Instruments and Experiences 047191763X

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INSTRUMENTS AND EXPERIENCES Papers on Measurement and Instrument Design R. V. Jones, FRS Honorary Fellow, Wadham and Balliol Colleges, Oxford

During his long career in the field of measurement and instrumentation, R. V. Jones has made considerable contributions to the sum of scientific knowledge. For this book, he has selected some of his key papers, and grouped them together as chapters. Each chapter has a common theme, and is introduced by a commentary written by the author which gives the scientific context of the research described and explains the methods and approaches used. The chapters themselves are grouped into two parts, which deal with different aspects of R. V. Jones' work. The first part consists of research papers, arranged to reflect the evolution of the author's interests, while the second part contains papers dealing more generally with topics in instrument design, with the history and philosophy of the subject, and with his work in education. Many of the papers are those in which the author's work in a particular field was first published. Among these are such diverse and fascinating subjects as his work on the measurement of the drag exerted by a moving optical medium (glass, for example) on light that is passing through it, and the measurement of small geophysical movements—like the eastwards tilt of the east coast of Britain at high tide, when the bed of the North Sea is compressed by the extra weight of water. Contents

Introduction PART I SOME CONTRIBUTIONS TO INSTRUMENT DESIGN 1 Pre-1939 work on the detection of infrared radiation 2 First projects at Aberdeen: crystal growth and optical levers 3 Elastic movements 4 Microbarographs 5 Capacitance micrometers 6 The radiation pressure of light 7 Measurements of'aether drag' 8 Rotary'aether drag' PART II TOPICS IN EDUCATION AND INSTRUMENT DESIGN 9 Some considerations in instrument design 10 Some trends in instrumentation Indexes

WILEY SERIES IN MEASUREMENT SCIENCE AND TECHNOLOGY

Chief Editor Peter H. Sydenham South Australia Institute of Technology South Australia

Editorial Advisory Board H. L. Daneman

L. Finkelstein

T. W. Kerlin

Santa Fe New Mexico USA

City University London UK

University of Tennessee Knoxville Tennessee USA

R. L. Moore

E. Szonntagh

Delaware USA

University of South Florida Tampa Florida USA

Instruments and Experiences: Papers on Measurement and Instrument Design R. V. Jones

I

*

Instruments and Experiences Papers on Measurement and Instrument Design

R. V. Jones, FRS

Honorary Fellow, Wadham and Balliol Colleges, Oxford

JOHN WILEY & SONS Chichester • New York • Brisbane • Toronto • Singapore

Copyright © 1988 by John Wiley & Sons Ltd.

All rights reserved. No part of this book may be reproduced by any means, or transmitted, or translated into a machine language without the written permission of the publisher

British Library Cataloguing in Publication Data:

Jones, R. V. Instruments and experiences.—(Wiley series in measurement science and technology). 1. Scientific apparatus and instruments I. Title Q185 68F.75 ISBN 0 471 91763 X Typeset by Thomson Press (India) Ltd., New Delhi, India Printed and bound in Great Britain by Anchor Brendon Ltd., Tiptree, Essex

To those who explore the frontiers of observation and measurement and, with them, the men and women of all occupations, races, nations and epochs who seek new achievements in their chosen fields.

Contents

xi

INTRODUCTION

PART I: SOME CONTRIBUTIONS TO INSTRUMENT DESIGN Pre-1939 work on the Detection of Infrared Radiation

Chapter 1

3

Commentary Original Paper: I

Chapter 2

‘Infrared Detection in British Air Defence, 1935-38’. (Infrared Physics, 1, pp. 153-162; 1961. Reproduced by permission of Pergamon Press Ltd.)

10

First Projects at Aberdeen: Crystal Growth and Optical

Levers

22

Commentary Original Papers:

II

III IV

(with J. C. S. Richards) ‘Recording Optical Lever' (J. Sci. Instrum., 36, pp. 90-94; 1959. Reproduced by permission of the Institute of Physics.) ‘Some Developments and Applications of the Optical Lever’ (J. Sci. Instrum., 38, pp. 37-45; 1961. Reproduced by permission of the Institute of Physics.) ‘The Optical Micrometer’ (Optical Engineering, 15, pp. 247-250; 1976. Reproduced by permission of the Society of Photo-Optical Instrumentation Engineers.)

34 42

55

Additional Notes:

1. ‘A Calibration Device for Optical Levers’ 2. ‘A Directional Stabilizer for a Laser Beam’ vii

63 65

viii

Contents

Chapter 3

Elastic Movements

Commentary

67

Original Papers: V

VI VII

VIII IX X

XI

Chapter 4

‘Parallel and Rectilinear Spring Movements’ (J. Sci. Instruni., 28, pp. 38-41; 1951. Reproduced by permission of the Institute of Physics.) ‘An Optical Slit Mechanism’ (J. Sci. Instrum., 29, pp. 345-350; 1952. Reproduced by permission of the Institute of Physics.) ‘Angle-spring Hinges’ (J. Sci. Instrum., 32, pp. 336-338; 1955 with an additional note from 33, p. 245; 1956. Reproduced by per­ mission of the Institute of Physics.) ‘A Large Optical Slit Mechanism Employing Spring Movements’ (J. Sci. Instrum., 33, pp. 169-173; 1956. Reproduced by permission of the Institute of Physics.) (with I. R. Young) ‘Some Parasitic Deflexions in Parallel Spring Movements’ (J. Sci. Instrum., 33, pp. 11-15; 1956) ‘Anti-distortion Mountings for Instruments and Apparatus’ (J. Sci. Instrum., 38, pp. 408-409; 1961) ‘Some Uses of Elasticity in Instrument Design’(J. Sci. Instrum., 39, pp. 193-203; 1962. Reproduced by permission of the Institute of Physics.)

72 79 87 95

105

117 122

Microbarographs Commentary

145

Original Papers: XII

XIII XIV

XV XVI

Chapter 5

‘The Velocity of Light in a Transverse Magnetic Field’ (Proc. Roy. Soc. A, 260, pp. 47-60; 1961. Reproduced by permission of The Royal Society.) (with S. T. Forbes) ‘A Microbarograph’ (J. Sci. Instrum., 39, pp. 420-427; 1962. Reproduced by permission of the Institute of Physics.) ‘Sub-acoustic Waves from Large Explosions’ (Nature, 193, pp. 229-232; 1962) (with S. T. Forbes) ‘Sub-acoustic Waves from Recent Nuclear Explosions’ (Nature, 196, pp. 1170-1171; 1962) Microbarograph Record of Waves from the Chinese Thermonu­ clear Explosion on June 17, 1967’ (Reproduced by permission from Nature, 215, p.672; 1967. Copyright © 1967 Macmillan Journals Ltd.)

149

164 180 188

191

Capacitance Micrometers

Commentary

Original Papers:

193

Contents

XVII XVIII

Chapter 6

‘The Measurement and Control of Small Displacements’ (The Bulletin ofthe Institute of Physics, pp. 325-336,1967. Reproduced by permission of the Institute of Physics.) (with J. C. S. Richards) ‘The Design and Some Applications of Sensitive Capacitance Micrometers’ (Journal of Physics E, 6, pp. 589-600; 1973. Reproduced by permission of the Institute of Physics.)

ix

197

219

The Radiation Pressure of Light Commentary

248

Original Papers:

XIX

XX XXI

XXII

Chapter 7

‘The Pressure of Radiation’ (part of a discourse at the Royal Institution, 6 March 1953: also published in Nature, 171, p. 1089; 1953. Reproduced by permission of the Royal Institution.) (with J. C. S. Richards) ‘The Pressure of Radiation in a Refracting Medium’ (Proc. Roy. Soc. A, 221, pp. 480-498; 1954. Reproduced by permission of The Royal Society.) (with B. Leslie) ‘The Measurement of Optical Radiation Pressure in Dispersive Media’ (Proc. Roy. Soc. A, 360, pp. 347-363; 1978. Reproduced by permission of The Royal Society.) ‘Radiation Pressure of Light in a Dispersive Medium’ (Proc. Roy. Soc. A, 360, pp. 365-371; 1978. Reproduced by permission of The Royal Society.)

258

268 289

306

Measurements of ‘Aether Drag’ Commentary

313

Original Papers:

XXIII

XXIV

XXV Chapter 8

‘“Fresnel Aether Drag’’ in a Transversely Moving Medium’ (Proc. Roy. Soc. A, 328, pp. 337-352; 1972. Reproduced by permission of The Royal Society.) “‘Aether Drag” in a Transversely Moving Medium' (Proc. Roy. Soc. A, 345, pp. 351-364; 1975. Reproduced by permission of The Royal Society.) ‘Radiation Pressure and “Aether Drag” in a Dispersive Medium’ (Nature, 277, p. 370; 1979)

317 334

349

Rotary ‘Aether Drag’

Commentary

351

Original Paper: XXVI

‘Rotary “Aether Drag’” (Proc. Roy. Soc. A, 349, pp. 423-439; 1976. Reproduced by permission of The Royal Society.)

354

Contents

X

PART II: TOPICS IN EDUCATION AND INSTRUMENT DESIGN

Chapter 9

Some Considerations in Instrument Design Commentary

Chapter 10

Some Trends in Instrumentation

Original Papers: XXVII

XXVIII

XXIX

XXX

XXXI

373

‘Instruments and the Advancement of Learning’. The text of the Thomson Lecture delivered at the Royal Institution, on 20 October 1966. (Transactions of the Society of Instrument Tech­ nology, March 1967, pp. 3-11.) ‘The Teaching of Instrument Science’ (A paper to the Symposium on the Teaching of Measurement and Instrumentation for Industrial Needs, at the City University, London, on 3 January 1969. A shortened vision appeared in Measurement and Control, 2, pp. 91-92; 1969.) ‘The Pursuit of Measurement’. (Text of the Kelvin Lecture to the Institution of Electrical Engineers in London, 24 April 1969. Reprinted from Proc. I.E.E., 117, pp. 1185-1191; 1970. Repro­ duced by permission of the 1EE.) ‘Impotence and Achievement in Physics and Technology’. (Text of the Cherwell-Simon Lecture in Oxford, 18 May 1965. Reprinted from Nature, 207, pp. 120-125; 1965.) ‘The Other Way Round' (Text of the Chelsea Lecture, delivered at Chelsea College, University of London, 6 November 1974.)

395

414

430

435

446 460

Index of Organizations

475

Author Index

477

Subject Index

482

Introduction The Chief Editor of this series of books on Measurement Science and Technology, Professor P. H. Sydenham, has suggested that there would be enough interest in my papers connected with instrument design for them to be selectively assembled: this book is my grateful response. Instrument design and performance have in various forms concerned me over a period of some 50 years. Even during World War II, when I was an intelligence officer, it was largely what I had learnt about instruments and observation that enabled me to pursue the activities described in Most Secret War. When I started in research the galvanometer was the most sensitive of measuring instruments: thermionic valves and photoelectric cells had certainly arrived, but the latter were suspect, all the more so because their design was largely empirical. For the creation of high vacua many of us had to start with a water pump, graduating up to a vapour diffusion pump if we were fortunate; and for the first time the limits to instrumental precision set by Brownian motion were being encountered, while in Cambridge the scale-of-two counter was being invented by Wynn-Williams. The transistor, the laser, the multilayer dielectric mirror, the ion pump, the Josephson junction, the infrared photocell, the charge-coupled device, the integrated circuit, and the optical fibre, all lay in the future. My own path was to take me only around the fringes of most of these major developments, for teaching and other duties precluded participation in the large efforts involved. Instead I had to concentrate on problems that escaped the attention of large teams, and which could be taken up and laid aside, rather like knitting, when other duties intervened—and which, like cheese or wine, might actually benefit by being undisturbed in the meantime. With basic interests in optics and in sensitivity of detection and in precision of measurement, I was naturally led to appreciate the importance of mechanical design, and the frequency with which optical and electronic design of the highest quality could be let down because too little attention had been paid to mechanical detail. So it has been rewarding to see how designers have now generally come to realize that only by mechanical design of outstanding quality can they use the intrinsic precision of such devices as the laser, or take microcircuitry to the limits of miniaturization.

xi

xii

Introduction

My research papers have tended to fall in groups reflecting the evolution of my interests: optical levers, elastic movements, microbarographs, capacitance micro­ meters, and radiation pressure and ‘aether drag’. To selections from each of these groups I have added introductory commentaries telling why and how I came to approach the problems involved, including the pitfalls into which I sometimes tripped. The papers and their commentaries form Part I of the book. Part II consists of papers dealing with more general topics in instrument design. In Chapter 9 I have outlined some considerations that experience has taught me to value, while in Chapter 101 have attempted to indicate some current trends in the field. To these chapters I have added the texts of a few lectures given over the past twenty years which may throw some historical, philosophical, and educational sidelights. Since the book consists largely of papers which have been reproduced in their original form ‘warts and all’, allowance may be needed from time to time for the dates at which they were written, particularly in view of the advance of techniques and knowledge in the meantime. Moreover, while my life has been concerned with physics and instruments both inside the laboratory and beyond, and despite the length of time over which my experience has been gained, it has been in limited fields. The resulting defects will be obvious in this book, but I hope that it contains enough interest, instruction and entertainment to lead others to benefit from what I have learnt. I am much indebted to Professor P. H. Sydenham who encouraged me to embark upon the book, and whose support has been essential to its publication. And I am grateful to Messrs. Wiley, particularly to their editor, Miss Ellen Taylor, and Miss Irene Cooper. It is a happy coincidence that Miss Taylor’s grandfather, the late Reverend Henry Taylor, was a personal friend in the days when I started research, and he helped to man my exhibit at the Physical Society’s Exhibition of 1934 which is mentioned on page 5. Wherever possible in the original papers I tried to acknowledge those who had helped me. Nearly all the work was done in the Natural Philosophy Department of the University of Aberdeen, where, from among my academic colleagues, I was fortunate in the collaboration of W. J. Bates, S. T. Forbes, A. H. Holbourn, D. A. Jones, B. Leslie, C. W. McCombie, M. A. Player, J. C. S. Richards (particularly for his support in electronics and radiation pressure) and I. R. Young. I was also fortunate to have the skilled technical support of‘Aberdeen workmen’, the able successors of those to whom in his day James Clerk Maxwell had paid tribute (p. 22), among them particularly H. Barber (head of the workshop), G. Coull, A. M. Gardner, A. Gibbs, G. Paget, G. Shepherd and T. Wratten. I am grateful to them all.

Aberdeen, 1988

R. V. Jones

PART I SOME CONTRIBUTIONS TO INSTRUMENT DESIGN

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CHAPTER 1

Pre-1939 Work on the Detection of Infrared Radiation When I started as a research student in the Clarendon Laboratory at Oxford in 1932 there was no specialized theme such as the Cavendish Laboratory had in nuclear physics at Cambridge. Instead each research student was started on a project which might well have no connection with any work elsewhere in the laboratory—we were very much on our own, and ‘chucked in at the deep end’ and left to see whether we could learn to swim, research-wise, for ourselves. Usually the project was something that happened to appeal to our Professor, F. A. Lindemann, at the time that we were starting; but his interest was liable to wax and wane as the months went by, as we showed greater or less promise, and as other developments diverted his interest. [For appreciations of Lindemann and his laboratory in the 1930s, see Thomson (1958), Berman (1987), Jones (1987a, b)] Actually I had worked on a minor problem for Lindemann in 1931 before I graduated. This was to create two small virtual and coherent sources of light, as in a microscopic Young’s experiment, so that the fringes in the resultant interference pattern would be so separated that a microscope objective moving across the pattern would alternately span two bright fringes separated by one dark fringe or one bright fringe flanked by two dark. In the first case, according to Abbe’s theory, the microscope would resolve the sources to show that they were two and separate; in the second there should be no resolution, and only a single source should appear. So the resolving power of the microscope, as defined by Rayleigh and Abbe, should be a function of the region of its field of view to be resolved as well as of its numerical aperture. The sources could either be two slits a few microns in width and separated by 10 microns or so, ruled in an evaporated layer of silver, or two 10-micron diameter wires lying side by side, with light from a distant source imaged on them as cylindrical mirrors. I achieved no success in the two or three weeks that were available; we had no satisfactory means of ruling silver films, and the 10 microns tungsten wires could not be persuaded to be compliant bedfellows. A further attempt to produce the slits photographically

3

4

Pre-1939 Work on the Detection of Infrared Radiation

also failed for want of resolution in the lenses and emulsions currently available. Such was my introduction to precise instrumentation. The failure remained with me for more than 20 years, when I realized that the effect could be tested far more easily with a telescope than a microscope and I was at last able to send photographs to Lindemann (by then Lord Cherwell) showing that he had been right. Photographs of the effect can be seen in Paper XVII. On entering into serious research I hoped to find, or be given, a problem which would present a theoretical as well as an experimental challenge. That it should involve precision was not an explicit consideration, although this would certainly have had a subconscious appeal based partly on a childhood association with the Brigade of Guards, and partly on the spirit of nineteenth-century physics of which Maxwell had written in 1871: The opinion seems to have got abroad that in a few years all the great physical constants will have been approximately estimated, and that the only occupation that will then be left to men of science will be to carry on these measurements to another place of decimals.... But the history of science shews that even during that phase of her progress in which she devotes herself to improving the accuracy of numerical measurement of quantities with which she has long been familiar, she is preparing the materials for the subjugation of new regions, which would have been entirely unknown if she had been contented with the rough methods of her early pioneers.

In fact the selection of a problem for me was based on less lofty considerations. The Clarendon had, in its very limited range of equipment, a small infrared spectrometer, and the researcher who had been working with it had tired of it. He suggested to Lindemann that someone fresh should use it to detect deviations from Lambert’s cosine law governing the emission of thermal radiation from metallic surfaces; and on the other side he suggested to me that besides the experimental challenge there would be the theoretical one of interpreting the results in terms of the new quantum mechanical theory of metals. So it was decided that I should inherit the infrared spectrometer. When, within a few weeks, I came to use it I found that its thermopile detector had been broken. There was no money for a replacement, and Lindemann proposed that I should try to make one, using an idea he had for making a multi­ turn helical conductor by cutting a helical thread in an insulating material and then evaporating metal on to it in vacuo, the thread and source being so disposed that metal was deposited on to one side only of the V-groove of the thread. The idea was promising enough, but our vacua (starting with water pumps which we had to make ourselves by glassblowing) were not high enough, and so I could not achieve straight-line propagation of metal from the source to the thread. (Incidentally, rivalry between the Clarendon under Lindemann and the Electrical Laboratory under J. S. E. Townsend was so strong that when the latter gave a lecture course on high vacua, the former proposed to give one on higher vacua.) I myself then proposed to achieve the same result by cutting a thread in bakelite at

Pre-1939 Work on the Detection of Infrared Radiation

5

500 turns per centimetre and winding constantan wire of 10/zm diameter into the thread. Then, by copper-plating over the constantan for the major sector of each turn and thus shorting it out, we might make a thermopile with as many as 500 constantan-copper junctions along each centimetre of its length. This I succeeded in doing within a few weeks, only to find that the whole scheme was flawed because string galvanometers, with which Lindemann had proposed the thermopile should be used, were so insensitive. I reacted by going to the other extreme from multi-junction to single-junction thermopiles, which I termed ‘thermoelectric cells’. Ultimately these worked well enough to show at the Physical Society Exhibition in January 1934, and I published my first paper in the Journal of Scientific Instruments later that year (Jones, 1934a). To complete my doctoral thesis I needed a problem to demonstrate that my thermal detectors were useful, and I found a more interesting one than Lambert’s Law in the mystery of the infrared absorption bands of coloured fluorites. This arose from a misfortune that turned into a blessing, when a new fluorite window of seemingly high optical quality, with no visible colour, was incorporated into my detector, which then seemed to lose sensitivity. It transpired that the spectrometer had been set at about 6 microns wavelength, and this happened to be in the trough of an absorption band in the fluorite. Now although it was well known that natural-coloured crystals of fluorite showed absorption bands in the infrared, such bands in natural colourless fluorite had not before been reported. Then I found that if coloured crystals were heated carefully, the colour would disappear. Someone else had evidently made the same discovery and turned it to financial advantage since the colourless material was much more valuable because of its clarity both in the ultraviolet and the infrared: but its infrared absorption bands were not removed by heating; neither was its ultraviolet fluorescence. The optical microscope makers who had provided the original window had bought the treated fluorite and had used it in good faith, so the latter part of my thesis turned into a detective problem which revealed how they had been duped. The infrared absorption bands in coloured fluorite seemed to be related to the presence of rare-earth impurities, some of which were slightly radioactive and which could thus have generated F-centres over geological time; the F-centres disappear on heating, but not the impurities which gave rise to them and which cause the infrared absorption. Another incidental development in my thesis arose from a desire to increase the sensitivity of mirror-galvanometers, following an idea by Wilson and Epps (who had also provided the idea of the multi-junction copper-constantan thermopile). This was their ‘thermo-relay’: it was subsequently developed by Moll and Burger (see p. 43), and it could be further improved by the substitution of photoelectric cells for thermopiles in which light reflected from the galvanometer mirror falls on two opposing photocells so that any rotation of the mirror increases the light falling on one cell and decreases that on the other. Perhaps by instinct, or perhaps

6

Pre~I939 Work on the Detection of Infrared Radiation

by luck, I made the system very compact, which proved to have advantages that I did not appreciate until I was able to take up the work again after the war, when it was to lead on to experiments on radiation pressure and ‘aether drag’. The 1934 instrument was described in the Journal of Scientific Instruments (Jones, 1934b). In searching through the literature concerning thermal detectors I became interested in the history of their development, and so included an appendix in my thesis describing the evolution of the thermopile. Only subconsciously at the time could I have recognized the force of the observation more than 2000 years before by Dionysius of Halicarnassus* that ‘history is philosophy teaching through examples’, but the interest then generated in the history of instruments, and their impact on science and society had held me ever since. An instructive incident in my pre-doctoral days arose from a request for help from a research student in zoology, who was desperately trying to measure the temperature of a hibernating bat. It transpired that although bats copulate in the autumn the resulting embryos do not develop until the following spring, and this might possibly be due to the temperature of bats being lower during hibernation. The method employed by the zoologist to measure the temperature was to insert a thermocouple in the rectum; but, as he explained to me, every time he tried to do so the bat woke up. When I saw his thermocouple I was not surprised: it was as large as a knitting needle, so presenting as painful a shock to the bat as would a broomhandle to a man subject to an analogous procedure. The solution was simple enough—I made him a delicate thermocouple which the bat could retain in place indefinitely without discomfort. It then struck me that the zoologist’s previous attempt provided a memorable example of an instrumental principle that I had learned from Lindemann’s enthusiastic exposition of Heisenberg’s recently formulated Uncertainty Principle: that any act of observation inevitably affects the system being observed. Although in macroscopic systems the resulting disturbance is usually far less disconcerting than that experienced by the unfortunate bats, it can proportionately be so on the atomic scale, as Heisenberg showed by his thought experiment of trying to observe the position of an electron with a gamma-ray microscope. In the summer of 1934 I started work in the University Observatory on the infrared spectrum of the sun, at the suggestion of Professor H. H. Plaskett, who had encouraged my interest in astronomy, but this spell in pure research ended after only a few months I became drawn into the problem of detecting the infrared radiation from the engines of aircraft. This was intended as a supplement to radar; and as for the associated politics and manoeuvring the story can be found in Most Secret War (Jones, 1978). Although I came into the problem independently of Lindemann, it transpires that he was well justified in claiming priority for conceiving the proposal to detect aircraft by infrared, for a letter has recently come to light which was dated 8 April, 1915, from Mervyn O’Gorman, ♦who learned it from Thucidides

Pre-1939 Work on the Detection of Infrared Radiation

7

Superintendent of the Royal Aircraft Factory at Farnborough to Professor H. Callendar at the Imperial Institute which begins ‘The bearer, Mr. Lindemann, is working at the Royal Aircraft Factory. He is investigating methods by which aircraft can be detected at a distance, and hopes to obtain some results by the detection of long wave radiation... .* At the outset in 1935 my own work started with thermopiles, galvanometers and photoelectric amplifiers, but I had realized that if the detectors could be made to respond quickly enough, which required very thin receivers, the incoming radiation could be ‘chopped’, and the resultant change in e.m.f. (or in resistance, for a bolometer) could be made to generate a signal which could be amplified electronically. Moreover the action of chopping had automatically put a label on the incoming radiation, and this could be recognized at the output end of the amplifiers by some device (in my case a vibration galvanometer) tuned to the chopping frequency. I was not the first to use the idea, for G. M. B. Dobson and D. S. Perfect had done something similar before 1930 which they described that year in a paper to a Physical Society discussion on Photoelectric Cells and their Applications, entitled A method ofcomparing very small amounts oflight by means ofa photoelectric cell and a valve amplifier, subsequently published by the Physical and Optical Societies. They used not a tuned galvanometer, but a commutator on the output of the amplifier driven on the same shaft as the chopper: this developed into what became known as ‘phase-sensitive detection’, a seminal idea for which subsequent literature had hardly given them due credit. To make fast detectors would require the receivers to be of the order of a tenth of a micron in thickness, and I hoped that these might be made by evaporating silver through suitable masks on to a thin film of collodion. Also by vacuum evaporation it should be possible to deposit thermocouple elements of bismuth and antimony, and so I attempted to make what may have been the first integrated circuits—but whereas today’s circuits have elements no more than a few microns wide, my elements were some hundreds of times wider. The resulting thermopiles failed for two reasons: breakdown through failure of the collodion substrates as they aged, and insensitivity because the deposited antimony and bismuth elements had higher resistivities and lower thermoelectric effects than the bulk metals. I therefore reverted to the alternative of making elements reduced in thickness from the bulk metal, and so spent much effort in rolling wire into strip, starting with wire around 30 microns in diameter and with the ‘rolling’ process mainly causing it to spread sideways into strip which, in the most successful cases, might be as thin as 0.1 micron, and 0.5 millimetre or more in width. I published the method and its results in the Journal of Scientific Instruments (Jones, 1936), and took up the work again in the 1950s without significant improvement in performance despite much effort. Since this latter work has not been published it may be worth mentioning its new findings here. The rollers that I had used in 1935 had a fine-ground surface finish, from which one could hardly expect to produce

8

Pre-1939 Work on the Detection of Infrared Radiation

foil much thinner than the roughness of the surface, and so in a specially designed machine at Aberdeen twenty years later I tried rollers finished to a mirror polish, but they refused to roll the strip to less than a micron or two’s thickness. On replacing them with fine-ground rollers I was able to repeat, and slightly improve on, my pre-war results. The explanation turned out to be that the process was not conventional rolling, which depends on the longitudinal squeezing of material as it passes through the rollers, as in the old-fashioned domestic mangle. This process might be expected to become more effective as the diameter of the rollers is reduced; but even though I had built a machine with rollers of 2 millimetres diameter, in contrast to the 5 centimetres I had used before, these would not roll out below about a few microns thickness, even though the strip was softened by heat treatment after every passage through the rollers, so long as these had a polished finish. What appeared to happen was that there was not enough ‘squeeze’ in the rollers, even at 2 millimetres diameter, to produce much longitudinal extension, and the strip could not expand sideways because it adhered to the polished surfaces of the rollers. When, however, these were in effect ‘grooved’ by the fineground finish of a cylindrical grinder, there were enough minute spaces for the material to flow locally sideways (I simulated the effect macroscopically by cutting grooves in the wooden rollers of a mangle, using plasticine as the material being rolled). Unfortunately, the fact that the rollers had to have an appreciable surface roughness for the sideways spreading to occur also resulted in the strip becoming scarified and ultimately breaking up if an attempt were made to work it down to much less than 0.1 pm thickness. Using the rolled strip of constantan and suitably copper-plating it, I made copper-constantan thermopiles containing 200-300 junctions in an area of about 17 x 15 millimetres with a time constant of 0.02 second. Details can be found in a paper in the Journal ofScientific Instruments (Jones, 1937). I used one of these piles in my first experiments to detect thermal radiation from an aircraft at Farnborough in 1935, but I later discarded the design in favour of a simpler one. The idea was, however, taken up by the National Physical Laboratory and used for many years for microwave power measurement. In fact some 30 years later, when I was a member of the Committee for Research in Measurement and Standards, we were shown the NPL’s apparatus, and the design of its thermopile seemed familiar to me. When I asked the demonstrator about its origin he said that they had taken the idea from an old paper by R. V. Jones; puzzled by the consequent ripple of amusement among the other members of the Committee, he sought the explanation and tentatively asked, ‘You’re not R. V. Jones, are you?’ When I replied that I was, he said, ‘we knew that there was a Jones in the party, but we did not think that it could possibly be you because we imagined that you had passed on years ago!’ He added that they had followed my recipe scrupulously, and in 1970 were still using the composition of the copper plating solution and the current density exactly as I had given them in the paper.

Pre-1939 Work on the Detection of Infrared Radiation

9

One reason why the NPL had heard nothing more of me in the subsequent literature was that my further work on infrared could not be published because of its relevance to aircraft detection. This consideration no longer held after the war, and so in 1961 I published a summary in volume 1 of the new journal Infrared Physics; it is reprinted as Paper I. REFERENCES

Berman, R. (1987) ‘Lindemann in Physics’, Notes and Records of the Royal Society, 41, 181-189. Jones, R. V. (1934a) ‘The design and construction of thermoelectric cells’, J. Sci. Instrum., 11, 247-257 Jones, R. V. (1934b) ‘A simple method of attaching a photoelectric cell relay to a Moll galvanometer’, J. Sci. Instrum., 11, 302-303 Jones, R. V. (1936) ‘The production of metallic films’, J. Sci. Instrum., 13, 282-287 Jones, R. V. (1937) ‘Radiation thermopiles of quick response’, J. Sci. Instrum., 14, 83-89 Jones, R. V. (1978) Most Secret War. Hamish Hamilton Jones, R. V. (1987a) ‘Lindemann beyond the laboratory’, Notes and Records of the Royal Society, 41, 191-210. Jones, R. V. (1987b) ‘Oxford physics between the wars’. In The Making ofPhysicists (ed. R. Williamson). Adam Hilger Maxwell, J. C. (1871) ‘Introductory lecture on experimental physics', in Scientific Papers of J. C. M., Vol. 2, pp. 241-255. Cambridge University Press Thomson, G. P. (1958) ‘Frederick Alexander Lindemann, Viscount Cherwell’, Biograph­ ical Memoirs of Fellows of the Royal Society, 4, 45-71

PAPER I

Infrared Detection in British Air Defence, 1935-38 INTRODUCTION The work done in Germany and in America on infrared detection for millitary use in World War II has been published in some detail; it has recently been reviewed by Arnquist.(1) The purpose of the present account is to outline what was done in Britain between 1935 and 1938 on the application of infrared means to the detection of aircraft. The account is largely a personal one since, until I was joined in 1937 by Dr. G. L. Pickard (now Director of the Institute of Oceanography in the University of British Columbia), the total effort was solely mine. It has not been published previously; publication before the war would have been unwise, even though we had by then decided to concentrate instead upon radar. Afterwards, most of our results, some of which seem to have been ahead of contemporary work elsewhere, had been duplicated and improved upon; we therefore left them unpublished. However, in view of recent references (Blackett(2)), and for comparison with Arnquist’s(1) account, it may now be of some interest to set our pre-war work in perspective. The details can be substantially verified from official documents. Commander Paul MacNeil

In February 1935 I was developing thermopiles and bolometers in the Clarendon Laboratory at Oxford in order to investigate, with Professor H. H. Plaskett in the University Observatory, the infrared spectrum of the sun at high resolution. At this time I was approached by Commander Paul MacNeil, formerly of the U.S. Navy, who was trying to interest various governments in his infrared detector, which was based on a Moll thermopile with a mechanical circuit interrupter and an a.c. amplifier. His thermopile had broken down, and he needed a replacement urgently for a demonstration to the Royal Air Force at Farnborough. I made several thermopiles for him, using evaporated elements of bismuth and antimony on collodion film, but they broke fairly quickly and did not reach a very high 10

Infrared Detection in British Air Defence, 1935-38

11

sensitivity, mainly owing to the poor electrical properties of the evaporated films. MacNeil’s trial at Farnborough failed, but I subsequently saw him detect aircraft at some hundreds of yards on the ground at Croydon Airport. The Tizard Committee When I told the Head of the Laboratory, Professor Lindemann (afterwards Lord Cherwell) of my activities, he remarked that he had proposed infrared as a means of detecting aircraft in 1916, and that he would like to see me working on the technique officially for the British Government. General interest in scientific aids to air defence had already been aroused—both publicly and privately—by Mr. Winston Churchill, Sir Austen Chamberlain and Professor Lindemann; and two official committees (the Air Defence Research Committee, under the Committee of Imperial Defence, and the Committee for the Scientific Survey of Air Defence, under the Air Ministry) were coming into operation. The latter Committee had Mr. (later Sir) Henry Tizard as its Chairman; at its first meeting, in January 1935, it had listed infrared as a possible means of detection, but this was rapidly overshadowed by radar, following Mr. (later Sir) Robert Watson Watt’s demonstration at Daventry in February 1935. As is well known, relations between Professor Lindemann and Mr. Tizard—once most cordial—had already become strained; and it may have gratified the former to have a detection technique being developed in his own laboratory which had been dismissed by the Tizard Committee, particularly since he might reasonably claim to have invented it. At that time, moreover, the possibilities of radar as an air-to-air system were not completely clear, and it was therefore prudent to consider seriously any alternative means.

TRIALS OF NOVEMBER 1935 The Tizard Committee doubted whether infrared techniques were worth developing, in view of the ease with which aircraft engines could be screened, but Lindemann (who had now joined the Committee) contended with reason that there must be much energy radiated from the hot exhaust gases, and he insisted that trials should be undertaken, although it was known that an investigation by Dr. A. B. Wood in 1926 had yielded negative results. The new trials, which were to have been made by Dr. J. S. Anderson of the National Physical Laboratory with myself as an independent observer, were in fact carried out with the personal roles reversed, because I at least had some infrared equipment whereas the NPL had virtually none; we therefore used a copper constantan thermopile made by the method of Wilson and Epps (Jones(3)), and a galvanometer amplifier (Jones(4)). The trials, which began on 4 November 1935, bore out Wood’s results: with the 500 h.p. piston engines and infrared detectors then current, there was insufficient infrared radiation from the exhaust gases, at least outside the atmospheric

12

Pre-1939 Work on the Detection of Infrared Radiation

absorption bands, although there was considerable energy radiating from the hot surfaces of the engines.

TRIALS OF JUNE 1936

Despite this negative result, the Tizard Committee—rather surprisingly, in the strained circumstances—asked me to continue the work on a full-time basis, with the object of developing an airborne infrared detector. I had already (November 1935) outlined several schemes dependent on the use of fast thermoelements or bolometers. All the schemes involved the comparison of one element of the field of view with an adjacent element, so as to eliminate as far as possible the effects of the background, and to show up the presence of point sources. The simplest scheme used a pair of adjacent thermoelements connected in opposition to the input transformer of an a.c. amplifier; an image of the target was to be oscillated at about 20 times/sec from one thermoelement to the other by rocking the concave collecting mirror. The most elaborate scheme used the same optical system, but the opposing thermoelements were to be replaced by an a.c. bolometer bridge operating at 500 c/s, with power fed into the bridge by making two of the arms secondary windings of a single transformer. The carrier frequency was to be amplified, and the low-frequency modulation due to any warm or cool source of small area in the field of view was to be detected after amplification. This bolometer was rather similar to the one used in 1941 by Zeiss in the Warmepeilgerat 15. In the first few months of 1936 I made both thermoelements and bolometers, but decided to concentrate in the first place on the former, owing to their greater simplicity, and to the need for rapid results; some bolometers made from platinum foil proved comparatively noisy, and there was no time to investigate the source of the noise. The system was tested on the ground at Farnborough in June 1936, using an 11 cm diameter collecting mirror, giving a 4° x 1.5° field of view; it was a rather simpler version of the equipment described in more detail below. The trials involved the measurment of the relative distribution of energy radiated by typical aircraft, and the detectability of aircraft in flight against clear sky and clouds; the speeds make odd reading now—one target aircraft, a Westland Wapiti, sometimes making runs at 70 m.p.h. Aircraft were satisfactorily detected in flight during daylight at ranges of about one mile; ground measurments showed that a range of two miles would be possible at night. Cloud edges in the field of view caused disturbances, but these were reduced by ten to fifty times by the interposition of a selenium powder filter in front of the thermoelements. The same filter was also used to eliminate the effect of reflected light from the aluminium-painted aircraft, since this was found to reach a magnitude compar­ able with that due to a hot engine. A fluorite filter was used to cut out radiation beyond 9 n wavelength, to improve the differentiation against cloud. It was decided that the trials were sufficiently encouraging to proceed to the next stage, which was to build an instrument to mount in an aircraft. The main

Infrared Detection in British Air Defence, 1935-38

13

El

eS (f)

(e)

(d)

(b)

(c)

(a) Figure 1. (a) Section through infrared detector for airborne trials, showing general scheme of scanning system (lower compartment) and indicating system (upper compartment), (b) Axial view through upper compartment, showing form of rotating shutter H, and appearance of screen S in absence of a target, (c) Layout of 4 element thermopile, actual size about 10 mm x 10 mm; each element consists of constantan (unshaded)-manganin (shaded). Dotted line indicates track of image when target is dead ahead, (d) Frontal appearance of diaphragm D (enlarged); quadrantal sector of H shown dotted, (e) Appearance of screen when target is dead ahead, (f) Appearance of screen when target is in right sector of field

improvements in this instrument were its larger field of view, a circle of about 30° diameter, and an indicating device for showing the direction of the target relative to the line of sight. The scheme is outlined in Figure 1. Scanning system

The collecting mirror M was 11 cm in diameter, and of 5 cm focal length, with the four-element thermopile T approximately in its principle focal plane. The mirror was mounted somewhat askew on a rotating shaft R, whose axis of rotation

14

Pre-1939 Work on the Detection of Infrared Radiation

passed through the centre of T. Thus when the shaft rotated, the image of a distant point on the axis of rotation traversed a circular path on the surface of T, generating four equal thermoelectric pulses of alternate sign as the image fell on each element in turn. An object on the line of sight thus gave rise to a regular alternating output of twice the frequency of rotation of the shaft. An object in the field of view off the line of sight gave a signal for that part of the rotational period in which its image was made to fall on the thermopile, whose output thus contained phase information giving roughly the direction of the target relative to the axes (up/down and right/left) of sight.

Thermopile The four elements of the thermopile (Figure (1 c)) were mounted to fill a square of 10 mm size. Each element was in the shape of a 45° set square, and was made of constantan-manganin foil; this foil was made by butt-welding a rod of constantan to one of manganin, and rolling the composite rod into foil, the rolling direction being in the plane of the weld. The ultimate thickness was about 0.1 p., and the best time-constant achieved (in air) was about 10 msec; an averagely good time constant was 25 msec. The shape of the thermoelements was suitable both to the geometry of the scanning system and to thermal efficiency considerations. The four elements were connected in series to the primary of the input transformer of the amplifier, adjacent elements being connected in opposition. Various metallic blacks were tried, but the curious fact was found that—providing the thermopile was operated at normal atmospheric pressure, as it was in this apparatus—a carbon black from a camphor flame had little harmful effect on the time constant.

Windows

Various materials were investigated, the objective being an infrared transmitting material obtainable in sufficient strength and size to permit a 6 in. aperture in the fuselage. Mica of 30 p thickness, reinforced by a grid of thin steel wire, was fairly successful; but it was decided to grow single crystals of silver chloride and bromide (thallous chloride, bromide, and iodide were also considered, but were not actually grown, because they were said to produce disconcerting physiolog­ ical effects). Single crystals of silver chloride and bromide were grown in 1937 up to about 250 g in weight, and windows made from them. The surfaces were protected from actinic action by evaporated layers of selenium. This work was the origin of the crystal-growing programme undertaken at Aberdeen after the war ended. Amplifier

Several four-stage amplifiers were built, but a three-stage R-C coupled amplifier was sufficient for operating the indicating system. The thermopile

Infrared Detection in British Air Defence, 1935-38

15

(resistance about 10 Q) was coupled to the amplifier by a mumetal-cored 1:300 transformer made by Ferranti Ltd. The transformer was screened by a double mumetal shield surrounded by a £ in. soft iron screening box, but there was still some trouble with inductive pick-up in the transformer, and with low-frequency microphonic effects.

Indicating system

In the 1936 trials, the indicator was a simple vibration galvanometer, tuned to the oscillation frequency of the mirror. More elaborate schemes involving phase­ sensitive rectifiers of the commutator type were contemplated, but the urgency for a demonstration of the possibilities of infrared detection made the simplicity of the vibration galvanometer a key factor. Similarly, for the 1937 trials, a direction indicating system involving a cathode­ ray presentation was considered, but it was found that the vibration galvano­ meter could be adapted for the purpose. The galvanometer G was mounted in a brass tube on top of that containing the mirror and thermopile. A small projector system P was used to illuminate the galvanometer mirror, and an image of the diaphragm D (immediately in front of P) was focused on the screen S after reflexion in the galvanometer mirror. The diaphragm D had four circular holes in it, arranged at the corners of a square with one diagonal vertical (Figure 1(d)); a rotating shutter H was mounted in front of D, with a quadrantal sector cut in it so that light from only one of the holes in D passed through H at any one time. The shutter H (made from a ‘Meccano’ sprocket wheel) was coupled to a similar sprocket wheel on the axis of rotation of the main mirror M and the phasing of the sector arranged so that the sector ‘selected’ the top illuminated spot on the screen S as the image of the target was at the highest point of its circular path on the surface of the thermopile T. If a target was dead ahead, the path of its image crossed all four elements of the thermopile, and four kicks in turn were delivered to the galvanometer. This was rather less than critically damped with a natural frequency twice that of the mirror rotation frequency, and thus gave four kicks (two forwards and two backwards) for each revolution of the mirror. Each kick was indicated by the appropriate light spot on S, and the appearance was as indicated in Figure 1(e); owing to the frequency of repetition of the kicks, each light spot appeared to be drawn out into a continuous band. The amplitude of the bands gave an indication of the strength of the target. If a target was to one side, the kicks during a cycle of the mirror varied in magnitude, and caused the pattern on the screen to change, in such a way that a rough estimate could be made of the direction in which the apparatus had to be turned in order to look directly at the target (Figure 1(f)). A lens L was added to project a direct picture of the view ahead on to the screen S, so that both visual and infrared information was available on one screen. The indicating system worked well, at the expense of the integration that was

16

Pre-1939 Work on the Detection of Infrared Radiation

lost by damping the galvanometer, and operating it at a high frequency. The rotation frequency of the mirror was about 20c/s, and the galvanometer was operating at twice this frequency; the thermopile elements had to respond substantially in about 1/80 sec.

Results

The noise limit of the thermopile and amplifier, when used with a vibration galvanometer giving an effective integration time of about 1 sec, was rather better than 10 8 W noise equivalent power; when used with the direction indicator, the performance was not as good. One ‘demonstration’ with the instrument was to project the light spot from a hand torch on to a black screen about five metres distant, and to ‘scan’ the screen with the infrared detector after the torch had been switched off. It could detect the region that had been warmed by the incident light one minute after the light had been switched off. In the field, ground-to-air detection ranges of about two miles were achieved, using the 11 cm diameter mirror. In flight, the range fell to rather less than 800 m on single-engined aircraft owing to the increased noise level arising from microphonic disturbance. The flight test was on 27 April 1937, when a Vickers Vincent (Bristol Pegasus III air-cooled engine, 635 h.p.) was detected in the broadside-on position at about 500 m range. This may have been the first occasion on which one aircraft was detected in flight from another by infrared means. Unusual observations in the evening of 30 August 1937 on an ArmstrongWhitworth Whitley aircraft with Armstrong-Siddeley Tiger engines suggested that radiation from the engines might not be the only means of infrared detection of aircraft in flight. This aircraft was on the ground and was being viewed from the top of a building; it was found that the galvanometer deflexion decreased as the engines were warmed up. It turned out that the aircraft had been standing under cover all day, and that its fuselage was therefore at a lower temperature than the ground beneath. It was therefore giving a large indication which was substan­ tially reduced as the engines warmed up; this led to an investigation of the possibility of detecting aircraft by the temperature difference between fuselage and background. Calculations showed that with the 1937 apparatus an aircraft of frontal area 100 ft2 travelling at 150m.p.h. should have been detected from the front at about one mile against a clear night sky background, using aerodynamic heat alone. This possibility was therefore proposed as an alternative means of aircraft detection. In the foregoing connection, we measured the effective black-body tempera­ tures of the clear sky and of low cloud at night in Oxford, by the simple method of hanging a suitable black body from a gibbet, and viewing this against the background of cloud or sky with the infrared detector. When the black body was adjusted to the appropriate temperature, the signal from the detector vanished,

Infrared Detection in British Air Defence, 1935-38

17

since the detector measured any difference in radiation coming from different parts of the field of view. At ground level, the effective clear sky temperatures measured (in September 1937) were about — 5°C; cloud temperatures were about + 16°C. THERMIONIC DETECTOR AND PICTURE DEVICE

An attempt was made to develop a thermal picture apparatus. The principle was to replace the normal cathode of a photoelectric image converter by a thin foil blackened on one side, and coated with a low-work-function thermionic emitter, such as caseium-silver oxide. The increased thermionic emission caused by heating due to the incidence of infrared radiation would cause a brightening of the appropriate region of the fluorescent screen of the image converter, which could thus be made to register a thermal picture. A Secret Patent was granted for this device: crude trials in 1937 with a tube made by EMI Ltd. showed corase images of hot sources. It was also proposed to develop a simple detector using thermionic emission, with chopped incident radiation, in place of a thermopile or bolometer, but the development ceased when a policy decision stopped infrared work in the Air Ministry in 1938. PULSED SEARCHLIGHT AND OPTICAL RADAR Another device which was outlined in some detail was the pulsed searchlight. This arose out of our attempts to eliminate the back-scattered glare from a visible searchlight, by using an infrared system, with an image converter. There was no improvement, because any reduction in back-scatter was offset by the compara­ tive insensitivity of image converters, but the work stimulated the conception of a pulsed searchlight, with an image converter which was to operate as a shuttered detector by the appropriate pulsed application of its accelerating potentials. The device could be used as an optical radar, or as a picture device sensitive only after the back-scattered radiation from the nearer part of the searchlight beam was behind the receiving lens, thus eliminating the glare from the nearer part of the beam; it was also contemplated, alternatively, to sensitize the image-converter to light scattered back from the portion of the beam beyond the target, so that this might be seen in silhouette—rendering the system effective against black-painted aircraft. The original memorandum, dated 27 January 1938, reposed in an Air Ministry file until October 1939, when the Tizard Committee expressed interest, but it was then thought too long term a project to start.

INFRARED MEMORY TUBE A further device which was of some subsequent interest was an infrared operated memory device. The research establishment at Bawdsey had asked for

18

Pre-1939 Work on the Detection of Infrared Radiation

ideas to improve the signal-noise ratio for radar in the presence ofjamming. One obvious way was to integrate many successive sweeps of the cathode ray tube, since the echo from an aircraft then showed up as a ‘nick’ in the baseline, cutting into the illumination above the baseline caused by the noise. The use of slow decay phosphors in the screen was suggested for this purpose, but it was found that these phosphors could not be stimulated effectively by electron bombardment. Sir Thomas Merton therefore suggested a twocomponent screen, consisting of a fluorescent component which was stimulated by the electron beam, and a phosphor component which was stimulated by the light from the fluorescent component. This device was very satisfactory, and is still in use. Anothor scheme, which originated with me in 1938, was to use the type of phosphor which is primed by ultraviolet light, and which emits the stored energy as visible light when triggered by an infrared photon. Trials showed that electron bombardment was effective in priming this type of phosphor, and that a ‘memory’ tube could thus be built which stored up many sweeps of the time base, which could all be shown up simultaneously by flooding the screen with infrared light. This device has since been successfully brought into use.

PHOTOCONDUCTIVE DETECTORS While photoelectric detectors would have been much more convenient for aircraft detection, we thought that we should very probably have to cool them for successful operation. Our attempts in that direction were much inhibited, partly by the shortage of time, and partly by the advice of a Government Establishment, which had worked on infrared photoconductive cells for eighteen years and which stated that it had tried a very wide range of substances, and that none had been found to go much beyond the Case ‘Thalofide’ surface. Despite, therefore, Lange’s publication of the photoconductivity of lead sulphide out to about 3 /z wavelength, little was done in this field in Britain, most regrettably, before 1944; although I did construct an apparatus for evaporating and evaluating photo­ conductive surfaces at the Admiralty Research Laboratory for the late C. A. Luxford to use in 1939. TERMINATION OF THE OXFORD INFRARED WORK

By September 1937 infrared was shown to be a feasible means of detecting one aircraft from another, but it had two important disadvantages. It would not work through cloud, and it gave no range information. Neither disadvantage was in itself fatal, but in March 1938 the system was abandoned on the recommendation of the Tizard Committee in favour of radar. The decision was substantially a correct one: I at least had all the time realized that radar was potentially a much more powerful technique, and had been rather surprised to be told, as I was in 1936, that it was not very promising for airborne use. It transpired that the doubt

Infrared Detection in British Air Defence, 1935-38

19

had arisen because at that time it was uncertain whether radio pulses of less than a few microseconds duration could be generated, and whether a receiver could recover sufficiently rapidly from the effects of the transmitter pulse to give a minimum range of less than a mile. Had these defects materialized, we might have had an awkward gap between the range at which radar became ineffective and that at which the pilot’s eyes could take over. It was only in view of this that I had spent the effort in developing an infrared method. At the time that we stopped, in March 1938, we were probably ahead, as may be judged from the records, of any other country, with the important exception of the photoconductivity work in Germany. We believed that the ultimate solution would probably lie in a fusion of the 1.5 m radar and infrared philosophies, leading to a system using a wavelength long enough to penetrate cloud, and short enough to be focused. On 4 November 1937 I had concluded that ‘salvation lies between 1 and 10 mm’ in a report to the Director of Scientific Research at the Air Ministry. Unfortunately I could not see the ultimate way of generating and detecting radiation of these wavelengths; but the experience contributed to the general background that led to the initiation of the microwave programme of 1938/9. It has recently been represented that radar and infrared were two rival methods of detection, backed respectively by Sir Henry Tizard and Lord Cherweil; and that, had the latter been able to dictate the prewar policy, Britain would have had no radar in 1940. This picture is false: Sir Robert Watson Watt has stated how valuable Lord Cherwell’s support was to him in radar and, as for the notion that Sir Henry Tizard considered infrared of no account, I may record that in October 1937 he asked me whether I would transfer from the Clarendon Laboratory to Imperial College, where he was Rector, and continue the infrared work there. There never was, in fact, any serious rivalry between radar and infrared, nor could there have been. The former technique, obviously powerful, had a large research team(for those days) devoted to it; the latter, obviously slender, for much of the time had only myself. I had no doubts about the unrewarding prospects, but it was simply a matter of duty for someone with the appropriate experience to try to take infrared methods to their limits in case apparently better methods failed. This is a familiar type of decision where military matters are involved, although it is rarely the happiest of experiences to be the one to be selected to attempt the ‘forlorn hope’. In retrospect, I should have been saved much personal effort had the work been stopped, as I expected that it would be, after the 1935 trials, but it might have been expecting too much to get a clear decision in the circumstances. WINDOW

An example of Sir Henry Tizard’s foresight was his recommendation in July 1937 that ‘The experiments now being conducted at Oxford on infrared radiation

20

Pre-1939 Work on the Detection of Infrared Radiation

should continue in the hope that they will have an application to air defence other than the detection of aircraft from the air.’ In a curious way this speculation came to be justified by an unforeseen, but satisfactory, outcome; this was ‘Window’, the resonant tin foil strip decoys designed to confuse radar. The idea was such an obvious one that it may well have been invented by many people independently; but the first discussion of it known to me, and certainly the one that ultimately led to its use in World War II, occurred in 1937. At that time, infrared detection was being rightly criticized for being unable to work through cloud; but it struck me, in drawing a comparison betwenen the weaknesses of the two methods, that radar, even on a clear night, might be almost as impotent as infrared would be in cloud, if the air space were cluttered up with resonant dipoles at a density of the order of a few per ‘resolution volume’ of the radar system, and that this might be achieved with relatively small effort. The original idea, with the comparatively long wavelength then used by our radar stations, was to suspend wires of the appropriate length from small balloons, so that they would remain in the field of view for a considerable time. When Professor Lindemann told me in the autumn of 1937 that the Tizard Committee were contemplating the stopping of infrared work, I therefore pointed out this vulnerability of radar to him; as a result, I believe, Mr. Churchill raised the matter with the Air Defence Sub­ committee of the committee of Imperial Defence. No notice appears to have been taken of the suggestion at that time; certainly no trials were made. In the autumn of 1940, after Mr. Churchill had become Prime Minister, Professor Lindemann was able to insist that some trials be made. The results were so striking that there were many misgivings that we might harm ourselves by using it since it would show the Germans an easy way of neutralizing our radar defences; the argument went on for more than two years (and an almost parallel argument was going on in Germany). Ultimately the matter went to the Prime Minister for decision. He held a meeting in June 1943, in which it fell to me to present the final technical case for using the device. Having heard the arguments on both sides, he gave the decision: ‘Open the Window!’. CONCLUSION

In some ways, the infrared work of 1935-38 was a lost effort; it was essentially of a short-term nature since we believed, rightly, that we had very little time. It could not be published early enough to influence other infrared work, although it anticipated several developments elsewhere; and it diverted my effort from completing in 1937 what would probably have been the first superconducting bolometer. It may, however, have contributed something to the soundness of our general decisions in scientific aids to defence, and it brought me into contact with many defence problems (Jones(5-7)) in a way which was to prove unexpectedly helpful when the war occurred.

Infrared Detection in British Air Defence, 1935-38

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REFERENCES

1. 2. 3. 4. 5. 6. 7.

Arnquist, W. N.» Proc. Inst. Radio Engrs., N. Y. 47, 1420 (1959) Blackett, P. M. S., Nature, Lond. 185, 647 (1960) Jones, R. V., J. Sci. Instrum. 13, 84 (1937) Jones, R. V., J. Sci. Instrum. 11, 302 (1934) Jones, R. V., Journal R. U. S. I. 92, 352 (1947) Jones, R. V., Research, Lond. 9, 347 (1957) Jones, R. V., A Physics Anthology (edited by N. Clarke), p. 121, Chapman & Hall, London (1960)

CHAPTER 2

First Projects at Aberdeen Crystal Growth and Optical Levers

The war, with its prelude and aftermath, took me out of research for 10 years, after which I came to Aberdeen and found a large university department with a heavy teaching commitment and a small staff entirely inexperienced in research. In considering how we could best get going I was much aware of several limiting factors. In the first place what would normally have been the most creative years for a physicist had for me been occupied by problems of defence; and in the second place my education had been weak in theoretical physics. Moreover, in 19461 was starting a year late on my contemporaries who had returned to universities in 1945, and who in many cases had only left them for defence work 4 years after I had. The former consideration decided me not to go into radioastronomy, which would have been a natural bent, because flourishing groups were now already very active at Cambridge and Manchester, and it would be pointless to attempt competition. As for what we might be able to do in Aberdeen, there were two attractions for me. The first was that in 1927 my predecessor, G. P. Thomson, had performed his famous experiment that demonstrated the wave nature of the electron, and the second was the fact that I could depend, as he had done, on a good workshop: and on my election to the chair I was delighted to find that it had once been held by James Clerk Maxwell and that he had done some of his best experimental work in Aberdeen, including his ‘colour box’ which anticipated Littrow in its prism arrangement, and which can still be seen in the Cavendish Laboratory. Maxwell had written in 1858: ‘I am happy in the knowledge of a good tinsmith, an optician, and a carpenter’ and ‘I have got a new model of my theoretical rings*, a credit to Aberdeen workmen’. I hoped that we could maintain the tradition. Before we could start on any research, though, there was the pressing problem of teaching. It was part of the Scottish tradition that the professor should himself teach the first-year classes; and it was also part of that tradition that these classes •by which he demonstrated his celebrated explanation of the structure of Saturn’s rings.

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First Projects at Aberdeen

23

should tend to unruliness. It was therefore essential to win their interest right from the start, and so I resolved to show some of the most fascinating demonstrations that I had encountered. Some of these had been shown me by the great German master of demonstrations in physics, R. W. Pohl, when I visited him in Gottingen shortly after the war. One was the phenomenon known as Mach’s bands, with which I hoped to show the students that however convincing might seem the evidence presented to them by their eyes, it was desirable to check it by the more objective observations that could be made with instruments. Pohl’s demonstration was to rotate a pattern consisting of a white star on a grey disc (Figure 1) fast enough for the eye to be unable to follow. What you would then expect to see is a uniform white central region out to the radius of the circle on which fall the inner points of the star, and a uniform grey region beyond the circle containing the outer points. In between you would expect that there would be a continuous shading from white to grey, as the percentage of grey at any intermediate region increases with radius. In practice you do indeed see this but in addition you ‘see’ two circles which are not really there—one ‘whiter than white’just inside where the shading to grey begins, and another ‘greyer than grey’, just beyond where the shading ends. And it is not an effect of rotation, for the rotating disc can be photographed by a time exposure (Figure 2), and the effect persists in the still photograph: the circles seem so real that you can put your finger on them. But, if the photograph is examined with a microphotometer, this

I

Figure 1. ‘Star’ pattern before rotation

First Projects at Aberdeen

24

-

■

ISHS

iw-

Jj ■■■

■

■r

Figure 2. Visual effect of rotating the star pattern of Figure I. showing Mach's bands

shows a gradation of density from 100 per cent white to 100 per cent grey, just as you would expect, with no trace of the two fictitious circles that the brain so convincingly suggests from the signals coming to it from the eye. There must be some mysterious over-compensation in the human vision system which causes an overshoot when it comes to the end of a density gradient. Besides astonishing the class as much as it had previously done me, the effect turned out to be unexpectedly topical in the Nobel prizewinning work which G. P. Thomson had carried out at Aberdeen which resulted in his famous photographs of the diffraction pattern of electrons from gold foil, which showed concentric circles. Many workers would have rushed to publish such spectacular evidence that electrons had wavelike properties: but Thomson knew of the

First Projects at Aberdeen

25

phenomenon of Mach’s bands, and delayed publication while he made or acquired a microphotometer to check that his circles were real. What he thereby lost in priority of publication was happily compensated by the Nobel recognition of his objectivity. In this work Thomson warmly recognized his debt to the skill of his Aberdeen instrument-maker, C. G. Fraser; and one of their many exchanges is worth recounting for the lesson it illustrates in the value of ingenuity in measurement. The practical problem was to find a crystalline material so thin that electrons could pass through it, when they might be expected to show a diffraction pattern by transmission. Knowing that gold leaf is one of the thinnest materials available, Thomson wondered just how thin it was. Fraser offered to measure it, and proposed to fetch his micrometer, which was one of the ordinary variety. The Professor told him that it would be a waste of time, because the thickness of the gold leaf was not only less than the 10 microns separating the smallest divisions on the micrometer scale, but smaller than one of the actual index marks separating the divisions. But the instrument-maker persisted in fetching his micrometer, and then proceeded to fold the leaf repeatedly, doubling its thickness each time until he reached the stage where it would give a readable measurement on the micrometer and then dividing this by 2 to the power of the number of doublings. A cheering corollary to the story is that when G.P. received the Nobel Prize one of his first acts was to send some of it to his instrument-maker. In 1946 there were only nine staff including myself, three being under 21 years old, one with shortened wartime degrees, and more than 300 students, and so teaching had to occupy most of my efforts for the first few years. Fortunately nearly all the students had seen war service and had come to university determined to make the most of the opportunity—and so teaching, although onerous, was a rewarding pleasure. In research I would obviously have to be the mainspring for any ideas that would provide research themes for my department, and so I raked over the debris of experience with which the war had left me. My appreciation for precision had been enhanced by the achievements of our German opponents and by contact with a REME major, Andrew Fell, who had been seconded to my staff in 1944, and who was rapidly becoming a leading watch designer. He introduced me to a range of watchmaking tools that would also be valuable for instrument work. As for a major research theme, I should like to have pursued experiments which might throw light on fundamental points of physics. As an example, I had from undergraduate days admired Lebedew, who in 1900 had performed the first experiment that detected and measured the minute mechanical pressure exerted by a beam of light on a mirror, but since his experiment had been done more than 10 years before I was born, it would be no contribution to physics to repeat it. Another possible experiment that I had thought of as an undergraduate was to direct a beam of light through a rotating glass disc and try to detect the sideways drag of the moving glass on the beam of light passing through it. Again, the

26

First Projects at Aberdeen

expected effect was minute, and it turned out that the same experiment had been contemplated by Oliver Lodge, who quickly decided that it lay far beyond the bounds of technical possibility, since it would require a ‘disc’ of glass 1 metre in diameter and 3 metres thick rotating at 3000 r.p.m. to produce a sideways drag of 1 micron, which he thought would be the smallest deflection that it would be possible to detect. As it was, I ultimately succeeded in doing this experiment and others of comparable difficulty: but it would not have been sensible to attempt them from the state of the experimental art in 1946. So I had to find something less ambitious but preferably useful. The one field where I had ‘had the edge’ of most of my contemporaries in Britain before 1939 was in infrared detection—but this advantage had already been lost because some of them had been able to pursue infrared spectroscopy during the war where, towards the end, they were able to explore the new lead sulphide detectors brilliantly developed by our German opponents. The two possible areas in which I might not yet have been overtaken were photoelectric amplifiers for galvano­ meters and the crystals of infrared transmitting materials which I had artificially grown during the work on aircraft detection. Since Donald Stockbarger at MIT had in the meantime shown that it was possible to grow crystals of fluorite (calcium fluoride) by similar means perhaps we could emulate him by growing fluorite at Aberdeen. Further, we might be able to incorporate rare earth impurities into them, and see whether these would produce the infrared absorption bands that I had observed with natural crystals in my pre-doctorate days. Moreover, if we could grow large crystals these would have many applications both in physics and in technology. Clearly the physics of solid state was going to be a major field of scientific attack during the next 20 or 30 years, and crystals would provide nearly ideal materials for the necessary investigations: and even if we did not have enough theoretical competence at Aberdeen to make such studies ourselves, we could at least be of help to workers elsewhere if we could provide them with desirable materials. In addition, infrared transmitting materials were going to be increasingly required for the purpose for which I had originally grown crystals: ‘windows’ for infrared detectors for use against aircraft, for incorpor­ ation in the homing heads of anti-aircraft missiles which, following developments in wartime Germany, were likely to become a major factor in air warfare.

CRYSTAL GROWING So, growing crystals would be one theme, and infrared spectroscopy to investigate their properties would be another. For the first I would scale up my pre-war work to grow large crystals of such materials as sodium and potassium chlorides, which could be made into large prisms for infrared spectrometers, and as sodium iodide, which was just coming in as a scintillator for nuclear radiation detectors. For a further phase, in which D. A. Jones and R. W. H. Stevenson

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27

joined me, we would design vacuum furnaces to grow more difficult materials such as lithium and calcium fluorides, which could be used as prism materials for both ultraviolet and infrared spectrometers, and to simulate the coloured natural fluorites. Within a few years, partly thanks to generous help from Donald Stockbarger, we became one of the world centres for crystal growth, producing crystals up to 5 kilograms in weight of the fluorides of lithium, calcium, barium, magnesium, cadmium, manganese, lead and other metals, and later of niobates and other materials for lasers. We supplied these crystals to many laboratories, as far east as Calcutta and west as California, and we showed other laboratories how to grow ionic crystals for themselves, including my old laboratory, the Clarendon in Oxford, and the Royal Signals and Radar Establishment at Malvern, who gradually took over the work from us. We made furnaces for industrial firms such as Mervyn Instruments (now defunct), Rank Taylor Hobson, and British Drug Houses, who took over commercial production. In all our work the ‘green fingers’ of D. A. Jones, who had come with me from the Air Staff to Aberdeen, were an important factor, and after his untimely death in 1975 our work on crystals came to an end. Relatively few publications resulted from the work as regards the actual growing of crystals, although many papers were published by our collegues in Aberdeen and elsewhere to whose experiments they were vital. The prolonged wrestling with practical difficulties, and the consequent development of ‘know­ how’, would have made unfashionable reading in the atmosphere then current in scientific circles, which tended to rate a proliferation in physical theory above a major contribution to experimental technology; we were still suffering from the tradition of Aristotle who, in his Politics, refused to accept training in crafts as a part of formal education at any level, for such knowledge will ‘absorb and degrade the mind’. One of the few technical papers that we published on crystal growing was on ‘Growth of cadmium fluoride crystals from the melt’ (D. A. Jones and R. V. Jones, Proc. Phys. Soc., 79, 351-357, 1962). Some of our efforts in crystal growth, particularly with lead and cadmium fluorides, may even now, in 1987, not have since been bettered elsewhere, but since this book is concerned mainly with instrument design rather than with materials, a detailed survey would be out of place here. Two incidents, though, may be worth passing notice. Lead fluoride proved difficult partly because, as with so many chemicals supplied even at the highest grade of commercial purity, it was insufficiently free from impurities, and even when it was it tended to dissociate at the temperature necessary to melt it, and thus lose fluorine into the surrounding vacuum. The melt was thus left with excess lead, which came out as small inclusions in the lead fluoride crystal on solidification. The resultant crystal appeared to be blue by transmission, and at the same time it scattered pinkish-red light out to the side. The inclusions could therefore be regarded as resonators for red light, which they then re-radiated, while blue light was thus extracted from the incident beam. This was exactly the reverse of what normally happens in the

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First Projects at Aberdeen

atmosphere, which scatters blue light from the sun, giving rise to the blue sky and reddened sun. In our crystals we had accidentally simulated what happened on those rare occasions when particles of suitable size are present in the upper atmosphere, for example particles of gum in the smoke from forest fires, to resonate to red light and thus to cause the legendary ‘blue moon’. A corollary to this is that the sky should simultaneously appear pink, but I do not know of any confirmatory reports. We were able to demonstrate our simulated blue moon at a Royal Society soiree in 1956. In fact the effect was troublesome with lead fluoride, and required delicate control of the pressure in the vacuum furnace to avoid it. The second incident was also concerned with the purity of our materials, where at one time we found a suitable supplier in the small East London firm of Thomas Tyrer, a subsidiary of Albright and Wilson. They told us that one of their steady, albeit bewildering, demands was for potassium nitrate cast as small images of the Buddha, to be sent out to India. Ultimately they found that the demand came from Buddhist priests, so that if a villager asked a priest for help when his tree yielded poor fruit, the priest could supply him with the image to bury at the foot of the tree, where the fertilizing effect of potassium nitrate would artfully encourage the villager’s faith in the miraculous powers of the Buddha. All this, though, lay in the future when in 1946 I had to restart on infrared work. For a spectrometer I chose one of the early double-beam instruments, the model D209 by Hilger and Watts, partly because the firm’s Director of Research was Dr. A. C. G. Menzies, who was formerly Professor of Physics at Leicester and who as a Group Captain in the Royal Air Force had been head of the Operational Research Centre on the Air Staff where our wartime cooperation had developed into the warmest of friendships. While the spectrometer was being built I proceeded to develop photoelectric galvanometer amplifiers so as to have them ready to measure the small currents coming from the thermopiles in the spectrometer. Hilgers, though, ran into an unexpected difficulty. They found that they could no longer make prisms of the same optical quality as previously because they could no longer work the surfaces (of rocksalt, which is a soft material) to sufficient flatness. Their design had been an ingenious one with a 30° prism in a Littrow design in which the radiation to be dispersed enters the prism via the hypoteneuse surface and is refracted so as to strike the next surface (the longer of the other two sides) normally. Here it is reflected, and is returned, almost along its incident path, the double passage through the 30” prism giving the same total dispersion as for a single 60° prism, but using only half the amount of the expensive prism material. As usual, though, one rarely gets something for nothing, and in this case the price to be paid lies in extra care that has to be taken in working the reflecting back-surface of the prism. For, supposing that light is passing through a surface of refractive index n, and this surface is inclined at a small angle 0 off normal, the deviation of an incident ray is roughly 0(n — 1) or, typically 0/2. But if the surface is a reflecting one, the deflection is 20, and so to

First Projects at Aberdeen

29

keep any local deviation to within some specified limit a reflecting surface has to be worked to a precision four times that for a refracting surface. Why could Hilgers no longer achieve the necessary extra precision? After their current generation of workers had had several unsuccessful attempts they finally brought out of retirement the old worker who used to make the prisms. I was told that he succeeded first time, and the other workers then tried to emulate his technique. He himself could not explain why he could do it and the others not; the only conclusion that they and he could think of was that there was some difference in the way that he lifted the prism off the lap in the final sweep of his polishing action; but for others to repeat his technique proved so difficult that the firm gave up the design. I tell this story partly to explain what then happened to my own work (for while waiting for the spectrometer I took the optical lever work for galvanometers well beyond my original intention) and partly to show how, even in the seemingly cold and precise field of scientific instrument construction, the individual skill of a single human being can make a unique contribution. D. A. Jones, whom I have already mentioned, was another example: and it has been my good fortune to encounter others, from the one man in the Pyrex factory who in pre-war days could blow bubble-free lengths of pyrex tubing to the men who beat out gold foil to a thinness that for years no machine could equal. OPTICAL LEVERS

Although the pre-war work on aircraft detection had shown promise in the use of electronic amplifiers for measuring small currents in low-impedance circuits, and although electronics had advanced rapidly during the war, a galvanometer operating at the limit set by Brownian motion was still much superior in sensitivity to the best thermionic amplifiers in 1946. While awaiting the delivery of an infrared spectrometer from Hilgers, I therefore decided to see how well a galvanometer system might be made to perform under conditions of adequate stability, and with the benefit of photoelectric amplification of the deflections of its mirror. Adequate amplification was available, and the ‘noisy* nature of the output suggested that the system was easily reaching the useful limit set by the random motion of the galvanometer mirror as its suspension was bombarded by the molecules of the surrounding air. But it was all too easy: and so I checked the performance of the optical lever and amplifier with a large fixed mirror where true Brownian movement should be negligible. The random fluctuations in the output were still there, and so must be due to some fault in the measuring system itself; moreover they were many times greater than Brownian theory would have predicted. At first I blamed the photocells which were used to detect the movement of the beam of light reflected from the mirror, but every now and again there was an abnormally large ‘kick’ from the secondary galvanometer measuring

30

First Projects at Aberdeen

the unbalance in output from the photocells. Then I noticed that the kicks coincided with flashes of light from the beam as the last surviving housefly in an Aberdeen winter flew through it. This made me wonder about the obscuring effects of dust particles in the beam and, finally, whether the random variations in the density and refractive index of air in the beam might divert small fractions of the light from one photocell to another. I was encouraged in this idea by pre-war experience with the Admiralty when considering the feasibility of an optical barrage across a harbour mouth with an infrared searchlight on one side pointing to a photocell on the other, so that the passage of any intruder would be indicated by an interruption of the beam. The device was of little use in practice because the random variations of refractive index in the intervening atmosphere were often enough to divert the beam almost completely from the photocells, causing the searchlight to appear to twinkle like a distant star. The suspected effect in optical levers was easily tested by ‘fanning’ air near the beam, and also causing pressure pulses to cross the beam by rapidly opening and closing the laboratory door. These tests proved positive. As a final check on air movements in the beam as the primary cause of the trouble, the obvious step was to surround the optical lever system with an evacuated jacket, and the trouble then disappeared. Now one of Rutherford’s dicta was that whenever you get a result that surprises you ‘Try it again!’ Although this particular result was hardly surprising, I did try it again by letting air into the vacuum jacket—and then I had a genuine surprise: the system was almost as quiet as it had been when evacuated. Now I had really to try everything again, including the physical removal of the vacuum jacket, when the disturbances at once returned: but they again disappeared when the jacket was replaced and made to act simply as a shroud enclosing the unevacuated optical path. This was a useful finding, for it was much simpler to operate the system with suitable shrouds than with an evacuated container. The question then arose: ‘Why did the shrouds work?’ The trouble had almost certainly been due to convection currents in the light path, and it transpired that at least two earlier experimenters had had trouble with convection currents and discovered a similar cure. The first was H. Cavendish (1798) in his classic measurement of the gravitational constant, G, when he found that convection troubles could be reduced by enclosing his torsion balance in a narrow container. The second was C. V. Boys (1895), also in a determination of G, where he concluded that convection current troubles were roughly proportional to the fifth power of the linear dimensions of the enclosure. Such a finding would, incidentally, tend to explain why I had not found them serious in my optical lever amplifier of 1934, where by luck or instinct I had made the system so small that it could be entirely mounted on the galvanometer itself. The effect of the containing shroud in inhibiting convection currents was, incidentally, largely explained in 1954 by Sir Geoffrey Taylor, as can be found in Paper III. In the work on optical levers between 1947 and 1950 I attempted to achieve a high sensitivity by using a strong light source, a 48 watt bulb; this entailed

First Projects at Aberdeen

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problems of thermal distortion in the mechanical components of the system, which were minimized by water cooling sufficiently to achieve the stability necessary for the long-term observation of Brownian movement in galvano­ meters. And since the spectrometer was still not forthcoming from Hilgers (partly for the prism trouble mentioned on page 28) it seemed worth while to check that the r.m.s. potential energy in the galvanometer suspension was the expected ^kT(k is Boltzmann’s constant). The check had some topicality at the time since it had been argued elsewhere that a galvanometer coil was subject to two independent sources of thermal noise—that due to the bombardment of the surrounding air molecules, giving a random motional energy of r.m.s. |kT, and also another due to the independent fluctuations of electronic currents in the coil giving another random motion corresponding to a random electrical energy of r.m.s. %kT: and since the electrons and the air molecules belonged to independent systems, the combined effect on the coil should be x |kT. C. W. McCombie, then a research student and later professor of physics at Reading, joined me in these measurements, which showed that the mean potential energy in coil was \kT and not times that figure. In fact our observations provided another way of determining Boltzmann’s constant within a few per cent, and the coil could simply be regarded, following Einstein, as an enormous molecule which had on average %kT of energy in each of its two degrees of rotational freedom, one kinetic and the other potential. This work was described in Jones and McCombie (1952). The physical explanation of why the r.m.s. disturbances remained at even though two sources of noise were present, was that the second source of noise also introduced extra damping (viscous air damping or electromagnetic damping) in exactly the right amount to keep the r.m.s. deflection at the same value as for a single source. We in fact recorded the r.m.s. deflections of a galvanometer coil in runs lasting from 10 to 20 minutes under heavy electromagnetic damping, and critical damping, and then air damping alone. Only in the last case, when the galvanometer was an open circuit, did the r.m.s. deflection differ by more than 1 per cent from the value predicted by Gustav Ising (1926) when he assumed that the r.m.s. potential energy stored in the coil suspension would be |kT. The 14 per cent excess that we recorded on open circuit was probably due to seismic and other disturbances, and we concluded (from some 6000-7000 measurements):

That Ising’s limit is the correct one, that the extended theory given in this paper is confirmed, that galvanometers can be reliably operated at the theoretical limit, and that drifts and fluctuations due to external causes can be made small compared with those arising from thermal agitation. The work on the optical lever itself was published in the Proceedings of the Physical Society (Jones, 1951). Besides the realization of the effects of convection currents, and how they could be reduced, the two other main items that emerged were the value of elastic movements as a means of making small mechanical displacements (which will be described in the next chapter), and the use of grids to

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improve the sensitivity of optical levers. I came to them by a ponderous process of thought and trial which were described in the paper, only to find that A. H. Pfund had proposed them as early as 1929. My optical system, though economic in components and reliable in performance, was cumbrous in construction and subject to pincushion distortion which allowed only grids consisting of con­ centric circles to be used. The cumbrous construction was mainly due to the relatively large sizes of the iron-selenium photocells and to the size and heat dissipation of the light source. A much neater construction was possible when better solid-state photocells became available. The first of these were the Schwarz photocells developed at Hilgers, and in 1959 I described, with J. C. S. Richards, a new optical lever system making use of them. That paper is reprinted as Paper II. Within a few years still better cells were available, along with transistors largely due to advances in silicon technology. Moreover, the optical techniques for amplifying the effects of small deflections could be applied in a variety of systems for observing relatively obscure phenomena such as radiation pressure and ‘aether drag’, and a device for detecting infrared radiation by the expansion its heating would cause in a thin metallic strip. Some of these applications were summarized in the Duddell Medal Address to the Physical Society in 1961, which is reprinted as Paper III. One interesting point that emerged in the paper was that the system could detect much smaller rotations than might have been expected from the Rayleigh criterion for the resolving power of the optical lever mirror. In fact, as remarked in the paper, the Rayleigh criterion could be exceeded by a factor of 106 to 107, the limit being set by the fluctuations in the number of photons falling in the two halves of a single diffraction pattern and not by the ability to detect the presence of a second pattern nearly superposed on the first. Rayleigh (1885) himself had considered the optical limit of sensitivity, which followed from his classic definition of resolving power to be that angle which involved a rotation of the mirror such that its edges moved by a quarter of a wavelength; but he appears to have overlooked the possibility of detecting much smaller rotations by not worrying about resolving power and contenting oneself with locating the optical centre of the diffraction pattern. I myself had first become sensitized to the possibility in 1937, not through optical levers but through the prospect of establishing the direction of a target aircraft by radar from another with a far greater precision than would be expected from the Rayleigh limit of an antenna system small enough to be carried by the second aircraft. A few further developments were described in the Parsons Medal address of 1967 which is reprinted here as Paper XVII. These included arranging the optical lever mirror to act as one component of a corner reflector, so that to a first approximation the zero of the lever system does not change if all other components of the system move as a whole relative to the mirrors, and the zero only moves if the angle between the two mirrors is changed by one of them rotating. This could have the advantage of depending primarily for long-term

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33

stability on the relatively small region around the mirrors rather than on the stability of relationship between the mirrors and the rest of the system. Another feature that was incorporated in the later optical levers was the optical micrometer, by means of which the image of one grid could be translated across another by inserting a thin parallel-sided glass plate in the intervening optical path, and rotating the plate through an appropriate angle. In some optical experiments to be outlined in Chapter 7 I used the optical micrometer as a very convenient method of making a small calibrated displacement; but its use required more care than I at first gave it, and the resultant errors were significant enough to merit publication to save others from repeating them; and these were published, with a brief history of the device going back to James Watt, in Jones (1976), reproduced here as Paper IV. It has been gratifying to find that these developments in optical levers have been useful to others, including such skilled experiments as Pierre Connes (1985) and the late Jesse W. Beams (1969). Two further notes will conclude this commentary on optical levers and optical micrometers. They outline two unpublished items and are reprinted here as additional notes to Paper IV. The first describes a simple device for calibrating an optical lever by direct rotation of a mirror through a small angle, and the second an application of an optical micrometer in tandem with an optical lever to stabilize a laser beam in direction. REFERENCES Beams, J. W. (1969) ‘Determination of the gravitational constant G’, Phys. Rev. Lett., 23, 655-658 Boys, C. V. (1895) ‘On the Newtonian constant of gravitation’ Phil. Trans. Roy. Soc. A, 186, 1-72 Cavendish, H. (1798) ‘Experiments to determine the density of the earth’, Phil. Trans. Roy. Soc., 88, 469-526 Connes, P. (1985) ‘Absolute astronomical accelerometry’, Astrophys. Space Sci. 110, pp. 211-265 Ising, G. (1926) ‘A natural limit for the sensibility of galvanometers’. Phil. Mag., 1,827-834 Jones, R. V. (1951) ‘Some points in the design of optical levers and amplifiers’, Proc. Phys. Soc. B, 64, 469-482 Jones, R. V. (1961) ‘Some developments and applications of the optical lever’, J. Sci. Instrum., 38, 37-45 Jones, R. V. (1967) ‘The measurement and control of small displacements’, Bull. Inst. Phys. Phys. Soc., 326-336 (October) Jones, R. V. (1976) ‘The optical micrometer’, Opt. Eng., 15, 247-250 Jones, R. V. and McCombie, C. W. (1952) ‘Brownian fluctuations in galvanometers and galvanometer amplifiers’ Phil. Trans. Roy. Soc. A, 224, 205-230 Jones, R. V. and Richards, J. C. S. (1959) ‘Recording optical lever', J. Sc. Instrum., 36, 9094 Pfund, A. H. (1929) Science, 69, 71. Rayleigh (1885) ‘Optical comparison of methods for observing small rotations’, Phil. Mag., 20, 360-361

PAPER II

Recording Optical Lever (with J. C. S. Richards)

INTRODUCTION

The use of cadmium selenide as a photoconductive material has led to the production of small sensitive photocells (e.g. by Schwarz*1*) and such cells have been applied by Thulin*2* to optical levers. The present paper describes work at Aberdeen with Hilger-Schwarz and similar cells which has improved the sensitivity of optical levers by more than an order of magnitude over the best previously described.*3* The previous performance had been obtained by using barrier layer or vacuum photocells with a 48 W projection lamp as the light source, giving a limiting useful sensitivity of about 10“ 9 rad for 1 s response time. Apart from the brightness of the source, the main factors in reaching this sensitivity were the attainment of high mechanical and thermal stability, and the elimination of convection currents in the optical path. The same factors hold in the present design, but the problems have been simplified by the small size and high sensitivity of the photocells. A 4 V 3.2 W miner’s cap lamp bulb now suffices as the source, and the low heat dissipation of this lamp combined with the general reduction in size of the optical lever system has facilitated the design of a compact amplifier which can detect changes in angle of about 10“10 rad in the orientation of a 2 mm2 mirror with a response time of about 0.25 s. The first co-author has been responsible for the optical and mechanical details, and the second for the electronics. OPTICAL DESIGN

The optical design follows that of previous amplifier*3* with modifications to concentrate the reflected light from the mirror on to the small area (twice 5 x 1 mm) of the differential photocell. The system is shown in Figure 1, and the construction in Figure 2. The source S is a miner’s cap lamp bulb rated at 3.2 W at 4 V, krypton-filled, 34

Recording Optical Lever

O

Gl

35

*-l

X, x2

Figure 1. Schematic plan of optical lever. Components L2, P,XxX2 are shown also in elevation

type MES 408 K; these lamps last for months when continuously run. They give sufficient light to operate the photocells, without developing so much heat as to require any special cooling arrangements. An image of S is focused by the condenser lens (which consists of two plano-convex lenses each of 70 mm focal length and 32 mm diameter, separated by 5 mm) on to the mirror M, which can be as small as 1 x 1 mm, but is preferably of twice to four times this area. M is attached to the system whose rotation is to be measured. The light reflected from M diverges until it reaches L2; this lens is a condenser exactly similar to Lt, and it focuses an image of M, and therefore of S, on the surface of the double photocell (type FT435 by Hilger and Watts Ltd.). Interposed between Lx and M and L2 is a subsidiary system consisting of a lens L3 (an achromat of 100 mm focal length and 25 mm in diameter) near the mirror M, and two grids Gj and G2. Gi is illuminated by converging light from the condenser Lt, and is so situated that the rays diverging from any point on Gl are made parallel by L3; after reflexion from M these rays return through L3 to be focused in the plane of G2. Thus an illuminated image of Gx appears in the plane of G2; this is the standard auto-collimating position, and results in a distortion-free image at unit magnifi­ cation. If Gi and G2 are replicas, G2 will stop or pass the light from Gt depending on their relative positions transverse to the light path. G2 is split into two halves, separated by the width of a bar in the grids, so that when the right

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First Projects at Aberdeen

half of G2 passes the light from Gn the left half of G2 stops the light from GP Viewed from L2, the effect is that of a vertically divided field, in which the halves go alternately black and white in anti-phase as the mirror M is rotated. A narrow angled prism P, conveniently made of Perspex, is placed behind one half of L2 with its vertex horizontal, to divert the light from this half sufficiently downwards to fall as a focused image of S on the lower half X2 of the photocell. Light passing through the other half of G2 and L2 is undiverted, and forms a focused image of S on the upper half X1 of the photocell. The arrangement thus results in an image of S falling on each half of the photocell. As M rotates, both images maintain their positions but their intensities change, due to the varying degrees of overlap between the image of and the two halves of G2. The system is operated with about a 50% overlap on each half, giving equally intense images on Xi%2. A slight rotation of M causes one image to brighten, and the other to darken; the bridge circuit containing Xx and X2 then registers an out-of-balance current. In order to adjust the transverse position of the image of Gx, an optical micrometer plate O2 is placed in front of G2. This plate is of plane parallel glass about 2 mm thick, and is rotated about a vertical axis by a mechanical micrometer; rotation of O2 through a few degrees causes sufficient transverse shift of the image of Gi to cover the whole range of a grid spacing. Since the interposition of O2 somewhat upsets the symmetry of the optical system, a fixed plate of similar thickness, is placed at the corresponding position in the incident beam. This reduces any loss of image quality due to O2. The grid spacings selected depend on the application; finer grids give greater effective sensitivity at the expense of maximum measurable rotation. A limit to the fineness of the grid that can be used is set by the aberrations of the system and by the diffraction at M; generally between 5 and 70 lines/cm are suitable. The system will measure linearly rotations up to about one quarter of the angle subtended by one grid bar and space at the focal length of L3. The grids can be made in several ways; the method employed here has been to make a wire grid, and then to take replicas by evaporating aluminium through it on to glass. The illumination on the photocell surfaces, when a mirror of 2mm2 area is used, is some hundreds of lux. This is sufficient to ensure satisfactory operation of the photocells.

CONSTRUCTION The construction is outlined in Figure 2. It is sufficiently substantial to avoid any danger of components shifting undesirably. The baseplate is of mild steel, | in. thick. The lens holders are also of mild steel; slotted holes in their bases allow sufficient movement for approximate focusing. A screw focusing mount for lens L3 gives a fine adjustment of the image of G! on G2. The glass plate Ox and the grids Gi and G2 are gripped in slotted holders (of brass or steel) by their lower edges, and screwed to the baseplate. The optical micrometer plate O2 is mounted

37

Recording Optical Lever

X| X2 Gi

O2

G2

Figure 2. Construction of optical lever. Overall length of base plate — 20 cm

on a cross-strip pivot (out of sight beneath the baseplate) which is rotated by the micrometer visible at the side of the baseplate. The source S and the photocells XxX2 are mounted as shown, appropriate adjustments being allowed by slotted holes for the fixing screws. The supply leads are soldered directly to the lamp so as to avoid any danger of variable contact. An exterior cover (shown broken) is provided, along with a centrally placed screen, to reduce the circulating volumes for convection currents. This screen and the inside of the cover are painted black to reduce the amount of scattered light reaching the photocells; they also improve the cooling of the lamp by radiation.(4) For the greatest suppression of convection currents, it may pay to seal the gaps between the lens holders and the cover and screen with paper baffles. The conical Perspex nosecap (shown dotted) on the front of the lens L3, and extending between it and whatever screen surrounds the mirror M, is particularly important. Without the nosecap, convection currents and dust particles drifting across the gap will cause much disturbance. The distance between L3 and M should desirably be as small as possible, to minimize aberrations, but the system is designed for this distance (which plays the part of the ‘working distance’ in a microscope) to be as much as 25 mm, so that the lever will work satisfactorily when the mirror is part of an instrument which does not permit a closer approach,

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First Projects at Aberdeen

as for example when it is in a vacuum jacket. There is little point in giving exact dimensions of the components in the lever, since the simplest procedure in constructing a similar instrument elsewhere would be to pick lenses from stock which are approximately of the dimensions given in this paper, and then ascertain the most suitable layout on an optical bench. The whole instrument is mounted on three adjustable screw feet, so that it may be suitably levelled with respect to the mirror. The assembly is surrounded by a Perspex case to reduce the effect of external temperature variations and draughts, and is run continuously to keep the internal temperature steady. ELECTRICAL ARRANGEMENTS

The twin photocell is connected with two load resistors in a Wheatstone bridge network. For purposes of initial design each half of the illuminated photocell (type FT435 by Hilger and Watts Ltd.) may be assumed to have a resistance of 20 kQ; the actual resistance will, of course, vary with individual photocells and optical arrangements. Thulin(2) has shown that the sensitivity of such a bridge may be made independent of the lamp brightness by feeding the bridge from a constant current rather than a constant voltage source. However, if maximum zero stability is to be attained in the present arrangement it is necessary to keep the lamp voltage as constant as possible (within ±0.1% say) in order to eliminate thermal effects, and Thulin’s technique then offers no great advantage. If the apparatus is used only occasionally, secondary batteries may be used to supply the lamp and, since the current through each half of the photocell is limited at 2 to 3 mA by considerations of thermal dissipation, dry Leclanche cells may be used to feed the bridge. The out-of-balance signal from the bridge can be observed on a galvanometer; assuming that the galvanometer resistance is appreciably less than the bridge resistances, a sensitivity of 10"8-10-9 A/mm is adequate. However, for most purposes it is more convenient to apply the output from the bridge to a pen recorder via an electronic amplifier, and to derive all power supplies from the a.c. mains. Because of the high sensitivity of the photocells, it is not difficult to design a suitable amplifier. Even under the best conditions noise and short term drifts in the optical lever are likely to give an output from the bridge of more than 100 ^V. Two different electronic amplifiers and lamp power supplies have been used. In both amplifiers a transistor input stage is employed, since the output impedance of the bridge is comparatively low. The second and output stages of the first amplifier use thermionic valves, and the power supply for the lamp is the same as that described by Richards(5) apart from the obvious modifications to reduce the output from 6.3 V to 4 V. Although this arrangement performs satisfactorily, a more compact and elegant apparatus can be made if transistors only are used. The complete circuit of the second electronic amplifier is given in Figure 3. The first stage consists of two transistors in a push-pull configuration with a large

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