. At the time it was recognized that the private effort of individual people could not be sufficient for solving existing physical problems. Whereas in the case of astronomy governmental support had long been taken for granted, other scientific areas had not enjoyed such treatment. Serious economic difficulties, which needed to be alleviated, had already appeared in the case of precision technique. Extra efforts made in England, France, and also in Russia to deal with this matter had generated additional pressure on Germany to initiate corresponding measures. In order to illustrate the historical background, we offer a selection of discoveries and inventions made during the period since the foundation of the Siemens & Halske Company in 1847 up to 1883: 1847 1849 1850

Helmholtz: Principle of energy conservation Telegraphy between Berlin and Frankfurt (Main) Bunsen: Gas burner Clausius: Second law of thermodynamics

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1854 1856

1858 1859 1861 1866 1869 1873 1876 1877 1879

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Geissler and Plücker: Electronic discharge tube Joule–Thomson: Refrigeration by adiabatic expansion of a gas Helmholtz: Handbook of physiological optics König and Clausius: Molecular theory of gases Bessemer: Converter for steel production Plücker: Cathode rays Bunsen and Kirchhoff: Spectral analysis Reis: Telephony Abbe: Microscopy Siemens: Self-exited generator (dynamo-electrical principle) Hittorf: Deflection of cathode rays in a magnetic field Mendelejew and Meyer: Periodic system of the elements Maxwell: Treatise on electricity and magnetism Linde: Ammonia refrigeration machine Boltzmann: Entropy as a measure of the thermodynamic probability Edison: Carbon filament lamp

In principle, today, even after more than 120 years, the memoranda still possess an interesting actuality. Therefore, in this chapter we reproduce in large part five memoranda, which were written between April 1883 and March 1884. In this case it was in particular Werner Siemens and Hermann von Helmholtz, who had forcefully advocated the establishment of a stateoperated research facility. Vote of the Herr Geheimer Regierungsrath Dr. Siemens (April 1883)

Werner

No country in the world has done as much for the scientific and technical education as Germany and in particular Prussia. This is well recognized everywhere and the German educational system serves as a model for all countries. In this case, considering only its material interest, Germany has done perhaps too much, because German scholars and to a higher degree German technical experts are distributed over the whole world, and due to their acknowledged competence they increase foreign competition exerted on German industry. Also, the highly educated scholars and technical experts remaining within the country find employment commensurate with their knowledge only to a small degree. According to the number and the education of its scholars and technical experts, without any question Germany should be leading in the natural sciences and technology ... . The reason why in general this expectation has not been fulfilled apparently

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lies in the fact that in the case of scientific research as well as of technicalinventive work, the time was unfavorable for creative powers existing to such an excessive extent. In the case of technology, since the decree of German patent law, this unfavorable situation has improved considerably. Since, because of the latter, inventions are placed under effective protection, inventors and manufacturers can invest effort and money in the careful preparation of new inventions, hoping — often in vain — for large future profit, because imitators no longer are allowed to use it without incurring painful effort and costs. There is hardly any doubt that the apparent upswing of German industry in recent years essentially must be considered a result of the now existing patent protection. The inventions and improvements are no longer, as before, immediately introduced abroad, where — to the great disadvantage of German industry — they once obtained patent protection, thus costing Germany its lead and reputation worldwide, forcing it to be satisfied with imitations. (After a brief discussion of the problems arising in many cases from an improper evaluation before patenting, Siemens continued): This danger can only be prevented by a stronger development of scientific research and simultaneously by a greater restriction of patent permission. Research in the natural sciences always provides a solid foundation for technical progress, and the industry of a country will never reach and keep an internationally leading position, unless at the same time it is at the top of the advances in the natural sciences! (Emphasis added). Achieveing this represents the most effective way to raise the level of industry. German natural science has always occupied an awe-inspiring position. Moreover one cannot go wrong if one assumes that it is only due to the high level of education in the natural sciences in Germany, that German industry was able to maintain its position in spite of its unfavorable situation. On the other hand, one must admit that in our case the progress of the natural sciences by no means corresponds to the extent of our scientific education. ... It appears that this is due to a deficiency in our state-operated institutions. In our case science still is in the same situation that technology found itself in prior to the protection of inventions. With obvious success the state has concentrated all its power on the support of scientific education. Its educational institutions generate a large number of highly educated natural scientists whose career is nearly always teaching. Scientific research itself is never a vocation within the governmental organization; it is only a tolerated private activity of scholars, in addition to their job, teaching. Individual experimental stations, which exist be-

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cause of special urgent needs, and also academies, which admittedly are devoted to scientific research, but are only active in terms of a secondary occupation and are not equipped with the facilities required for performing experimental studies, do not change much regarding this point. The professional scholars of the academies, in addition to their teaching obligations, are nearly always overloaded with scholarly business to such an extent, that — according to the statement of one of our foremost natural scientists — they have to stop being scholars. ... However, it must be considered a waste of national power that highly talented scientists, as they appear only rarely, are heavily burdened by professional jobs which others might carry out even better, and are for the most part thereby lost to science itself, to which they would be of invaluable service if they could devote themselves to it completely. However, more significant is the fact that so many talented and highly educated young scholars do not find an opportunity to carry out scientific work. The laboratories of the universities and schools as a rule are open to them only until they have completed their scientific education. Yet these institutions are destined and established for teaching and in general are not at all suitable for delicate and extended scientific investigations. In the majority of cases it is the sad consequence that scientific projects, which would newly revive and fertilize whole areas of life, remain undone, and that in the struggle of life talents are not developed or go unrecognized, which under more favorable circumstances could have led to great accomplishments honoring and materially benefitting the country. This applies in particular to experimental physics. Chemistry is more intimately connected with industry, which offers rewarding employment and opportunity for research activities to many educated chemists. This is helped by the fact that the space and equipment for chemical studies can be obtained much more easily than that required for extended experimental physical investigations. Therefore, the German chemical industry has always been able to maintain a scientific and technical level corresponding to the German educational level. Unfortunately, this does not apply to the case of experimental physics. Here, England has reached a clear predominance because of its wealth existing in many circles and the preference of the English for scientific activities. Prosperous Englishmen have established a large number of private laboratories, in which they themselves work eagerly and provide an opportunity to competent scientific experts for carrying out larger projects. In spite of the fact that scientific education is relatively sparsely distributed in England, nevertheless, this country has still made great accomplishments and has developed an unusually large number of talents of first rank. In

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recent times, England, France, and the United States, the countries which are our most dangerous competitors, have recognized the great importance of scientific superiority for material interests and are eagerly attempting to improve the educational level in the natural sciences ... and to create institutions which favor scientific progress. ... France has always cared greatly about education in the natural sciences. In that country the situation is similar as for us. Although in France scientific education is well organized and the knowledge in the area of the natural sciences is widely distributed, and although the conservatoire des arts et métiers is an institute which essentially should serve scientific–technical investigations, recently yet one found it necessary to establish a large new institute, which is destined exclusively for scientific studies. Therefore, it must be feared that the lead we still have, in better organized teaching of the natural sciences and in a more widely distributed scientific education, will soon be lost, and that in the future we no longer will be leading in scientific progress, if the latter is not also state supported. State-supported organizations would have to satisfy the double purpose of promoting research in the natural sciences in general and of supporting industry in terms of solving the scientific technical problems and questions which essentially affect its development. In addition to universities and technical colleges, laboratories should be established which would be directed by highly talented men and would be equipped plentifully with supplies in order to be able to carry out experimental studies of all kinds with the highest possible precision. For working in these institutes only particularly gifted, scientifically completely educated people should be admitted to perform certain investigations, which they themselves would have to propose or which are assigned to them to work on. ... This combination of scientific research and technical application can be realized much more easily in the case of chemical production; the rapid development of the chemical industry in Germany and the dominant position it still occupies presently in the world owes essentially to this circumstance. However, much less favorable is the situation of business based on mechanics. Exact physical investigations demand much more expensive instruments and particularly suitable space; hence, they are considerably more costly and time consuming, and furthermore they require a much larger extent of knowledge and capability on the part of the personnel carrying out the studies. Therefore, in the case of the branches of industry having a mechanical base, such a coincidence of theory and practice favorable to progress will exist much more rarely. ... If one looks at our current situation, the necessity of such a state-organized

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research ... institute is quite convincingly clear. One decade ago precision mechanics had lost its earlier superiority to a highly disturbing degree. The apparent evils were so large and alarming that at the time the state institutions, which could not satisfy their requirement of precision instruments at home any more, initiated the foundation of a state organization for promoting and upgrading domestic precision mechanics. Admittedly, since that time an essential improvement in this matter has occurred; however, it will not regain its earlier high position, unless the help planned originally really comes. One speaks of long series of experimental studies of the composition and the fabrication methods of the different types of glasses needed for thermometric, electric, and other purposes, of their physical properties, and the exact determination of their constants. Missing are extremely necessary, similar experimental studies of the properties of metals and their alloys, of the gradual changes of these properties due to external effects (temperature change, vibrations, electric effects, etc.), of elastic and frictional situations. Also missing are detailed studies of the specific [electric] conductance of metals, of the induction constants of insulators and of their insulation constants at different temperatures and at high electric voltages. ... Yet still much more important success is expected from the actual research activities, which had to be connected with the Institute for upgrading the precision mechanics planned earlier. If one looks at the development of new branches of industry or at the essential reshaping of existing ones, then one sees that usually this happens in steps. As a rule this follows new scientific accomplishments, providing new goals or new incentives to industry. Recent examples of this are: the complete change of pyrotechnics due to the regenerating heating system, and of the steel industry due to the Bessemer process; the inestimable increase in value of the German mostly phosphorus-containing iron deposits due to the Thomas dephosphorization process; the anilline- and alizarine-fabrication, which has very favorably affected Germany’s international balance of trade. The possibility of the low-cost generation of large electric currents by means of the dynamo will exert a similar reshaping effect upon the mechanical industry. Meanwhile the transmission of large amounts of power by means of electricity and its use at other locations for different technical services have been added to the rapidity of the propagation of electric effects, which has already influenced our cultural life quite drastically. This alone clearly signifies the necessity of an organization by the state for experimental scientific investigations. Although the principle of the dynamo was already published in January 1867 by the Academy of Berlin and the major technical consequences it would

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lead to pointed out, it still took more than a decade for worldwide industry to succeed in working out this principle far enough for it to be applied to practical life. ... Up to now the technical consequences resulting from the application of arbitrarily large electric currents in the different branches of industry cannot be overlooked; however, the country which does it first will thereby obtain a strong lead over all others. Therefore, when the support of scientific progress by the state in this area is discussed, one is dealing with important questions of the national economy. In addition, the necessity for the definition of certain electric units for traffic and of permanent institutions for the control of these units has turned out to be inevitable because of the large-scale application of electricity. Although theoretically and practically these units were established and applied for the first time by Germany, an organization responsible for carrying out difficult scientific projects with the completeness required by their practical application is still missing, and we run the risk that England and France may outrun us in this concern. Already the burning question of electric units inevitably calls for the most rapid establishment possible of an organization to provide suitable locations and facilities for scientific experimental investigations. Compared to the invaluable benefit expected to come from such a well-financed and suitably staffed organization, the invested financial means are likely to be negligible. (At this point, we note the following. Many of the most important pioneers of modern physics whose work we will touch on in this book were born within the period of only five decades in the middle of the 19th century and educated just as the world of classical physics was taking shape: Maxwell 1831, Boltzmann 1844, Faraday 1845, Röntgen 1845, Becquerel 1852, Michelson 1852, Lorentz 1853, Planck 1858, Lenard 1862, Nernst 1864, W. Wien 1864, Rubens 1865, Mme Curie 1867, Rutherford 1871, Einstein 1879, O. Hahn 1879, v. Laue 1879, N. Bohr 1885 and Schrödinger 1887.)

Vote of the Chief of the Trigonometric Department of the Royal Survey, Herrn Oberstlieutenant Schreiber (May 1883) The Dependence of the Royal Survey on the Advances in the Area of Precision Technique The Royal Survey has to carry out and to process extended measurements with an annual budget of about one and one half million Marks. Therefore,

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it is highly interested in the general improvement of precision technique. ... The survey has to cover the Prussian State with a network of trigonometric points, which completely satisfies administrative needs. Annually 200 square miles of a triangulated area shall be completed with 10 points per square mile, i.e., every year 2000 points must be determined. All points must also be determined regarding their altitude. For this purpose every year 31 universal instruments and theodolites and 5 levelling instruments are in action, all of which are equipped with a telescope and a water-level made of glass (the universal instruments and theodolites also with microscopes) and which belong to the most excellent of their kind, and partly even to the first-rate instruments. These trigonometric activities are immediately followed by the topographical ones. Annually 200 square miles are topographically picked up at the scale of 1 to 25 000, in which case more than 100 instruments are used, which are also equipped with a telescope and a water-level made of glass. Finally, for the copying of the topographic records and of the numerous other maps many precision instruments ... are used, among which in this case the photographic instruments are particularly relevant. From this it follows immediately, that the shortcomings of the glass regarding its application for optical purposes and for the fabrication of water-levels ... not only result in the most sensitive inhibition of the works of the survey, but also must reduce the quality of it. Every year it happens that during the activities in the field without any obvious reason the water-levels lose the reliability of their reading and become quite useless. Each time such a case leads to a considerable delay, because the glass tube must be replaced by a new one, and the latter must be investigated regarding the value and accuracy of its reading. However, if the problem is not recognized early, then the resulting errors remain in the work already performed, or the latter must be discarded and repeated. Regarding the telescopes and microscopes, admittedly so far the properties of the glass used have not generated technical faults of such a sensitive kind as in the case of the water-levels; however, frequently there appear cloudings of objectives, requiring their replacement by new ones. On the other hand, by far the largest profit the survey would draw for its work on the improvement of optical glass comes from the fact that it allows an increase in the power of the telescopes without increasing their dimensions. The latter are limited quite narrowly because of the requirement of easy transport of the instruments, which reduces their brightness and enlarge-

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ment to a certain degree and can only be exceeded by improving the glass quality. However, the enhancement of the power of the telescopes regarding this point ... would present an invaluable advantage for the ease and rapid progress not only of the activity of the survey but in general of all geodetical measurements. ... The determination of a large number of points within an extended area must always be preceded by the construction of a fixed frame covering the whole area with connected chains of the largest possible triangles. This fixed frame represents the main network of triangles, and the survey has to place such a one over the whole State of Prussia. In addition to the resulting practical purpose of the main network of triangles — according to the organizational statute of the survey — it should also completely satisfy the scientific requirements. In the case of science accuracy is an end in itself; therefore, it goes without saying that for the scientific part of its task the survey must strive for the highest degree of accuracy. However, here it is particularly important that the highest accuracy of the measurements of the main triangles reached at the moment appears still much more imperative in the case of the practical purpose of the latter. Few difficulties exist when covering an area of several hundred square miles with a network of triangles in which no errors of disturbing magnitude appear, as it is difficult to avoid such if the area to be triangulated amounts to several thousand square miles, and if the results shall serve not only scientific, but predominantly practical purposes. For scientific purposes so far, one made do with calculating large triangulations for the time being in individual parts (chains), and postponing an exact matching. However, particularly in the case of this matching procedure, the largest errors by far appeared due to gradual accumulation, and they still become much larger, if one cannot wait until everything is finished, but instead must calculate the measured parts every 2 to 3 years and adjust them to those parts already finished, as is essential in the case of each triangulation intended for practical purposes. Regarding the practical part of its task, the survey is in such an extremely unfavorable situation, that the degree of accuracy considered completely satisfactory for scientific purposes today is by no means sufficient to avoid very large angular distortions with certainty. Thus it is that within the individual chains, as they are offered by the survey for scientific purposes, there appear only very few angular corrections of more than 1 sec and only a single one of more than 2 sec, whereas due to the

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matching condition even 10 sec had to be exceeded. In the case of the measurements of the main triangles, still more important than the mentioned faults of the glass, another problem existing in the area of precision technique must be considered, which has represented a serious difficulty for the survey for more than 10 years. This is the uncertainty existing in the determination of the temperature and of the length of metal rods depending on it, in particular in the case of measuring rods of the base. (After discussing the problems associated with different metals serving as metal thermometers, Schreiber continues): If now it appears without any doubt that zinc is one of the metals having a most irregular expansion, the question still exists, if and which two metals yields a better metal thermometer than zinc and iron. In this case it is important that the two differ as much as possible in their expansion. ... Since zinc expands more strongly than any other metal, it is questionable whether this advantage compensates for the disadvantage of the higher irregularity. To be sure, one has already employed other metals for the construction of base measuring rods; however, so far one has not found out if they work better than zinc and iron. Instead, the general distrust against metal thermometers in some countries has already led to the return of mercury thermometers, previously replaced by metal thermometers because of their inaccuracy. ... The practical part of the present situation is the fact that one can measure a trigonometric base line no more accurately than to the two-hundredthousandth part of its length, whereas this accuracy immediately increases up to its 10- to 20-fold magnitude as soon as the question of thermal expansion of the rods is solved. For 12 years now the survey has had the urgent need to have a new base instrument at its disposal, because that of Bessel is outdated and primitive compared to those constructed more recently. However, as long as the difficulty mentioned is not overcome, the worst instrument yields the same accuracy as the best, and the remaining advantages of a new instrument are not worth the time and the cost of its construction ... . Therefore, the survey felt compelled to carry out its base measurements, of which three have happened during the past 12 years, using as before Bessel’s instrument.

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Vote of the Herr Geheimer Regierungs–Rat Prof. Dr. von Helmholtz (June 1883) About the Tasks of the Scientific Section of the Projected Physical–Mechanical Institute In the memorandum and the associated votes, which were presented during the last meeting to the commission installed by the High Ministry for the discussion of the organization of a physical–mechanical institute, as far as I see, the aspects which relate to the promotion of precision technique are essentially completely emphasized, and a quite appropriate organization of the institute has been proposed for this purpose. I only want to permit myself to emphasize in more detail the essential importance of the scientific section of this institute as well, not only for the further development of pure science but also for the promotion of precision technique. For the purely scientific research there exist a number of important tasks which cannot be solved by the private means of individual workers or by the laboratories of our universities founded for the purpose of teaching, since to cope with them they require on the one hand expensive instruments and space, and on the other hand also more free working time of experienced and competent researchers than normally can be made available without support by public finances. So far it had been nearly exclusively astronomy, whose care has been taken up by the state in terms of separate institutes and observatories devoted primarily to scientific research and, only secondarily, to teaching. ... Aside from the fact that astronomy has caused a total revolution in our whole world-view because of the ideas it is giving us about the construction of the world system, our navigation, [and] the civil and the historic calendar essentially depend on it, as do the art of practical optics, the higher watch-making industry; all improvements of the lengthand angle-measurements resulted directly from the tasks given to it, as well. ... In the documents presented to the commission it has already been discussed in detail what is required by the astronomical and the related geodetical investigations. The tasks to be handled for this purpose by precision mechanics and the help to be provided are clearly and completely listed therein. However, these two directions of scientific activities already have their purely scientific institutes and organizations, and what they feel lacking, therefore, is restricted to the inadequate technical support of their work by

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the art of mechanics. However, also in the other branches of natural science, in particular that of physics, there exist a number of quite similarly important tasks, which so far either are solved only highly insufficiently, or, corresponding to the increasing knowledge of that science, from time to time must be taken up again, using improved methods and requiring considerable external financial means for their implementation. Admittedly, following my suggestion, the memorandum has been completed to some extent regarding this point; however, the commission has felt that a few more detailed remarks about this part of the matter would be advantageous. In order to first mention a few physical tasks closely connected with the astronomical–geodetical tasks, I list the following: 1. The exact determination of the intensity of gravity and the comparison of this intensity at different locations on the surface of the Earth. During recent years the International Commission for the European Measurement of Degrees has discussed this subject many times, since it would yield an important control of the geodetical levelling and would be extremely important for the determination of the local irregularities of the surface of the Earth. However, so far a laboratory is missing where the preliminary experiments on the desired higher improvement of the observation methods can be carried out. 2. The absolute measurement of gravitation, or the determination of the average density of the Earth. In this case so far only a moderate degree of accuracy has been reached. By means of such a measurement the masses of the celestial bodies would be reduced to the same scale as the terrestrial ones, in the same way as one uses the length measurements of the meridian and the observation of the passage of Venus in order to reduce the cosmic distances to a terrestrial length scale and to determine exactly the magnitude of the gravitation. The latter determination is extremely important for the general physics and for the derivation of a temporal scale independent of the probably varying rotation of the Earth. 3. The velocity of light can be determined within terrestrial distances and, as the previous measurements have shown, with an accuracy which may not remain behind that probably reached by the passages of Venus. This determination is again suitable to reduce the cosmic distances to a terrestrial length scale, and since over the next hundred years no passage of Venus will occur, it would be quite advisable to continue work along this direction. 4. In the theory of the magnetic effects of electric currents a velocity ap-

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parently exactly equal to the light velocity plays a fundamental role. W. Weber refers to it as the critical [velocity]. Its equality with the light velocity appears to indicate an essential internal relationship between the optical and the electrical behavior. Insight into the mysterious parts, in particular of the electromagnetic phenomena, appears to have found a guiding line in this case, which probably will lead us to its deepest level. In the case of electric technology, the exact knowledge of this velocity Weber mentioned is of great practical importance as soon as electric currents and electric charges become effective simultaneously. (Here we note the visionary character of the remark concerning the (fundamental) role of the velocity of light. We will come back to this point briefly in Chapter 12 in connection with the actual definitions of the units of length and time).

5. This point is followed by the studies dealing with the electric scale units taken up many times already. They have been recommended by the two congresses in Paris in recent years. However, their final definition is still due, because the majority of the corresponding studies had to be performed using insufficient external means. 6. The theory of thermodynamics, of the forces generated thermally, is actually based on the measurements of the pressure and the density of gases and vapors at different temperatures and on the measurement of the amount of heat dissipated in this case. The most reliable determinations of this kind which we have so far are given by V. Regnault. For this work he could benefit from the means of the porcelain factory at Sèvres, of which he was the director, ... Later when after 1848 the financial means were held up, he received very considerable sums from England from private sources supporting this work. Nobody before or after him was able to work with similar financial means, and since he was an excellent and intelligent researcher, his accomplishments in this field are of outstanding value. However, already now one could repeat some parts of this study using improved methods, [and] avoid newly discovered sources of error; furthermore, a number of questions remain unanswered, ... which already made themselves felt as a highly disturbing gap, considering the influence of thermodynamics upon nearly all chapters of physics. These shortcomings will become more and more noticeable, and a repetition of the investigations by Regnault will soon be urgently needed. The whole development of our more modern technology is based on the application of the heat engine, whether it utilizes vapor, hot air, or gas; therefore, these questions are also of great economic importance.

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These are some of the most important problems as they exist just now; without any doubt each decade will generate new ones no less important and having a similar character. ... Because of the listed motivations, on the occasion of the inquiry by the High Ministry regarding the determination of the galvanic unit of [electric] resistance, the Academy of Sciences has also allowed itself to recommend a request of the establishment of a physical Observatorium organized similarly to the [astronomical] observatories. The connection of such an Observatorium to an institute for the promotion of precision mechanics would be relatively strong and natural. ... To me the above remarks appear to emphasize that an institute of physical precision measurements is not only necessary, and will become more and more so, but also that the planned Institute of Precision Mechanics will be able to develop in a way far more fruitful and stable, if new large tasks are constantly proposed by the organization itself, rather than if it should remain completely dependent on external requirements, the number and direction of which being difficult to estimate, in particular at the beginning. Just because of this it appears important to me that right from the beginning the scientific section should be connected with the technical [section]. Memorandum concerning the Foundation of an Institute for the Experimental Promotion of Exact Natural Science and Precision Technique. (Physical–Mechanical Institute) of June 16, 1883 I. General. During the negotiations of the sub-committee appointed at the end of the year 1873 by the Central Board of Directors of the Survey in the State of Prussia, by whom in the year 1874 the “ Proposals for upgrading scientific mechanics and instrument science” have been presented, as well as in those documents which have served as the starting point of these negotiations, it has already been emphasized that admittedly at the time a particular economic emergency of precision technique represented the most urgent cause for the introduction of support by the state for the corresponding industrial and scientific interests, ... that in the basic motivation for the operation of

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precision technique ... there would be included very serious requests for the state to turn its attention in the future to the care of precision technique not only occasionally but instead systematically. In particular it had been pointed out that the necessary and highly promising further development of precision technique is confronted more and more by such tasks and problems, the handling of which by the private sector is not profitable, since in the presence of large difficulties and costs no immediate financial success and no immediate increase of productivity is expected, whereas on the other hand ... there exists the highest probability that the handling of the corresponding tasks and problems ... will bring the richest profit and success during the course of later development. ... Precision technique is confronted by larger and larger difficulties to satisfy convincingly the severe requirements which it must meet regarding the quality and the reliability of its materials; since in the case of the increasing mass production of many materials being indispensable but at the same time serving general needs, the special requirements presented by the latter needs have started already to lead to an unmistakable decline of many essential properties of the materials which are highly important for precision technique. ... For some time, in addition to individual geodesists and physicists, there were almost only observatories which could perform lengthy and thorough series of investigations about the accuracy of the instruments and the quality of the measuring techniques, because to these scientific institutes was still given the greatest freedom of immediate jobs and duties, and because particularly in the case of research projects resulting from the nature of astronomical problems and extending over longer periods, the mentioned economic profit, achieved by a more complete study of the instruments and measuring techniques, was more clearly visible. However, also the observatories, in particular the German ones, have started now to experience very perceptible restrictions of their free experimental activities dealing also with larger problems of the future, [this restriction being] due to the increase in the teaching duties and due to the increasing obligation of their personnel to all kinds of scientific collaboration in the more exact tasks of administration (public time service, time signals at the shores, nautical extrapolations, surveying, meteorological and magnetic service, units and weights, etc.). As a result, in Prussia it has already appeared to be advisable in this field, to found a separate institute, the Observatorium in Potsdam, in a completely isolated location and totally free of all immediate teaching duties and of all other official obligations, which during the course of its development more

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and more explicitly has been given the task to serve exclusively the more basic experimental program and the measurement problems of astronomy and astrophysics ... without consideration of the ephemeral success and of the immediate benefits. Just in the same sense, but to a still higher degree, in addition to the care of precision technique, now it appears urgently required that in the case of the whole field of exact natural science in Prussia, at least the nucleus of an institution be created by means of a physical Observatorium, which in time will improve the mentioned imperfectness of the existing organization before ... perhaps suitable personnel starts to be missing. ... By the way, how much recently also in other countries the necessity is recognized to assist experimental research and precision technique associated with it, without restriction due to educational and administrative purposes, using the means and ways of the state, follows from many public proposals and from the corresponding approvals, for example, in England from the funds for experimental studies of general interest transferred some time ago to the Royal Society ... , as well as from similar temporary and rather aimless steps, the whole purpose of which is denoted by the slogan “endowment of research”. Also abroad there exist already individual institutions which admittedly do not completely and exclusively serve the tasks discussed above, however, which are financed such that they can serve also the experimental and theoretical support of precision technique; for example, the Conservatoire des Arts et Métier in Paris, which became famous because of the critical handling of many relevant studies; ... further, the physical Centralobservatorium in St. Petersburg, in which Kupffer at his time carried out investigations highly important for precision technique. Recently in France to the largest extent steps were taken for the special care of experimental research and technique by means of the foundation of an important state institute, which originated from the stimulations generated by the electrotechnical exhibition in Paris, and which at first had been financed by the excess funds remaining from it, but now constantly shall be developed further using large financial means. During this whole period in our case the perfection of our teaching organization has been perhaps the most striking aspect of the development. And if it is justified ... that recently compared to other nations we have clearly fallen back in terms of the purely inventive, original accomplishments in the relevant areas of research and technology, certainly this would find its explanation partly in the strong pressure of the teaching functions on the research activities already emphasized above. ...

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At present in particular the state of glass fabrication presents very considerable uncertainties and material losses for precision technique. The orientation based on momentary success, on the immediately appropriate [steps], ignoring such economic considerations which also include the element of continuity, just in the fabrication of this material ... over a long time has gained a dominating influence. Aside from an attempt taken up to improve the bare long-term stability of this material at the expense of other properties, changes in the chemical composition of the glass, which turned out to represent clear advances regarding the economy of the fabrication as well as the ease and reliability of the design, were introduced gradually; however, regarding the behavior as a function of the temperature and the stability against influences of the air [they represented] a step backwards of the most deplorable kind. It happens more and more often that optical glasses, the most careful polishing of which had taken much effort and costs, already after a few years become dirty with a frightening ease and rapidity, sometimes only temporarily, but yet to such an extent that during the necessary periodic cleaning the perfection of the surface suffers more and more, sometimes a restoration becoming even impossible. Thermometric glass tubes which are calibrated most perfectly depend on the effects of temperature changes to such an extent that the actual temperature can be derived from its readings only by numerical calculations, which take into account also the influence of the previous temperatures, sometimes even this being impossible with sufficient accuracy. Also it appears to happen sometimes that sweating of the glass surfaces at the inner walls of the tubes causes disturbing inhibitions and distortions of the thermometric movements of the mercury. Probably some part of the more serious perturbations of the barometric measurements due to anomalies of the capillary action, happening more frequently for some time, must be traced back to similar states of the glass surfaces. Water-levels made of glass, i.e., glass tubes the inner walls of which one has prepared by highly accurate polishing, making them invaluable instruments for the measurement of the inclination angles versus the horizontal plane, more and more frequently give out by sweating at the walls, which disturbs the movements of the air bubbles versus the scale division, at first in a way which is difficult to recognize and, therefore, more dangerous, [and which] later cause very severe inhibitions of these movements, such that the great effort spent for the polishing of these water-levels is completely lost sometimes. ...

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Furthermore, in the area of optical glass there still exists a peculiar emergency, not only regarding the German astronomers but in general regarding the future development of armed viewing. Admittedly, in Germany there exists an important location in Munich for the fabrication of optical glass; however, for its more general needs its precision technique depends upon two foreign locations in Paris and Birmingham for the fabrication of optical glass, since this location essentially works only for the binoculars fabricated by its owner. At these two locations this branch of fabrication, which requires quite special equipment and experience, is almost monopolized, and, of course, there one is neither inclined nor in a position to perform difficult and costly experiments with uncertain success for the purpose of the further improvement of optical glass. And yet such improvements and the chemical and physical studies absolutely necessary for this ... represent one of the most urgent demands for the economy and the progress in scientific research. The present state is characterized by the fact, that — instead of a rational investigation and improvement of the still-existing imperfections of optical glass — for a further increase of its power of collecting light, enormous increases of the lightcollecting areas are demanded and are being worked on. On the one hand, this increase of the dimensions of optical glasses to a large extent leads to an increase of the expense and heaviness of the instruments; on the other hand, precisely in practical astronomy a number of evils, connected with the spatial variations and fluctuations of the temperature of the surrounding air, are enhanced so much that in many cases the gain in power of the glasses achieved with enormous financial means becomes quite illusory or even turns around to the opposite. Previous attempts to achieve a better performance, having a reduced sensitivity to inevitable external situations and requiring a more reasonable financial effort, by means of improvements of ... essential properties of optical glass and by an impractical increase of the bare dimensions of the glasses, so far in the hands of private people, have reached the threshold of at least partial success; however, without any doubt the accomplishment of a more complete ... improvement of the performance of optical glasses has turned out to represent a task the solution of which can only be expected with some certainty ... if substantial financial means are reliably promised. In the case of Germany at present the fact is that soon German astronomers will be forced ... to apply for a large telescope, at least for the observatory in Potsdam, having the same dimensions as existing already or soon to exist in Washington, Vienna, and Pulkowa, and that in this case many hundred

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thousand Marks must be approved which, in large part, would have to be paid to a famous optician in North America, unless in the near future ... it is possible to develop the chance, due to the fabrication of better optical glass, that ... kinds of glass with much smaller dimensions but better qualities can be given preference for most research tasks. In this case ... it will only be necessary to promise to particularly competent people, who have already occupied themselves with investigations of a corresponding kind, considerable material support after the submission of valuable results in this field, and that for this purpose a state institute, exclusively established for mechanical–physical research, will be charged with the testing of such results and with all experiments and measurements which can support the development of such a fabrication. ... However, it will be necessary to establish, in the same way as in the case of thermometers, a testing office for optical glass and systems of optical glass, which provides confidence to those interested in science and technology upon payment of modest fees, ... which they cannot obtain by themselves within their economic conditions. In the case of all other applications of glass for precision purposes, in particular also for its application in electrical insulations and similar cases, one will deal with quite similar improvements ... . Also the strain phenomena in glasses will deserve most detailed examination. Presently other severe evils exist in an analogous way in the area of the metal industry. ... In the case of the bronzes it has been in particular the increasing use of zinc, which has enhanced these uncertainties and evils, but also in the case of the fabrication of iron and steel, regarding the thermal behavior, the surface qualities, the elasticity, etc. of the materials to be used, precision technique ... is in an evil situation; for example, presently almost never can it rely on a somewhat confirmed knowledge of the thermal expansion behavior of its materials. Thermal compensations of pendulums and chronometers almost never can be carried out by numerical calculations, but only by trial and error, since in particular the expansion behavior of the applied materials — steel, zinc, brass, and bronze — are different in nearly every piece of the same material, depending on the previous metallurgical treatment and their indescribably varying composition. The large anomalies which can be seen more and more clearly in particular in the expansion of zinc but also in some sense of steel and iron, sensitively impair all delicate length measurements as well, such that also due to this point our surveying is confronted by difficulties which cause not only enormous effort and costs, but also paralysing uncertainties and quarrels within

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the circles of experts. ... The assistance of the Institutions of Weights and Measures, which is taken up frequently against the mentioned evils, and which has been provided, for example, by the Normal-Calibration Committee in terms of comparisons of measures and expansion measurements, cannot yet be provided to the full extent of the needs, if the original activity of these authorities, which they must take up primarily, ... should not suffer. Only the study of metals regarding their thermal behavior will be relevant for directly affecting the subject of measures and units, whereas also along many other directions there exist urgent and important tasks; for example, regarding metallic alloys most suitable for reliable magnetic instruments, regarding hardening, regarding elastic behavior (after effects) and resistance against chemical effects of different kinds, regarding the homogeneity and stability of the structure, etc.; finally, in the area of friction experiments and the like. For example, essential progress in the field of chronometry will be expected from the results of certain studies of elasticity. Therefore, experiments supporting the fabrication of suitable metallic materials for precision purposes, as well as the systematic testing and certification of such materials, and maintaining a certain continuity and uniformity of their fabrication, just in the same sense as in the case of glass technology, will represent important and urgent tasks of an institute financed by the state. In the area of physical research, first, electric studies of the units are urgently required. ... (After some more general remarks the memorandum continues): However, Germany and especially Berlin as a center of great electrotechnical activities is particularly suited for such a handling of the task, and one can hope, that ... later also the science and technology of other countries will not ask for a different experimental and testing authority of electrotechnical character and will turn to these institutions to the considerable economic advantage of German precision technique. Among other things studies of the so-called “critical” velocity [introduced by] Weber, which is fundamental to the theory of the magnetic effects of electric current, would have to be connected with these electric activities. Regarding other physical investigations, for which in our case suitable institutions are presently totally absent, one would have to emphasize an urgently necessary renewal and improvement of the most important thermodynamic measurements of pressure and density of gases and vapors at

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different temperatures, etc.; furthermore, absolute determinations of the gravitation or of the average density of the Earth, fundamental measurements of the light velocity, etc.. A second part follows on “More detailed proposals regarding the establishment of a physical–mechanical institute in Berlin”, which we do not present. At the end it says: The undersigned are strongly convinced that based on the above remarks the physical–mechanical institute can develop into a creation which promises not only to accomplish something important, but to form a true nucleus of national prosperity. Berlin, June 16, 1883

Oberstlieutenant Schreiber, Chief of the Trigonometric Section of the Royal Survey

Geheimer Regierungsrath, Professor Dr. von Helmholtz, Member of the Royal Academy of Sciences

Geheimer Regierungsrath, Professor Dr. Landolt,

Geheimer Regierungsrath Dr. Werner Siemens, Member of the Royal Academy of Sciences

Major von Goessel, à la suite of the General Staff of the Army

Geheimer Regierungsrath, Professor Reuleaux.

Member of the Royal Academy of Sciences

Professor Dr. Vogel,

Professor Dr. Paalzow.

Director of the Royal Observatory in Potsdam

Professor Dr. Doergens.

C. Bamberg, Mechanic and Optician

R. Fueß,

Professor Dr. Foerster,

Mechanic and Optician

Director of the Royal Observatory

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Memorandum concerning the Foundation of a “Physikalisch– Technische Reichsanstalt” for the Experimental Promotion of Exact Natural Science and Precision Technique The first proposals for the establishment of a state institute for the promotion of the exact natural sciences and precision technique were presented on July 30, 1872 by Professor Dr. Schellbach, supported by the Gentlemen von Helmholtz, Du Bois–Reymond, Paalzow, Bertram, and Foerster, and at the time have found the enthusiastic support of His Imperial and Royal Highness the Crown Prince. ... During the further developments Geh. Regierungsrath Herr Dr. Werner Siemens has offered to the Royal Prussian Mr. Secretary of Educational Matters to donate an area of 12 000 m2 to the Prussian State, if the latter would commit itself to take up the building, the equipping, and the maintenance of the necessary laboratories and other buildings for the Section of the projected Institute to be charged with fundamental scientific research. After he had been informed about the approval by the Prussian State Parliament required for meeting the stated conditions, Dr. Werner Siemens declared that he would also carry the costs of the construction of the required buildings, and that he would like to proceed at his own risk without requesting a guarantee of the reservation of the financial means in the annual budget of 1885/86, in order that a full building year would not be lost. According to his wish the Counsel of Building Technology in the Royal Prussian Ministry of Educational Matters was given the permission to help him with the design and execution of the buildings. This was the situation of the matter, when Dr. Werner Siemens, looking at the national importance of the plan and hoping for an implementation of it on a larger scale and with increased financial means, decided to extend the offer to Prussia and to the German Reich. The Prussian Mr. Secretary of Educational Matters expressed his agreement: his most vivid desire would be that one would succeed at all in getting started with the important institute. Afterwards Dr. Werner Siemens declared himself to be ready to donate to the German Reich the amount of half a million Marks in property or in capital for the foundation of an institute performing research in the natural science for technical purposes, and in the corresponding document he has summarized his views about the importance and the goals of such an institute as follows: (Here follows the document written by Siemens. In the first part he dis-

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cusses at length the role of education and natural science in Germany and other countries and the need of a research facility maintained by the government, and then he continues): Already a longer time ago these ideas have caused the decision of the undersigned to donate to the Royal Academy of the Sciences ... a larger sum of money for the foundation of a laboratory which should be devoted to fundamental scientific studies. However, when during the past year ... the necessity of an experimental facility serving exclusively the research in natural science was approved by His Excellency Mr. Secretary Dr. v. Goßler, but on the other hand in addition to financial problems the difficulty of obtaining a suitably located building site hindered the execution, I offered to donate to the state such a suitable plot of land of about 1 hectare area located in the Marchstrasse in Charlottenburg under the condition, that the state builds on it for the mentioned purpose at its own expense and finances the institution appropriately. Furthermore, I also offered to take up the building of the laboratory space hoping to avoid in this way additional loss of time. ... Finally I remark that with the offer of 12 million Marks in property or capital for founding the planned institute I only intend to provide a service to my country and to express my love of the science, to which I owe exclusively my advancement in life. Berlin, March 20, 1884 Dr. Werner Siemens Geh. Regierungsrath ... A Physikalisch–Technische Reichsanstalt having the tasks and facilities explained in detail further below would differ from the physical or other scientific–technical institutes, laboratories, and observatories of the universities and technical colleges ... of the individual federal states ... essentially in terms of the magnitude of the tasks and of the quality of the corresponding facilities, in addition to the important condition emphasized by Herrn Dr. Werner Siemens that the Reichsanstalt be kept free of any connection with teaching obligations. The scientific–technical institutions existing in the different states would lose neither their missions nor their financial means; so far none of the large missions, which shall be given to both sections of the Reichsanstalt according to the work plans indicated below, ... has been taken up by one of the mentioned state institutions, whereas all of the latter demonstrate the largest need of a thorough and complete handling of the corresponding

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tasks. ... Also taking into account that in several of its largest and most extensive services — telegraphy, military and navigational science and technology — the Reich is intimately involved in a deeper foundation and promotion of many physical–technical investigations, an institution as it is discussed will have to be established not as a facility of the state but instead as one of the Reich. As an important aspect of the handling of fundamental scientific– technical institutions as a matter of the Reich there is also the commercial and economic interest group within the whole German Reich. In the case of the only government support of precision technique exercised recently, namely in the case of the support provided by the Royal Prussian Government to the Herren Professor Dr. Abbe and Dr. Schott in Jena for scientific research about the deeper foundation of glass technology, initially for optical and thermometric purposes, there occurred already the necessity to go beyond the borders of the Prussian State. For handling the tasks discussed above in general terms the Physikalisch– Technische Reichsanstalt would consist of two main sections, the first of which, referred to in particular as the “scientific” section, would have to devote itself to research in the corresponding areas, whereas the second section, referred to in particular as the “technical” section, would have the task to further develop the research results along the technical side and to make them useful for scientific technology. For this purpose this section would carry out independent technical studies; it also would have to maintain permanent contact with the different branches of the relevant technology; finally in some sense it would represent an analogy to the technical experimental stations of the state institutions; however, it would differ from these ... due to the fact that because of the close connection with the scientific section of the institution ... it would have to make the preservation of the uniformity of the tests and certifications in the physical–technical area one of its main tasks ... . Without the creation of such a center for the conservation of the basis of physical measurements, as well as for the orderly and final settlement of existing differences, indeed there would be the danger that the results of the different experimental institutions ... could lead to an increase or at least to an intensification of uncertainties, as far as a controversy between the results of different public experimental institutions would be particularly embarrassing for the people involved.

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(A large number of pages follows, dealing with the tasks of the first (scientific) and of the second (technical) section, which had been worked out by Dr. von Helmholtz or Dr. Foerster. Also included is a detailed organizational plan explaining the board, the buildings, the personnel, and the operating expenses. Here we do not reproduce this particularly detailed part of the memorandum.)

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Chapter 3

The Start under President Hermann von Helmholtz

Administratively the new Reichsanstalt (Imperial Institute) was under the control of the Ministry of the Interior. However, essentially this control extended only as far as the budget. A much more important role was given to the board, which supervised all activities. It consisted of 24 experts in the field of physics and the precision technique. The board was to meet annually and to evaluate the activities of the previous year and the plans for the next year. The uppermost superior of all members of the institute was the president. He managed the physical section as its director and supervised the director of the technical section. Usually small groups of 2 – 4 people collaborated, working on a specific research subject. The physical section followed two general directions. On the one hand, it was supposed to take up scientific research subjects which required a greater effort in time, instrumentation, and material than could be expected from the universities and technical colleges. On the other hand, it should solve problems which had come up in the technical section. The technical section had to perform the following tasks: (a) advance precision mechanics, (b) certify the instruments for measurements and control, (c) fabricate instruments for the government which were not available from private industry, and (d) build apparatus parts for industry, if the latter is unable to do so. Altogether, one had observed a careful balance between the activities of the Physical–Technical Institute, the universities and technical colleges, and industry, such that competition between these was avoided as much as possible. In the end, Werner Siemens had donated to the Government a large area of 19 800 m2 with an estimated value of 566 157 Marks, located near the Technical College Charlottenburg for accommodating the physical sec35

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Fig. 3.1

Fig. 3.2

Outline of the PTR, 1884 – 1887.

Map of the area of the PTR (PTB Archives).

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tion of the Physical–Technical Institute. This donation was made under the condition that a building for physics and a residence for the president be financed by the German Government. In 1892 the German Government bought an additional 14 389 m2 of an adjoining property from Siemens for the amount of 373 106 Marks. So in 1893 the Physical–Technical Institute possessed a total of 34 189 m2 or, after subtraction of the necessary part for road construction, 25 739 m2 , having a total value of about 939 263 Marks. Prior to 1920, this represented perhaps the largest research complex for physics worldwide. Between 1887 and 1896 ten individual buildings, five for each section, were erected. Next to the physical institutes of the Universities of Berlin and Leipzig, the physics section of the Reichsanstalt represented the most expensive physics facility in Germany at the time. The total sum of 3 672 360 Marks invested in the buildings and the equipment of the Physical– Technical Institute in Charlottenburg exceeded by more than a factor of two the amount of 1 500 000 Marks spent by the USA until 1903 for building the National Bureau of Standards, and by a factor of six the amount of 600 000 Marks spent by England until 1902 for the National Physical Laboratory.

Fig. 3.3

The Observatorium (PTB Archives).

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Hermann von Helmholtz served as the first President of the Reichsanstalt. When he assumed this office in March 1888 he was 66 years old, with a highly distinguished career as a physiologist and physicist. In Chapter 13 we present a brief summary of his biography. His academic career extended over a period of 44 years. In his research he had dealt with the most abstract theoretical and mathematical problems, but he was also a pioneer in experimental work and in the development of instruments. At the time he clearly was the single most suitable candidate for the presidency of the Reichs anstalt offered to him by the authorities, and he had readily accepted. Similar to Werner Siemens, he felt that the pure science is the base necessary for technical developments.

Fig. 3.4

Hermann von Helmholtz in the year 1889 (Siemens Museum, Munich).

During the initial years as president, Helmholtz was occupied by much administrative work, and he had to watch over the building of the Reichsanstalt. However, beyond that he also continued with his own research in the area of theoretical physics, he conducted a seminar at the university, and he held numerous public offices. In this wide range of activities he

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benefited from his capability to attract many bright young people to act as his assistants.

Fig. 3.5 Residence of the president, built in 1889, damaged during the Second World War, and later demolished (PTB Archives).

From the beginning Helmholtz took special care that the results of the scientific and technical activities of the institute were published in major scientific and technical journals. Also he closely collaborated with the journal, “Annalen der Physik,” which quickly published the papers submitted by the Reichsanstalt. During the period 1877 – 1899 this journal was edited by Gustav Wiedemann, a board member of the Reichsanstalt and an old friend of Helmholtz’s. For the publication of the results obtained in the technical section Helmholtz selected the “Zeitschrift für Instrumentenkunde,” the official outlet of the “German Society for Mechanics and Optics.” Finally, for the announcement of official statements by the Reichsanstalt such as the legal definition of electrical standards and units, the “Centralblatt für das deutsche Reich” (Central Journal for the German Empire) could be utilized. It took Helmholtz almost six years until the physical section was completed in terms of personnel and the operating budget. Initially, this section consisted of three laboratories: for heat, for electricity, and for optics. In the following we indicate briefly the early activities of these three laboratories.

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Fig. 3.6

Main Building, Siemens Bau (PTB Archives).

The heat laboratory was the largest laboratory of the physical section. It was headed by Max Thiesen and mainly pursued the following lines of research: finding better materials for the fabrication of thermometers; increasing the accuracy of temperature measurements and extending them to higher temperatures; and determining the influence of temperature, pressure, and other parameters on the operation of heat engines. An important subject was the establishment of a reliable absolute thermodynamic temperature scale. Also the thermal expansion of water and mercury within different kinds of glasses was investigated with the aim of improving thermometers. Pyrometric experiments up to the melting point of platinum were carried out in connection with the goal of quantifying the intensity of light sources and of observing the influence of heat treatment on the magnetization of steel. The laboratory for electricity was headed by Wilhelm Jäger. One of its tasks was the determination of the basic electric units of voltage, current, and resistance, and the development of the necessary measuring instruments. It was the intention to reach a leading international position in this area and to break the French dominance. In 1893 at the International Conference on Electricity in Chicago, the laboratory gained international recognition for its presentation of the electric standards of the Ampere and

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the Ohm. Other work dealt with the magnetic properties of iron and steel, being important for the German steel industry. The Imperial Navy had asked the laboratory for help in minimizing the disturbance of the compasses of the Navy due to the magnetism of the steel used for ship building. The optical laboratory was under the direction of Otto Lummer, who was also in charge of the corresponding laboratory in the technical section. The primary task of this laboratory was the improvement of photometry. Furthermore, it was involved in the quantitative measurement of the light intensity and the development of a generally accepted standard for this quantity. At the time many groups were much interested in such a standard. Also the “Deutsche Verein für Gas- und Wasserfachmänner” (German Society of Experts for Gas and Water) strongly advocated this project. In addition, the German Navy was looking for improved photometric instruments and the solution of the problems caused by bad weather. The development of a physical or scientific light standard having the highest possible accuracy represented a great challenge. Eventually, this should also serve as the base for measuring the temperature of all thermally radiating bodies. When in 1894 Otto Lummer was released from his duty as head of the optical laboratory of the technical section, he joined his friend and colleague Ferdinand Kurlbaum in order to concentrate on the development of a sensitive bolometer needed in conjunction with the light standard. They succeeded in the fabrication of such a standard having an accuracy of one percent. Eventually, because of its radiation research the optical laboratory became still more successful and quite famous. This will be discussed in detail in Chapter 5. In addition to its research on photometry, the optical laboratory performed polarimetric studies. Polarimetry was an important subject for the German sugar industry and also for the customs authorities. They needed instruments for polarimetry that had been accurately tested and certified. Also in this case a suitable standard had to be developed for the reliable determination of sugar content. The “German Association for the Sugar–Beet Industry” had asked the Reichsanstalt for support in this matter. During Helmholtz’s presidency the physical section pursued a mixture of scientific work and of studies important for industry. In this way the activities of the Reichsanstalt helped the advancement of technology and demonstrated its usefulness to German industry. The technical section was directed by Leopold Löwenherz (1887 – 1892), who was followed by Ernst Hagen (1893 – 1918). This section was divided into separate laboratories for the following four areas: precision engineering,

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heat and pressure, electricity, and optics. In general, the technical section was heavily involved in the development of instruments and procedures needed for routine testing and the corresponding certifications. The laboratory for precision engineering was headed by Arnold Leman from 1887 until his death in 1914. It dealt, for example, with the exact measurement of the thickness of quartz plates used in the fabrication of polarimetric standards. After the norms for a uniform system of threads issued by the Reichsanstalt had been accepted by industry, threads were regularly tested and certified. Other work included measurements of the thermal expansion of metals and the testing of special alloys to be used for precision weights. Also standards for tuning forks were developed. The laboratory for heat and pressure directed by Hermann Wiebe had to perform the largest number of tests by far, namely for thermometers, most of which were used in medicine. Already in 1889 this workload became so heavy that the Reichsanstalt opened another testing office in Ilmenau in Thuringia, the center of the German industry for glass instruments and thermometers. The work of the laboratory for heat and pressure also included the development of technical pyrometers and the technique of temperature measurements at the furnaces of three glass factories in Thuringia. Furthermore, instruments such as calorimeters, barometers, manometers, and viscometers were tested. The laboratory for electricity was headed by Karl Feußner and concentrated mainly on subjects which were important for the German electric industry. At the time it contributed significantly to electric measuring techniques, for example, by means of the potentiometer developed by Feußner. The laboratory performed many electric precision measurements and tested and certified a large number of electric instruments. Finally, it carried out many tests for other laboratories of the Reichsanstalt and for government agencies within and outside of Germany. Also the current meters of the customers of the electric power station of Berlin were tested and certified by this laboratory. During the Presidency of Helmholtz the optical laboratory was directed by Otto Lummer (as the optical laboratory of the physical section had been). Its main tasks were connected with the German illumination industry, which experienced a strongly growing demand for light generation both within and outside of buildings. A need for reliable and portable photometric instruments existed, to which the optical laboratory turned its attention. The gas-illumination technique (promoted by the “Deutscher Verein für Gas- und Wasserfachmänner”), the testing of arc lamps for street

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illumination, the photometric analysis of different kinds of petroleum, and the generation of light with different colors were some subjects which occupied the optical laboratory. By the mid-1890s the Reichsanstalt had gained impressive momentum. It employed 65 people, among them more than a dozen German physicists. The scientific and technical results were published in first-rate journals, and its scientists regularly presented their reports at the meetings of the Physical Society of Berlin. When Helmholtz died in 1894, following a severe accident and head injuries he had suffered the year before, the Reichsanstalt represented a successful and highly promising facility.

Chemistry Building

Machine Shop

High-Current Building

Main Building

Low-Temperature Laboratory

Observatory

Director’s Residence Fig. 3.7

Administration

President’s Residence Overview of the PTR in the year 1937 (PTB Archives).

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Chapter 4

The Institute as a Model

Already soon after the foundation of the Physical–Technical Institute in Berlin it became obvious that it represented a much admired model. Worldwide the Reichsanstalt was the first research center supported by the state, the members of which were completely free of any teaching duties in contrast to scientists at the universities and technical colleges. The developments at the Reichsanstalt in Berlin were noted with great interest, particularly abroad. Its leading role as a model can be clearly recognized, for example, in the case of the foundation of the National Physical Laboratory (NPL) in England and of the National Bureau of Standards (NBS) in the USA. In England in 1895, Douglas Galton, at the time President of the British Association for the Advancement of Science, started to push for a national physical laboratory as a means of advancing physical measuring technique and the quality of precision instruments. He wanted to stop British scientists from having to go to Paris or Berlin for the calibration of their precision instruments. He had even based his considerations on detailed building plans and the organizational structure of the Reichsanstalt in Berlin. Together with a group of outstanding British scientists he demanded a scientific institution for the United Kingdom similar to the Reichsanstalt in Germany. In 1898 these plans were approved by the Treasury, and one year later the National Physical Laboratory could start its operation in Teddington. In the USA one had also been following the developments at the Physical–Technical Imperial Institute with great interest. Already in 1836 the Office of Standard Weights and Measures had been founded there in order to satisfy the demand in the area of metrology. However, still in 1897 the office employed only five people, such that extended investigations dealing with standards for measurements and questions of calibrations were impos45

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sible. Therefore, frequently in the case of questions of standards and calibrations the American experts had to turn to the Reichsanstalt in Berlin. Eventually, the physicist Samuel W. Stratton was given the task to critically evaluate the activity of the office and to develop a plan for its extension according to the example of the Physical–Technical Institute. Subsequently, Stratton visited the Reichsanstalt and laboratories in England and France. Additional visits of American physicists to the Reichsanstalt followed. In this way it became more and more clear that the American Government should establish an institution like that in Berlin. After important members of Congress and of the government could also be convinced, in March 1901 the Congress of the United States passed a law establishing the National Bureau of Standards. Samuel W. Stratton became the first President of the NBS. Already soon after taking office he visited Berlin again to inspect the Reichsanstalt. Today altogether 51 State Institutes of Metrology exist in as many countries. The leading institutes are: Istituto Nazionale di Ricerca Metrologica (INRIM, Italy) Laboratoire National de Métrologie et d’Essais (LNE, France) National Metrology Institute of Japan (NMIJ) National Institute of Standards and Technology (NIST, USA) National Physical Laboratory (NPL, England) Physikalisch–Technische Bundesanstalt (PTB, Deutschland) In addition there are associated establishments in 22 countries as well. All institutes and establishments are brought together in the Bureau International des Poids et Mesures (BIPM) with its seat in Sèvres near Paris.

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Chapter 5

The Optical Laboratory and the Birth of Quantum Theory

In the years after 1870 the beginning of electric illumination within and also outside of buildings was one of the main reasons for the rapid growth of the electric industry. At the time the development of the technique for the generation, transmission, and utilization of electric energy gained extreme importance. In a brief overview of the history of the generation of light in the 19th century we have to note that during the second half of the century a few new artificial light sources came into use: the petroleum lamp, gas illumination, and the electric light source. However, the breakthrough of electric lamps only occurred after Werner Siemens had discovered the electrodynamic principle of the generation of electric current in 1867. In 1879 in the USA Thomas Alva Edison constructed the first lamp having a carbon filament, and in 1881 he built the first light-bulb factory in Menlo Park, New Jersey. One year later in Germany Emil Rathenau founded a company with the intention to promote the wide use of Edison’s invention. In 1887 the Allgemeine Deutsche Elektricitätsgesellschaft (AEG) emerged from this company. At the same time the company founded by Werner Siemens concentrated exclusively on the fabrication of arc lamps. Hence, the first German factory producing light bulbs was opened by Rathenau’s company in 1884 in the Schlegelstrasse in Berlin. At the time in Germany Walther Nernst also participated in the development of the technology for electric illumination. The result of his work was the “Nernst lamp”, and in 1897 Nernst applied for a patent for this invention. The Nernst lamp contained a glowing piece made of an oxide mixture which was heated by an electric current. However, in order to achieve the current flow, this piece had to be heated by a separate heating device. Because of this, the operation of the Nernst lamp was relatively 47

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complicated, and the principle did not succeed in the long run. However, initially Nernst was able to sell his patent to the AEG. It is estimated that for about half a decade several thousand Nernst lamps were produced per day by the AEG.

Fig. 5.1

Laboratory for radiation measurements in the PTR around 1900.

Rapidly spreading artificial illumination also provided the motivation for establishing the optical laboratory at the PTR. In Chapter 3, dealing with the start of the Reichsanstalt under President Hermann von Helmholtz, we have already indicated the main tasks of the optical laboratory: the improvement of photometry and the quantitative measurement of light intensity, aiming at a generally accepted standard for this quantity. When in 1887 Helmholtz accepted the presidency of the PTR, he asked Otto Lummer to direct the optical laboratory. In 1884 Lummer had obtained his PhD, with Helmholtz acting as his advisor. Until 1887 he remained at the Friedrich–Wilhelm–University of Berlin as Helmholtz’s assistant and then moved to the Reichsanstalt together with the latter. Lummer’s co-workers were also former students of Helmholtz. At this point we mention in particular Wilhelm Wien and Ferdinand Kurlbaum, who had obtained their PhDs under Helmholtz in 1886 and 1887, respectively. Also Ernst Pringsheim had obtained his PhD under Helmholtz in 1882 and worked as a guest

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scientist in the optical laboratory of the Reichsanstalt from 1893 until 1904.

Fig. 5.2 Otto Lummer (German Museum Munich).

Fig. 5.4 Ferdinand Kurlbaum (Technical University of Berlin).

Fig. 5.3 Willy Wien (German Museum Munich).

Fig. 5.5

Ernst Pringsheim.

Around 1860 the physics dealing with the radiation emitted by hot bodies had seen important advances. At the time Gustav Kirchhoff formulated

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his radiation law, according to which for all bodies the emissivity for thermal radiation for all wavelengths and in all directions is proportional to their absorptivity. In 1854 as a professor of physics Kirchhoff had moved from Breslau (today Wroclaw) to the University of Heidelberg. In Heidelberg in close collaboration with Robert Bunsen he carried out spectroscopic investigations. In 1862 Kirchhoff created the concept of the “black body” as an ideal hypothetical body, which completely absorbs incoming (electromagnetic) radiation for all wavelengths. A black body or a black radiator is an ideal thermal radiation source and serves as a base for theoretical discussions as well as a reference source for experimental investigations of electromagnetic radiation. However, its technical realization is by no means trivial. In 1879 the Austrian physicist Josef Stefan had demonstrated experimentally that the total power W emitted by a black body is proportional to the fourth power of the temperature: W = σ · T 4. A theoretical foundation was given by Ludwig Boltzmann in 1884. Soon afterwards this law was referred to as the Stefan–Boltzmann law, and the factor σ as the Stefan–Boltzmann constant. In the year 1893 Wilhelm Wien had formulated his famous law about the dependence of the radiation intensity of a black body on temperature. According to this law, the product of temperature T and wavelength λm of the maximum emission in the radiation spectrum is constant: λm · T = konst. Wien demonstrated that the radiation within a hollow space of a black body can be defined “as a state of a stable thermal equilibrium.” If the spectral energy distribution of a black body is known for any temperature, then the energy distributions can be derived from this for all other temperatures as well. With his law predicting a shift of the spectral energy distribution with temperature, Wien had achieved an important step in the understanding of thermal radiation. However, on the other hand the theoretical description of the spectral energy distribution of a black radiator was still missing. In the year 1896 Wien also accomplished this part with his law of spectral energy distribution: ¶ µ −C2 . E = C1 · λ−5 · exp λT

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Here, C1 and C2 are constants. Wien had found this law by assuming a number of hypotheses and based it on the experimental data available at the time. Subsequently, a more rigorous derivation of this law was given by Planck. After joining the PTR in April 1891, Ferdinand Kurlbaum turned to the field of electromagnetic radiation, to which he would remain devoted for his whole life. Soon, together with Lummer, he worked on the development of a highly sensitive bolometer for radiation detection. Kurlbaum succeeded in the fabrication of new kinds of bolometers in which the change of the electric resistance of extremely thin metal ribbons due to the absorption of radiation is utilized. By rolling platinum foil between thin sheets of silver he was able to fabricate foils having a thickness of only 1 µm or even less. Using a meandering geometry of the thin foil, he achieved a strongly increased signal during the electric resistance measurements. By comparing the electric resistance change ∆R due to Joule heating with the change ∆R caused by thermal irradiation he succeeded in measuring the absolute value of the radiation power. The blackening of the ribbon required numerous experiments, initially using the soot of a petroleum flame. However, this only resulted in inhomogeneous layers. Finally, covering the platinum ribbon by platinum black led to the desired result. In 1898 the famous communication by Kurlbaum together with Lummer appeared about the electrically heated, absolutely black body, which for a long time served as the starting point of all measurements of light and thermal radiation at high temperatures. For the first time their measurements of the total radiation of the black body yielded reliable values for the Stefan–Boltzmann constant. The development and the fabrication of the black radiator used had taken no less than 3 years. In addition to the almost perfect spatial homogeneity of the temperature of the wall of the black cavity, the extraordinary challenge resulted from the desired reliability of temperatures of up to as high as 1900 K. At the same time Ferdinand Kurlbaum, together with Heinrich Rubens in the laboratory of the latter at the Technische Hochschule in Charlottenburg, carried out measurements of the radiation intensity of a black body in the range of long wavelengths up to beyond 20 µm. These measurements provided the final stimulation for Max Planck to propose his radiation law, and thereby represented the cause for the origin of quantum theory. During the Fall of 1904 Kurlbaum accepted an offer from the Technische Hochschule in Charlottenburg as the successor of Adolf Paalzow. In 1904 Otto Lummer went to the University of Breslau as Full Professor of Exper-

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Fig. 5.6

Heinrich Rubens.

imental Physics. 1905 Ernst Pringsheim followed him there as Professor of Theoretical Physics. Already in 1896 Wilhelm Wien had left the Reichsanstalt and had accepted the position of an Extraordinary Professor at the Technical College in Aachen. During the subsequent years he had appointments as Professor of Physics at the Universities of Giessen, Wurzburg, and Munich. Towards the end of the 1890s Max Planck had worked intensively on the entropy and the temperature of radiating heat, and he had closely followed the new experimental observations on the spectral distribution of radiation energy (Annalen der Physik 1, 69 (1900); 1, 719 (1900)). We quote from his latter paper, which had appeared in the Annalen der Physik, having been received on March 22, 1900: “... Although a conflict between

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observation and theory can likely be stated without any doubt only when the data of the different observers agree sufficiently with each other, the question hovering between the observers represented a motivation also for me, to clearly compile and expose to a sharpened criticism the theoretical conditions leading to the expression for the radiation entropy mentioned above and which certainly had to be changed, if W i e n’ s law about the energy distribution should turn out to be not generally valid. Here I want to report briefly the essential part of this ...”. In the legendary meeting of the German Physical Society of October 19, 1900 initially Kurlbaum had reported on the results of his radiation measurements carried out together with Rubens in the range of very large wavelengths. There could no longer be any doubt about the fact that in the range of long wavelengths and high temperatures Wien’s formula was completely in error. Already prior to the meeting Planck had heard of these experimental results, and, hence, had looked for a theoretical ansatz which could reproduce the experimental observations in the case of both limits of large and small wavelengths. At the meeting of October 19, 1900 following the report by Kurlbaum he could already propose his famous radiation law and suggest its being verified. The discussion after Planck’s lecture must have been highly motivating, since his request for having his formula verified was met on that same night. In his autobiography Planck wrote: “In the morning of the next day the colleague R u b e n s visited me and told me, that after the meeting still in the same night he had exactly compared my formula with his experimental data and had found everywhere a satisfactory agreement.” Planck’s new formula for the spectral energy distribution was: C1 · λ−5 ¡ C2 ¢ E= . exp λT −1 Here, C1 and C2 are the same constants as in Wien’s law of energy distribution. We see that in the limit C2 /λT >> 1 (small wavelengths) Planck’s formula approaches the Wien’s law. On the other hand, in the opposite limit C2 /λT