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Oral History Transcript — Dr. Ebenezer Cunningham

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Interview with Dr. Ebenezer Cunningham
By John L. Heilbron
At Cambridge, England
June 19, 1963

 
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Ebenezer Cunningham; June 19, 1963

ABSTRACT: Part of the Archives for the History of Quantum Physics oral history collection, which includes tapes and transcripts of oral history interviews conducted with ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Charles Glover Barkla, Harry Bateman, John D. Cockcroft, Paul Adrien Maurice Dirac, Albert Einstein, James Jeans, Joseph Larmor, Robert Andrews Millikan, John Joseph Thomson; University of Cambridge, and University of London.

Transcript

Heilbron:

Well, perhaps you could just say again some of the remarks you made about Larmor a minute or two ago.

Cunningham:

Yes, well, just speaking then about my personal contact with him. In my fourth or fifth year of graduate work, I attended some lectures of his which were called lectures on thermodynamics. He was a very bad lecturer, but he was very stimulating and as one thought about things afterwards, one saw that he was a real thinker. I became interested through him in the Lorentz transformation as some clue to the failure of the Michaelson-Morley experiments to detect any particular velocity of the earth through the ether. When I found myself in Liverpool with time to do as I liked, I found that in following up Larmor’s book on Aether and Matter, published in 1900, there was something more to be said. What we often call the Lorentz-Larmor transformation was an exact transformation of the Maxwell equations and therefore gave the clue to this failure to establish a precise ether. Now, during the year when I was attending Larmor’s lectures on thermodynamics, he spoke about the problem of the distribution of energy in the black body radiation --.

Heilbron:

This was before 1900?

Cunningham:

Yes, well, it would be 1901 or ‘02. No, no, 1903. Because I took my Tripos in 1902, and it was after that. That would be in 1903, probably. He clearly was puzzled by the fact that the ordinary kinetic theory of gases and temperature did not fit onto the distribution of energy in the black body radiation, which experiment he was showing. I think Jeans was involved in that to some extent. He had been writing about kinetic theory. I do remember in a vague way that Larmor in those lectures seemed to suggest that the only way out of the problem, the failure of the classical equations to agree with the experimental results, was in some kind of way a unitary theory of energy. That is to say that there was some elation between the energy emission and the frequency, a sort of germ of this quantum theory.

Heilbron:

Did he mention Planck, as far as you recall?

Cunningham:

He didn’t get as far as Planck in those days. Well, whether he’d been reading him or not, I don’t know. Certainly, I do associate and it’s rather vague in mind, now, that he hinted at some sort of connection between the energy emission from molecules, from atoms, and the frequency of radiation: some sort of integral relation, rather than the continuous relationship that the old theories had suggested. The germ of it was there.

Heilbron:

Do you remember by chance whether or not Larmor was sympathetic to this idea of Jeans that the reason we do not observe all the energy in the high frequency range of the spectrum is because there is no equilibrium, or equilibrium takes eons to establish itself and that our current experiments Just give us a non-equilibrium condition?

Cunningham:

No, I wouldn’t feel that in this situation.

Heilbron:

But that was a most curious notion of Jeans to circumvent the Planck proposal.

Cunningham:

No, he didn’t speak much of Jeans though they were more or less contemporary, but in fact I should say that Larmor was a deeper thinker than Jeans on the whole. But the other aspect of Larmor -- that was what I was saying about his book and this development of the transformation which was more or less associated with the name of Lorentz. Of course, he was very tentatively suggesting an electromagnetic theory of matter. He was basing it all on Maxwell, you see, but introducing the conception of the electron; it is funny to think about now, isn’t it.

Heilbron:

It is. Before, you were describing the reception of Larmor’s book.

Cunningham:

Yes, Larmor at the British Association, I think in 1904; it was in Cambridge. Horace Lamb was the President and he spoke of this book in relation to modern theories of matter and said that this book which had just been published called Aether and Matter, Adams Prize at the University of Cambridge, would more appropriately be called Aether and No Matter. [Laughter] It was true. He was feeling toward a purely electromagnetic theory but all on a Maxwellian basis plus this introduction of the electron, the smallest particle of electricity.

Heilbron:

Was it felt at all that the book was hard to read?

Cunningham:

Oh, it was.

Heilbron:

It was quite difficult.

Cunningham:

When I got free at Cambridge and was a junior lecturer and had some time to myself, I spent a long, long time chewing it over you see, going backwards and forwards and trying to see what the real significance of it came to. In the course of that, I discovered for myself that the Lorentz transformation was an exact transformation of the Maxwell equations. That seemed to me to be a really important point. Larmor told me that he had found the same for himself and I dare say, many other people had too.

Heilbron:

At this time you were in pure mathematics more or less?

Cunningham:

No, I was shifting over, you see, I was a general lecturer in anything at the University beginning with first year students. But this was where my interest was, rather than in the pure mathematics in which I had been thoroughly indoctrinated in Cambridge with Professor Baker.

Heilbron:

One of the things that particular interests us is the relation between the experimental physicists and the applied mathematicians and the pure mathematicians. It’s a distinction that seems a little awkward to draw sometimes.

Cunningham:

Yes, in those days I should say that the distinction was not so sharp as it is now. The normal honors course for students here treated them as one whole, and neither was more important than the other. The time was approaching when the emphasis in mathematics did go largely onto the pure side with Hardy and Littlewood and people like that and Baker in geometry. Larmor was a theorist entirely. He had no experimental background. The other person in Cambridge at this time, of course, was J. J. Thomson. Well, he, of course, began as a mathematician as Maxwell did. But J. J. Thomson and Maxwell became aware of the importance of the physical applications of these things and their mathematical relationships. Thomson, of course, went on to the experimental side, but it was always said that he couldn’t do an experiment himself.

Heilbron:

Well, he was successful in not doing them.

Cunningham:

He had an amazingly able assistant technician who would do any experiment he wanted for him, a man named (Everett), I think it was. But J. J. Thomson, himself, it was always said, couldn’t do any experiment, nor any experimental work himself.

Heilbron:

Did you ever attend any lectures by Thomson?

Cunningham:

No, I didn’t. I didn’t do anything on the physical side in those days. The Cavendish level was developing then, you see. But I think somehow that was sort of a birth of a new era that was going on, you see. Maxwell had gone a long way on the wave theory of electricity and light, and J. J. Thomson really is the man who got the experimental aspect of the electron theory going. That’s what it comes to, I think. He with his colleagues. I remember C. P. R. Wilson with his cloud chamber. I didn’t become aware of Rutherford until a good deal later when he came back here from Manchester.

Heilbron:

Well, that’s of course very reasonable. He wasn’t here at all.

Cunningham:

No, he wasn’t here in those days. But there were a great many 28 of the old school, like G. F. Searle and Alex Wood, and so on. They were the people who were running the physics lab at the time. Even Bragg hadn’t come on the scene then. But coming back to my own experience, I remember an occasion when, I think it must have been about my first or second year here in ‘99 or 1900, I can take you to a spot by the front gate of St. Johns -- within a foot or two I can take you there where I was standing -- when I heard a small group of people alongside discussing J. J. Thomson’s discovery of the electron as 2,000 times lighter than the atom of hydrogen. It’s as definite as that.

Heilbron:

Do you remember anything that was said?

Cunningham:

No, I just remember the fact. It sort of swept across me. This was breaking down all my assumptions, you see. You see, I was still Daltonian in my outlook, the Dalton indivisible particle of matter.

Heilbron:

Well, it’s a reasonable thing to think that an atom should be indivisible.

Cunningham:

The name implies it. This new conception that you can knock spots off it is another --. You see, that does date the beginning of modern physics so much in these sub-atomic things.

Heilbron:

Was there a general feeling that physics was going through a rebirth?

Cunningham:

Well, they weren’t aware of it, it wasn’t talked about much. These things were being fed into the atmosphere.

Heilbron:

What kind of an impression did radioactivity make? I suppose quite a large one.

Cunningham:

I couldn’t say much about that.

Heilbron:

And the X-rays as well.

Cunningham:

Well, now the X-rays -- they seem to me to have come later. Now there was a man named C. G. Barkla. Well, he was a chorister at Kings College here in my time. Wonderful bass voice.

Heilbron:

That’s something we didn’t know about in --.

Cunningham:

Oh, yes, great tall chap and wonderful bass voice. It was he who began to differentiate the X-rays into their different frequencies, the whole scale of them. He went on to Liverpool University to be professor there after I’d been there. He did a lot of his work there. Incidentally, just to get the background of dates in this business, when I went to Liverpool, the professor was Wilberforce and he was giving lectures on liquid air and the things it did, like making daffodils brittle and lead wire springy and so on. And that was a sort of modern physics then. But I don’t think that radioactivity had really reached the surface much in those days. I think Barkla did a lot.

Heilbron:

Of course, Barkla’s work on X-rays was later but the discovery was before the turn of the century.

Cunningham:

No, I think I would be right in saying that at that time we were very local in our outlook. We thought in terms of Maxwell and Larmor and J. J. Thomson as they developed here. There was a whole stream of development in Cambridge running parallel to the continental ones.

Heilbron:

Were there any sorts of seminars or meetings or colloquia in which the applied mathematicians could come into contact with the experimental physicists?

Cunningham:

I think they were developing gradually. I didn’t come across them very much. I think we were very individualistic in those days. Somebody said once, “What does a mathematician do in his research? Does he just shut himself up in a room with a piece of paper and think?” That was the picture of the mathematician, you see. J. J. Thomson was escaping from this, but I don’t think the development at that time during the first decade did come from the escaped mathematicians. It was growing up as a purely experimental thing and J. J. Thomson was the first link. Thinking over the people in the mathematical school and excepting J. J. Thomson, I don’t think of anyone but Larmor who in those days had realized the importance of the new experimental side, and he was almost Thomson’s contemporary. It was quite new, you see. Until 1900 practically, or until J. J. Thomson, the experimental work in the Cavendish laboratory was, I wish to say now, very old-fashioned.

Heilbron:

Were things about the same at London?

Cunningham:

Yes, I can only tell you what I came across at London. Ramsay was there and he’d just discovered argon, that sort of thing. That was the sort of thing that was talked about, you see.

Heilbron:

Still, my goodness --

Cunningham:

-- It was a step forward.

Heilbron:

It must have been 10 or 15 years old by that time.

Cunningham:

Yes, but there was the old chap himself, you see. (Laughter) Of course, in those days I was a youngster, I didn’t realize the significance of these things, but I remember him as a person; and Donnan doing physical chemistry. Ah, yes, now Trouton, do you know him?

Heilbron:

Oh, yes, the man with the heavy weights.

Cunningham:

Yes, he was professor of physics at that time. I was trying to remember what I associated with him. It’s gone a bit. But I should have said there wasn’t very much going on there. The applied mathematics was in a poor way at University College because the professor of applied mathematics was Karl Pearson, who had gone over into statistics and biometrics and so on. In fact, I was doing his work.

Heilbron:

And on your return to Cambridge, did you --.

Cunningham:

I came back to Cambridge in 1911 and in those days mathematics -- the faculty wasn’t even a faculty -- each college practically did its own teaching to its own students. And we all had to be jacks of all trades. The Tripos that we were teaching for included such things as geometrical optics, the old-fashioned positional astronomy, the fundamentals of spherical astronomy, and so on. But what you might call physical, astronomy had hardly come into being then. Eddington appeared on the scene and he got -- this was of course a bit later, 1912 thereabouts, when the more general theory of relativity came along -- he got very interested in that and was talking about it. But gradually he was developing the observatory here on the physical side instead of the positional aide.

Heilbron:

Where would one be examined on such things as mechanics? That would be in the Mathematical Tripos too?

Cunningham:

Yes, yes. I did a lot of lecturing on that side of things.

Heilbron:

How much kinetic theory would that include?

Cunningham:

Yes, that came in the more advanced subjects, the voluntary subjects, you see. In those days we had a part one of the Tripos which was the general thing including mechanics and so on, and then part two, which was generally taken by people in their fourth year. That would cover kinetic theory and electromagnetic theory of waves and so on but it hadn’t got as far as electron theory.

Heilbron:

And you would lecture therefore on those subjects.

Cunningham:

Well, I was lecturing in my early days here, first of all, on things general, but as time went on --. Yes, before the war, First World War, 1912 to ‘14, I was lecturing on electromagnetic waves and the classical electron theory as we then knew it. Very largely Lorentz’ stuff and I remember lecturing on the emission of radiation from vibrating electrons -- that sort of thing, you see.

Heilbron:

You must have lectured in part on the quantum theory, at least on the theoretical aspects of radiation.

Cunningham:

No, that was the funny part. After all, there’s a lot of story attached to this. In fact, I rather lose a sense of dates you see, because this horrible war broke in and I was switched off for a long time. I was an agricultural laborer for a couple of years --. I came back here at the end of the war, 1918, and then really the exciting times began. During the time, I would say, from 1912 to 1918, various things had been happening. The quantum theory had really been getting into people’s minds. Oh, who is the person I’m thinking of?

Heilbron:

At Cambridge?

Cunningham:

He was a Cambridge man and then he was away from Cambridge. But this idea -- well, it really followed out of Einstein and his photoelectric business; you know, the integral relation between the emission of energy and the frequency. What year was that, ‘06, ‘07?

Heilbron:

‘05. Same year as relativity.

Cunningham:

‘05. I always forget those numbers. That had got into people’s minds and when I came back having missed three years, I found that the world was new. Yes, for instance, you take the general theory of relativity; just before the war, Einstein and his colleague Grossmann, I think it was, had published their first paper on generalized theory. I said to myself, “No, the universe isn’t made this way.” [Laughter] It seemed too ridiculous, that Einstein would get a tame pure mathematician to tell how the universe was. But when I came back, you see, Eddington had seized on the general theory of relativity and had developed quite a bit there. I found myself left behind. I still was thinking very largely in classical terms except for this sense that there was a new loosening of concepts. After the war, for instance, I found myself put down still to lecture on the classical electron theory, and I asked people, “Is it worthwhile going on with this?” It was all on just the plain Maxwell equations, and so on. And they said, “Yes, it’s important, it’s fundamental.” So I went on and I, among other people, lectured on this particular stuff to Cockcroft and Dirac. [Laughter]

Heilbron:

Well, one can’t complain about the results.

Cunningham:

I remember one particular thing. I remember I’d been working out -- just as a pure result of Maxwell equations and vibrating electrons -- that in the radiation emitted, the electrical component arid the magnetic component were at right angles to one another and to the radius. This all turned out at the end of a long calculation, having evaluated the field as far as possible explicitly. I said to the class one day, I remember, “This is an extraordinarily simple result in the end, but why? Why should it work out like this?” A week later, a young man who had only been at Cambridge a year or two, a year, I think, yes, less than a year, came up to me and said, “Here you are.” That was Dirac. [Laughter] In his first year in Cambridge. It had its significance, I think. For instance, I worked out the thing, putting it in reverse, the radiation coming in and being absorbed by an electron -- it’s only a little altering of boundary conditions, which was quite interesting; by compromising between the two, you could get a non-radiating vibrating electron, which seemed to have some significance. Oh, it was all rather exciting, but the most exciting thing in a way was the emergence of people like Cockcroft and Dirac coming into the scene, I always feel my position at Cambridge was that of harboring some of these people. Cockcroft, for instance, sat for an examination at St. Johns College for exhibitions. He’d been at the Manchester Institute of Technology -- no, no, (Metropolitan Vickers) and he was trained at the Manchester School of Technology and they’d offered him something to come here and we had this exhibition to offer and he came up for the examination. We picked him out as a chap to have. Dirac, similarly, had been at Bristol University. He was recommended to us by the professor there who is a member of my college, and he said, “We’ve got a very good chap here. I want him to come to Cambridge.” And I said, “Well, let him sit for this exhibition exam.” So he did and we offered him an exhibition, but he hadn’t enough money to come, so he stayed on at Bristol and did almost an engineering course, I think it was. Then he got a grant from the DSIR, I think it was, and then he could come. Cockcroft came and he’d been doing electrical engineering. He did the Mathematical Tripos and then he went to work in the Cavendish laboratory. He worked with Kapitza. So he got a marvelous equipment of pure physics and math, some engineering. Dirac very quickly made a name here. But you see, I just felt they’d run away from me. I was lost. Actually, I was rather immersed in some administrative problems in college and that plus the interruption of the war left me rather behind. Well, now, are there any other questions I’m afraid this is rather rambling.

Heilbron:

Oh, no. We can straighten it out very easily. Well, in the direction of the quantum theory, I wonder whether there was a difference in the way people reacted to the strange ideas of the quantum and to the strange ideas of relativity. Was it easy here or difficult to accept one or the other?

Cunningham:

That’s rather difficult to say. I wasn’t in the center of the physical, of the laboratories. I was on the side of the mathematical school plus the administration in college. My general impression was, I should say, that people swallowed it. It was a new thing, but it was working. It was fitting in, you see.

Heilbron:

Did you discover that people objected much to relativity when you first began to work on it?

Cunningham:

No, I don’t think I did. I suppose I was the first to talk about it in this country, but mainly on the old restricted principle. I spoke about that at the British Association in (Portsmouth), I think it was, in 1910. I think it was the University Press that first asked me to write something about it, which I did; I had got that more or less ready for press when J. J. Thomson asked me to write a monograph, one for the (Longman) series of monographs, which came out about 1912 or 1913. I had written that more with thinking on the fundamental concepts and I found that the physics people welcomed that very much, and the experimental people, people like Appleton, for instance. I remember that he appreciated that very much. I don’t think that people reacted against it. I think the people did realize that here was a step forward. I remember A. N. Whitehead was quite interested in it. We were all puzzled, I think, when the generalized theory emerged largely because so many people could hardly tackle the mathematics.

Heilbron:

It’s very annoying to have to accumulate such a mathematical apparatus before you can --.

Cunningham:

Oh, yes, but the interesting thing was that the mathematics was there ready with Riemann.

Heilbron:

That’s happened several times, hasn’t it? With the matrices --.

Cunningham:

But the quantum theory, I think, was a bigger thing to swallow because we were able to let time and space be more relative but not discontinuous! I still feel puzzled about the young men, how they can more or less start at that stage without having been through the mill.

Heilbron:

It’s perhaps easier to start at that stage than -- Do you recall the first discussions of Bohr’s work?

Cunningham:

I heard Bohr lecture here for two hours. I was a bit ‘Bohred’. It was very difficult to follow. Yes, I remember there was a lot of talk. That goes back to 1912 or thereabouts, doesn’t it? And, of course, Rutherford appeared on the scene.

Heilbron:

I think he came here in 1919. Did one know of the Rutherford atom quickly after its --?

Cunningham:

Yes, I think that it came in quickly, in the sense of a sort of orbital system within the atom. People were able to swallow it. Even I could swallow that.

Heilbron:

But the radiation difficulty was --.

Cunningham:

Yes, that is where I got interested in this idea of a non-radiating vibrating electron but the switch over, jumping from one orbit to another, was a difficulty.

Heilbron:

Was there any question at all about the reality of atoms when you began your career; did you hear of these peculiar German notions of men like Ostwald and so on?

Cunningham:

I don’t think they’d reached us. I seem to associate something with Eddington as having thought of the atom as a very small (marble) or something like that. No, I think people comparatively easily assimilated these things -- the sense of the atom as a complex system of orbital electrons. You’ve got dates so clearly in your mind, when did Planck first suggest this?

Heilbron:

In 1900.

Cunningham:

Yes, I was trying to fix the date and I --.

Heilbron:

1900. I think the paper was published first in early 1901.

Cunningham:

I remember that there was a paper I read somewhere about that time but I couldn’t locate it in my memory.

Heilbron:

How closely did one follow the German literature?

Cunningham:

I don’t think very closely. The excitement here was all on the experimental side, I think. I mean the actual identification of the electron.

Heilbron:

You discovered for yourself Einstein’s articles.

Cunningham:

Yes. Actually there was a paper published in the --. Where was his first paper published?

Heilbron:

I think in the Annalen.

Cunningham:

Annalen der Physik I think it must have been there. I lit upon that quite by accident.

Heilbron:

Oh? It was an unknown name, probably.

Cunningham:

Yes, I couldn’t identify the exact date, but I think it was in 1906 that I lit upon it. And I went: (raps fist on table) “He’s got it.”

Heilbron:

Immediately.

Cunningham:

He’s got something, you see, and it’s not only he’s got something, but he’s opened up something. I wrote several small papers about it in the Philosophical Magazine. In fact, I wrote a comic paper, in a way, I think now, generalizing it from the idea of the four dimensional rotational to a four dimensional inversion. It was a definite generalization, but I couldn’t see any significance to it. Mathematically, it was a complete conformal transformation in four dimensions.

Heilbron:

So it was really through you that most people in England heard of Einstein’s new work?

Cunningham:

Well, I wouldn’t like to say so, but I certainly published the first literature. I don’t know whether you’ve seen the monograph -- it was called Relativity and the Electron Theory.

Heilbron:

This is your second book?

Cunningham:

The second small one, published in (Longman’s). The first book was raw and undigested, I think. So much so that when they asked me for a second edition, I said, “No, I don’t want to rewrite that.” So they printed up a second impression and then burned half of it. I think that was an important piece of work.

Heilbron:

To go back to this question of the relations of the mathematicians and the physicists, would you say that Rutherford’s coming here as head of the Cavendish made any large difference in that question?

Cunningham:

Rutherford wasn’t a mathematician.

Heilbron:

Oh, of course not. Exactly the question –-

Cunningham:

He was a man of imagination and he just plunged into experiments. This is my impression of him. And found out a lot of things. I think his son-in-law, R. H. Fowler, did a lot to stimulate people. He was a mathematician but he was very much associated with the experimental people.

Heilbron:

What college was he in?

Cunningham:

Trinity. He died fairly early. Very alive, very forceful person. He would have done a lot. He stirred up people. You see, there was so much development going on on the experimental side and Kapitza made a lot of difference trying to smash up the poor little atoms, and with his magnetic fields. Of course the epoch making thing was when Cockcroft really did knock a bit off and identify it. That’s interesting just as a personal connection with that. Cockcroft arranged a demonstration of this for Einstein’s benefit because he was going to be in Cambridge. He invited some of us down to the laboratory just to see the poor little atoms being knocked to pieces; Einstein didn’t turn up. Dirac was there and he’d asked me down. And he wanted to put someone in the cabinet, to see it happen, you see. So he put me in. [Laughter] And there it was. I was in the dark and there was a fluorescent screen and then there was a little spark on it. That was it. They said I came out too soon, they hadn’t turned the current off. I might have been electrocuted. Of course, there was great excitement when Cockcroft, working with Kapitza in the magnetic lab, tried this dodge of submitting the atom to a more intense magnetic field by -- you know that story? They designed a dynamo which was to be run up to a very high speed on open circuit and then short circuit it through an electromagnet, you see, so as to produce a very fast field but such a reaction on the machinery, I think it was brought to rest in a very small fraction of a second, but it was long enough to hurt the atom --

Heilbron:

-- and short enough to hurt the machine.

Cunningham:

Well, that was what we were all afraid. People were really afraid of doing the experiment. But here was Cockcroft, you see, who had been in electrical engineering, pure mathematics, pure physics; he calculated out stresses and all sorts of things, you see, and the thing was safe. It worked, but it was out of date, of course, like everything else. His original apparatus was fantastic -- an enormous array of glass tubes.

Heilbron:

This was the experiment with Walton?

Cunningham:

Yes, the one with Walton, yes, you see it was done electro-statically.

Heilbron:

Yes, I’ve seen pictures of it.

Cunningham:

Oh, fantastic, really.

Heilbron:

Do you recall what excitement there was over the positron discovery?

Cunningham:

Just by hearsay. I wasn’t tremendously -- I felt everything was gone beyond me by that time. Of course, I’ve seen a good deal of Freddie Hoyle but all these things were rather outside my sphere by then.

Heilbron:

What of the great excitement about the famous eclipse in connection with relativity?

Cunningham:

Well, Eddington went somewhere in the South Seas, wasn’t it, to -- wait a minute, my mind’s not got back there yet. Yes, it was a consequence of the general theory of relativity. It taught three things there, of course. One was the development from the equations to give an account of the residual -- you see how far I’ve got afield -- yes, the rotation of the orbit of --

Heilbron:

Of Mercury.

Cunningham:

Well, I remember when that first came out. I thought, “Well, now this is too good to believe.” Then Eddington went over to investigate the shift of -- the gravitational shift of the spectrum. An interesting personal connection with that -- I have a friend here who wanted to be taken over to the observatory to have a look around -- a young woman who was just interested to see what telescopes were like. So I said, “Come along, we’ll go and see, and find somebody.” We couldn’t find anybody but at the door of the (‘sheep-shanks’ house) I heard a scuffling and down came Eddington with his hair on end. He was just measuring the plates of his photographs. He’d just found them, you see, in the thick of it.

Heilbron:

He wasn’t interested in showing your visitor his telescope, was he?

Cunningham:

Oh, yes, he was kind, very kind.

Heilbron:

Is it the spectral shift or the period of Mercury’s rotation that really is off a considerable amount?

Cunningham:

I think the question raised was largely a question of shift of the spectrum.

Heilbron:

But you found the results convincing?

Cunningham:

Well, I wouldn’t say that T expected them to be exact or final, but there was something in it. It’s interesting that people lost interest in that (cause) for the moment, didn’t they?

Heilbron:

Yes, although there seem to be people who go back to it. It is true that for a number of years it really dropped out of the picture, didn’t it?

Cunningham:

Yes, what I don’t understand, because I haven’t been able to keep up with it, is the way relativity is worked in with the quantum theory. I suppose Dirac was the chap who was doing it.

Heilbron:

Yes, and it’s still very difficult.

Cunningham:

A satisfactory theory really isn’t completed.

Heilbron:

Well, there’s the business about the infinite self energies.

Cunningham:

Oh, I know, there’s a lot left to be worked through. Well, it’s been interesting, of course, to me just to be almost just an outside observer, seeing these chaps grow up and come into the field. It’s been a wonderful time, really.

Heilbron:

A colossal amount has come from Cambridge.

Cunningham:

And of course it has so rapidly spread all over the world. It’s bubbled up in various places. Of course, California’s in the picture so much nowadays, isn’t it?

Heilbron:

Oh, yes.

Cunningham:

Do you know Hoyle? He’s out in California quite a lot. He has some assignment out there. When I was out in the States and roaming around without any particular job, I dropped down to Berkeley, stayed in Berkeley a night or two and there was a chap, Bateman, did you ever talk to him?

Heilbron:

The mathematical physicist? Oh, yes.

Cunningham:

Well, he was a co-lecturer with me at Liverpool, actually, in our early days.

Heilbron:

He went to Pasadena, didn’t he?

Cunningham:

Yes, he went to Pasadena. Wait a minute -- I think it was Pasadena, but I know he took me over to see the two hundred inch reflector being ground. He was the most naive of people, very charming in his way. I remember once after we’d been lecturing at Liverpool, he came to me and said, “Cunningham, what do you do when people applaud you in lectures?” I said, “They don’t. What have you been doing?” He said, “I was drawing a picture –- I was lecturing on hydrostatics -- of water finding its own level; I drew a picture of a jug with some water coming out of it. There was such applause.” The porter used to go in and collect up paper darts after he’d finished. But he and I were talking together about relativity you see in 1904, or ‘05. He and I put some stuff together for a paper and sent it to the London Mathematical Society and they said, “Well, this is all right, but you’ve got to cut out this and this and this.” They cut out all of Bateman. He wasn’t a realist at any rate.

Heilbron:

The Phil at that time carried most of the shorter papers?

Cunningham:

Yes, the shorter papers. I published one thing in there which attracted some attention, about Stokes’ law of the fall of particles in a viscous fluid. I sort of got the idea in my head to consider the region midway between that of viscous fluid and kinetic theory of gases. You’ve got particles whose size is of the order of the distance between the molecules. That would mean that the ordinary Stokes’ law wouldn’t apply. I tried to make a modification of this Stokes’ law, allowing for the fact of the free path of the particles between the atoms, and that made a modification of the Stokes’ law which was interesting but I didn’t think it was important. But I had a most excited letter from Millikan, a bit afterwards, saying, “This answers the question. Now we do know what the charge on the electron really is.” It had straightened it out, you see, the estimates of the electronic charge had varied according to the size of the particles or something –-

Heilbron:

According to the size of the oil drop --.

Cunningham:

Yes, and he said, “Now, this pulls us all straight and we do get a uniform result.” Was a most exciting letter he wrote to me.

Heilbron:

Well, that’s interesting.

Cunningham:

In fact, this result that I got got quoted in the Annalen der Physik as the ‘Cunninghamsche Korrektur’. (Laughter)

Heilbron:

One could culminate a career on that. That must have come out just before Millikan’s final results?

Cunningham:

Yes, I should think so. It was connected with his final estimate of the charge. It had cleared up for him some of his problems.

Heilbron:

Well, it’s surprising he hadn’t asked somebody to investigate the question.

Cunningham:

Yes, it seems such an obvious thing, in a way and what I did I thought was rather crude, you see, but it did draw together -- out of the kinetic theory gave a result which would become Stokes’ law.

Heilbron:

Is that the function of the diameter --?

Cunningham:

Depending on the diameter of the particle.

Heilbron:

What is the increase in number of students in physics, say, between 1911 and 1930? Was it quite significant?

Cunningham:

I think it wouldn’t be only students of physics. You see before the first war, the number of students at Cambridge was just under three thousand, I think. Oh, take my own college. We were under three hundred, two hundred and eighty. After the war, it immediately jumped up to over four hundred and there it stayed between the two wars practically. But after this last war, it jumped up again enormously. I think there has been a steady draw toward the physical side as compared with the arts side.

Heilbron:

And did you find that in mathematics the applied mathematicians decreased, or at least did not increase proportionally to the --

Cunningham:

The total number taking the mathematical honors course has not increased very largely. Not in the finals. The first year increased largely, and many people went on to science. But the number of people who go right through with mathematics to their honors finals hasn’t increased so very largely and I would say that I think that -- this is rather a guess because I haven’t looked at the figures lately -- that the interest in pure mathematics has been maintained. It’s developed in all sorts of ways but the bias has been towards the physics side, but I wouldn’t think it’s been so very large. Of course, one has to think mainly in terms of the first-class people, really. One looks at the contributions made. But the sort of thing that has lost interest is the more classical side in all the things, the pure and applied.