George Paget Thomson

Heilbron:

If you would begin by telling us something about your own education.

Thomson:

I owe a great deal to the teaching of my mother, who was a remarkable woman She had a passion for physics, and in an age when women didn’t get much education she had in fact very little she somehow managed to get enough to actually work in the Cavendish for a short time, terminating when she curried my father. I was practically a single child, I have a sister, but she’s 11 years younger than I. My mother taught me until the age of nine, and taught me a great deal, in all kinds of subjects. Then I went to school at the local school here called the King’s College Choir School, originally founded by King’s College to provide choir boys. They only wanted fourteen choir boys; in order to make it a desirable school day boys were allowed in considerable numbers when I was in school there were over a hundred, I didn’t have anything to do with the choir I’m quite unmusical. I stayed there for the normal time, until the age of 14. Then I went to the local grammar school here, called the Perse, an old early seventeenth century foundation, where I was fortunate with my teachers. There was a brilliant, though slightly erratic, headmaster at the time who had gotten a number of interested people around him. The teaching was good, we weren’t overtaught.

Indeed the last two or three years I spent most of my time, really and truly, sitting in the great hall, where there was generally a master present marking papers or something, reading books and doing examples. Very often there was one of the two mathematics masters there with whom I could take up a thing when I was really stuck, but he was generally busy, and I wasn’t too much encouraged to. In science I was taught by a man unfortunately killed in the first World War, a man called Davies, who think was very good. He also left me alone to a considerable extent, but I did do some practical work, quite a bit of practical chemistry. All the chemistry I’ve ever done in my life was done at school. Then I came up to Cambridge to work in mathematics. This was my father’s influence — he maintained that mathematics was a very important thing and you would learn physics somehow, roughly speaking. I took mathematical Tripos, first part, in my first year. It is easy, or it was easy in those days, though it’s tightened up a good deal now. I took the second part, with the extra papers, in my second year. This was unusual; it would normally be taken in the third year. I then spent my third year –- a Cambridge degree in only three years — on physics. I had done a good deal of the physics in fact before, partly in the form of applied mathematics, because the mathematical Tripos at Cambridge included a good deal of applied mathematics. This was at that time classical mechanics. In the extra papers, what are called the “B” papers, I took things like electric waves; I remember going to lectures on those by Bromwich, so I knew a fair amount of that sort of stuff. And of course I had done experimental physics in school you see.

Heilbron:

Was there much in the ray of the new quantum theory?

Thomson:

No. I took my degree in 1913, then did a year’s a research at the Cavendish on positive rays, which is the subject my father was then working on. I remember the things he put me to work on was what you call now radicals, things like CH, CH₂, CH₃ and so on, the kind of things for which positive rays is now used so much in the oil industry. It was an apprenticeship year rather than serious research. I never published anything on it. Then the war came, so I stopped it. When I came back I decided it wasn’t worth going on with that. I did another positive ray thing, but it was somewhat different. I think tae quantum theory in my undergraduate days was something which was regarded by people with a good deal of reserve. The general view was that of course Planck’s formula was correct, there was no doubt about that. The difficulties of getting any formula for black body radiation which did not trail off into infinity at the blue end of the spectrum were also recognized. But it was felt that perhaps the rather drastic and apparently somewhat illogical procedure by which Planck got the answer was not by any means the final way of looking at it, and that some other method might be arrived at. All sorts of things were discussed; it might be some degree of graininess, in the aether, which prevented this from happening. The aether was still a possible thing to talk about.

Heilbron:

Do you recall other suggestions?

Thomson:

Well of course about this time the Franck and Hertz things were coming along, and I think these perhaps interested my father more than the black body thing because they were more in his line of country. He was inclined to look for the solution of the quantum thing in some kind of property of the electron, and particularly the electron in relation to the electric field. All his life he hankered after concrete lines of force and he thought that you could somehow or other manage it with lines of force. He produced several suggestions, they were quite good actually, but he never quite got it off. This was the kind of thing that was being talked about.

Heilbron:

There was also the suggestion of Jeans, that one had a non-equilibrium condition.

Thomson:

Yes, that was a suggestion. I remember that being discussed too — but it wasn’t a very nice suggestion. Everybody hoped it wasn’t true — it was rather a last resort. But I think the attitude towards the quantum theory definitely was one of reserve. It was felt that Planck had certainly wade a great discovery, but that his statement of it was provisional, and there would be a better explanation of it one day, perhaps quite soon. Then of course there were the Franck and Hertz things which did of course reinforce it very considerably, and about the same time, the specific heat thing.

Heilbron:

The specific heat thing was a little earlier than the Franek and Hertz, and then the photo effect business; was that at all discussed?

Thomson:

Yes, the photo effect business, that was a very serious point. Well, of course I had heard some discussion of it, but I didn’t have much discussion in the Cavendish, except for the very last year before the war; I was working in the class you see. I had most discussion with my father, and in June 1913 to June 1914 when I was working then of course I mixed more with the research students. Willie Bragg, for example, one of my life-long friends, was just beginning to make his big discovery, and this was terribly exciting. The photo-electric effect was much older, and my father had put forward a theory of this in 1903 –- speckled radiation –- and he believed that this was a stronger support for is idea of individual tubes of force. He tended to stress the photo effect as the –- indeed I think it is -– the irrefutable reason why one must have some sort of quantum theory. I mean, here one had a discrepancy of millions to one, and so on, and not in a complicated formula like the Planck formula, where one could wiggle around it somehow. They are impossible to get around, except in the way that Bragg tended to do by denying that X-rays were waves at all and saying that they were particles.

Heilbron:

Was there much sympathy for that view before Bragg?

Thomson:

Quite a bit, but it was definitely a minority view. My father never shared it, and I don’t think that many people at the Cavendish took it terribly seriously. It was talked about with respect, “A bright idea; not that I think he’s right, you know.”

Heilbron:

That about atomic models? Was your father’s model pretty much the accepted way of looking at things?

Thomson:

Well, is was giving place it that tine, in fact had given place by 1910 to the Rutherford one. My father’s theory is an interesting one — I’ve been writing a book about my father which I finished about, the first of January. This is a book I wrote at the request of Nelson’s publishers who are doing a series on the British men of science. I think my father’s life must be pretty well documented, what with Rayleigh’s life and his own Recollections, but still as I was asked to do it, I naturally had to. It involved looking into one or two things which I had thought I knew and found I didn’t quite know as well as I had expected. One of them is the question of his model. When I talked to him about it, a good many years after it was clear that the Rutherford one was right, he always said, “Well, but the planetary model is unstable, and besides of course it radiates.” The radiation objection is the one being stressed of course, and it was quite important at the time. But there did seem to be just perhaps a way out of it, and in one of the papers my father wrote he showed that the radiation from rings of electrons — he was always keen on rings of electrons — could be extremely minute. If my recollection serves, it goes down by a factor of 10¹⁶ or something of that kind, for a comparatively small number like ten electrons, it goes down enormously.

So much so that one would be able to get around this objection if you wangle it a bit somehow. At any rate I think that my father never felt that the radiation, the fact that if the particle went round in an orbit ought to radiate, but certainly didn’t, was all that impossible. On face value, yes, but you might perhaps be able to get around that somehow… It was merely a question of whether you had a Rutherford model with a nucleus, or whether you spread the electric charge all around and thereby get yourself a different field of force. He believed that a planetary system, in which the planets repelled one another instead of attracting by gravitation was unstable. Now I thought that this was a well recognized thing. But when I came to write the thing up I made inquiries and had a correspondence with Harold Jeffreys, who I found was the best authority on these matters. He said no, the thing is much more complicated than that; there are cases in which it would be unstable, and cases in which it wouldn’t, but apparently there is no general law. Well I tried to explain the thing, and I think I know the explanation. My father, who was a student of Maxwell, as you know he edited an edition of Maxwell, got it from Maxwell’s essay on Saturn’s rings. If you look that up you will find, of course, he does the stability quite elaborately, but there is another qualitative discussion which actually does consider the possibilities of the things that go the other way, and mentions that if the particles repelled one another the rings would be unstable. I’ve looked to see whether he ever wrote a paper himself on this and apparently he did not. So he was taking it on the basis of some other published work, and I think it must be Maxwell. I think in talking to him he mentioned Maxwell. I have a sneaking suspicion that my father may have been more nearly right than the astronomers… Instead of having an inverse square law force, if you suppose the particles are moving in a field of force as you would get inside a sphere of positive electricity, that is directly as the distance, then these problems disappear. And indeed he was never quite sure. He would do most of his calculations twice over, once with the things at rest and once with the things spinning around, that is, as though the electrons were in rotation through this imaginary field of force although they weren’t. I think he varied a little bit towards which way he inclined.

Heilbron:

Did he think Rutherford’s experiments were decisive?

Thomson:

Oh yes. Immediately. He accepted that more readily, for example, than he accepted the Bohr atom — which after all does take a good bit of accepting.

Heilbron:

It’s quite curious that in the same volume of Phil. Mag. in which Bohr’s first two papers appeared your father has a paper on an atom which has electrons moving in a field of force which is inverse square attractive except in certain tubes in which it’s inverse cube repulsive. By a suitable adjustment of constants he’s able to reproduce many of Bohr’s results wad account for the photo-electric effect.

Thomson:

Yes, that was the sort of model he was aiming at. He always had the idea of discrete tubes. In this case he attached them to the atom; sometimes he attached them to the electron.

Heilbron:

He refers once to Bohr in that paper.

Thomson:

Yes, Bohr was in Cambridge of course for a year. I’m not quite sure how much my father was really in touch with him during that period. He saw him sometimes I know, but I don’t really know how much he got in touch with his work. My father always was very dubious about that kind of quantum theory, at least that kind of atomic model quantum theory, I mean the Sommerfeld type of thing. I remember talking to him about it once. I gave him arguments for accepting the model, the evidence being in those days the original Bilmer series thing, fine structure constant, the Stark effect, and perhaps one other, I’ve forgotten. And he replied that it’s all very well, but when you come to look at it all these things depend on dimensional arrangements, and if you suppose that they depend in some way on the major constants like e, c, h and so on, then there really is only one answer they can have, and all that you’re playing with is a numerical constant. To a large extent he was right, because the actual values then given for these things are not now the accepted ones. I mean the new quantum theory has changed it.

Heilbron:

There was an interesting paper about 1910 or 1911 by someone who obtained the Rydberg as functions of e and h and m by using your father’s model in fact.

Thomson:

You can do these things. But one thing I must tell you because this did happen to me, and as far as I know it hasn’t come out anywhere else, but it is worth recording. This must have been immediately after World War I think it probably was the summer of 1919. I went for a summer holiday in Switzerland, we stayed at (Berne), with a lady who is now (???) and her mother; her brother, who had been killed in the war flying had been a very great friend of mine. We were joined by a friend of theirs, a young Swiss man, whose name was Balmer. I naturally one day in conversation with him said to him, “I don’t know whether you’re at all interested in physics, but your name is one much spoken of nowadays in physics.” And he said, “Oh yes, I know about that.” The story was that it wasn’t his father or his grandfather, but it was an uncle or a great-uncle. He was a curious person. I think he said he was a school master, very interested in numerology, things like the number of the beast, and the number of steps of the pyramid. He had a friend who was a physicist or a chemist, whose name is not recorded, and they were talking, and Balmer said, “Well, I’ve rather run out of things to do.” His friend said to him, “Well you’re interested in numbers, why don’t you see what you can make of this set of numbers that come from the spectrum of hydrogen,” and he gave him the wave lengths.

Heilbron:

Were others at Cambridge more receptive than your father to the quantum idea?

Thomson:

I think on the whole the young men naturally accepted it more completely, but you see there was nothing before the War that could be regarded as a quantum theory. There were a lot of odds and ends and the only connecting feature was the fact that they involved h, and some degree of discontinuity.

Heilbron:

And Bohr’s ideas?

Thomson:

Well the Bohr paper only cane out in 1913, and this was regarded as a very daring attempt to do the thing, it was rather like Picasso. You could either say it was nonsense or you could say this was the greatest thing that could be, or you could say something a bit in between, that probably there must be something in this, but obviously it isn’t in its final form, which certainly was true. I think this is how people took it on the whole. They were very interested in it, it was the sort of thing everyone was discussing, but I think there was not general agreement. Then there were other views put forward. Of course the period of the war made a considerable difference; quite a lot happened during the war in physics as well as other things. Immediately after the war Jeans brought out a report for the Physical Society on the quantum theory, and that had a considerable influence. He brought all the bits together. You also must remember that the interest to some extent of physicists in the immediate post war period was held by relativity, which was then quite new, even the special theory for practical purposes. The general theory of course practically nobody had ever heard of until peace came.

Heilbron:

There was considerable dislocation of English physics during the war, much more so than Continental physics. Was this due to a greater demand on the services of scientists?

Thomson:

Well, I don’t know quite what it was due to. I think France was equally dislocated. I don’t know why it wasn’t dislocated in Germany. Those were the days of universal service, either voluntary, so-called, or by conscription. In England the majority of the scientific people volunteered at once. Most of us were in the Army, even when we were doing scientific things — I was doing airplane work — but I did actually serve, a few weeks in the trenches. But Bragg you see was in France almost the whole time, doing sound ranging — he made sound ranging, so to speak. The people I knew were divided really into two main groups, one lot were doing sound ranging, another lot had something to do with airplanes. A few people, most or our older people, worked on antisubmarine devices. My father had a little to do with this, though I don’t think he actually did very much with it. The elder Bragg did quite a lot on antisubmarine things. So they were pretty occupied. In a way it was a bigger upset than World War II, from one point of view. I mean Cambridge had to shut down. In Corpus Christi, where I’d just been appointed fellow, I think there were only two undergraduates. I think one was an Indian and the other was just recovering from infantile paralysis. Well of course it wasn’t quite the same thing among the dons naturally, but most of the younger dons had gone. It did shut things down very completely, so when we curse back after the war, we had all the new stuff suddenly coming at us. We heard rumors of some of these things.

Heilbron:

Of course the journals were not available.

Thomson:

Well, we didn’t have time to read them. I’d learned a lot during the war, because it happened that Lindemann was at Farnsworth, and he was just back from Germany where he’d been working in Nernst’s laboratory and was all out in the solid state quantum theory and was a vehement apostle of it.

Heilbron:

The older quantum theory. He was, I believe, antagonistic towards the Bohr atom.

Thomson:

Well, I remember his talking about the Bohr atom. No, I don’t think he was. I heard a story about the Bohr atom which he told me. I can vouch for the truth of the story, I can’t, naturally, vouch for the truth of the fact. He was kicking himself that he had not invented it. He said he did do this, he had the same idea except for one important difference. He said that he thought that if an electron changed from an orbit of 4-quanta to on orbit of 2-quanta then it ought to radiate 2 quanta of radiation. And having once got that (???) it is much more reasonable, if that were the right thing, we should take this for granted now, this would be natural.

Heilbron:

As a matter of fact, in Bohr’s paper it almost looks that way for a while.

Thomson:

Yes, I think it’s the natural view to take. I think you’ve got to persuade yourself that the other one’s right. It won’t fit the facts. But in this incident I don’t think he was antagonistic. Of course he was a critical person, you see, and he was of course perfectly aware — you didn’t have to be extremely critical to be aware — of the inconsistencies in Bohr’s derivation of the Balmer series.

Heilbron:

As I remember, Lindemann was willing to tolerate half quanta too at quite an early time.

Thomson:

Yes, he was quite keen on half quanta. I remember long discussions about zero point energy, which is really half quanta.

Heilbron:

But these half quanta in his improvement of Einstein’s specific heat formula that he had done with Nernst?

Thomson:

Yes, they came in quite early, I mean you could almost have said he considered that he discovered them. He was rather keen on that. When we came back at the end of the war, relativity even in the special form was still a new thing and making a terrific furor as a popular thing. It was the first time, except perhaps for the actual discovery of X-rays, that physics become news. Everybody was giving lectures about it, writing or reading books about it, talking about it at dinner table, all that kind of thing. My father was approached by Lord Haldane whom he knew. Haldane said that the Archbishop of Canterbury was very worried about the influence of relativity and so on and had appealed to him, Haldane, for an explanation. Haldane told my father that he had done his test, and this was what he’d said… Haldane wanted to be reassured that he told the Archbishop the right thing, which I don’t think he had.

Heilbron:

That was just the special theory?

Thomson:

That was both. The special theory had been known to physicists before the war; I don’t know what year you give to the special theory.

Heilbron:

1905 was Einstein’s first paper. I think Cunningham started to publish on it quite soon thereafter.

Thomson:

Oh yes, it was going all the time. I remember discussing that with my father. He said of course it’s all right, it’s the right answer unquestionably. But because he was fond of the sort of pictorial type of physics, he preferred the Lorentz contraction, which gives the same answer. But that was there for awhile, and I remember listening to an undergraduate’s paper on relativity before the war, on the special theory. Then the general theory came in, we all tried to learn something about this, and at the same time the quantum theory became really real. Fortunately for me because I had these talks with Lindemann and so on the quantum theory was not so much of a novelty to me as I think it was to some of the other people. The photo-electric difficulty had been a well recognized thing for ten years before that. That was one of the sore thumbs of physics that had to be explained somehow. You could explain it the way Einstein did, or, indeed, my father’s thing predated Einstein by a couple of years, the speckled wave thing.

Heilbron:

When, after the war, the quantum theory became well known here, was it mainly in the form that Sommerfeld had given it or in this newer form of Bohr, the correspondence principle playing such an important role…

Thomson:

Oh yes, that mystical —. I would say that they were accepted both, they’re not incompatible in any way. I didn’t myself have much to do with the atomic side of the quantum theory until I read de Broglie’s paper which seemed to me to make the thing much more visual as it were. I never worked on the Sommerfeld thing at all — I did try to do one or two experiments on —. I’ve forgotten what they were now.

Heilbron:

Did de Broglie’s theory make much of an impression on anyone here besides yourself?

Thomson:

I don’t know that it did, but of course I wasn’t here at the time, I was at Aberdeen. I don’t think it did much. I think in retrospect I was in advance of my time, I think I paid more attention to de Broglie than probably anybody else in this country on the whole. Some people thought it was just nonsense.

Heilbron:

But that’s even better than in Germany where most people didn’t know of it at all.

Thomson:

It was published in England you must remember, not in Germany. As far as I know there wasn’t a corresponding German publication to the Phil. Mag.. That’s where I saw it…

Heilbron:

Then you combined it with some of your father’s notions.

Thomson:

Yes, I played around with it. As I say, I don’t take laurels for that except that I paid some attention to it.

Heilbron:

What did your father think of that?

Thomson:

He thought it was very interesting.

Heilbron:

And your development?

Thomson:

Well I think he was very pleased, largely because it was in the family, but it was more than that. Partly because it was the kind of mathematics he understood, and he felt this was now a kind of theory he liked, as it were, with nice differential equations.

Heilbron:

Then nothing much happened thereafter until the Schrodinger paper and the Oxford meeting?

Thomson:

The Schrodinger paper came before the Oxford meeting. And before that was Elsasser. I didn’t see that…

Heilbron:

This business with Elsasser is quite curious. I’ve never quite understood that. He discussed these matters with Franck and Born; he actually undertook experiments, and got no help at all. He was originally a student of Franck, and there are various tales about how the notion occurred to him about applying do Broglie’s ideas to Davisson’s experiments, and then he decided to try it. He had no experimental facility, and gave up after a few months and became a student of Born.

Thomson:

All I know of Elsasser — I never met him in my life — was that he wrote this quite short piece, which, as I say, I’d not read until afterwards, but which quite possibly may have influenced me indirectly because there was a good deal through the talk of those people at Oxford.

Heilbron:

Yes, before we started the recorder you were saying that you think now that that had perhaps a greater role.

Thomson:

Yes I believe the Oxford thing was really quite important. It was rather an unorganized meeting because the British Association, as you probably know, even more now than then, is not really a very high scientific thing, it’s more just quasi-popular. These people come rather because it’s a nice place to come to. The then Prince of Wales, afterwards Edward VIII, was the President. … At that meeting Perrin was there with his son, a boy in knickerbockers. He’s now the head of French Atomic Energy.

Heilbron:

In this paper of Elsasser, he also refers to the Ramsauer effect as one of the difficulties which the de Broglie theory might enlighten. Was that an important phenomenon in England too?

Thomson:

Yes, the Ramsauer effect was taken very seriously. Naturally by Townsend who was then professor at Oxford.

Heilbron:

He was quite antagonistic.

Thomson:

Violently antagonistic to the whole thing. I think that Townsend had obviously rather — though he was a very great physicist in his day — got himself into a hole. Townsend opposed to it possibly made those who might have been opposed to it less inclined to be. He overdid it, so to speak. It was quite obvious to everybody that Townsend had missed a very important discovery, namely that of Franck and Hertz and went around saying the quantum theory wasn’t true. And then all this elaborate business with Ramsauer.

Heilbron:

I don’t know about Townsend’s relations with the Ramsauer effect.

Thomson:

Oh he disliked it intensely. Townsend as you know had done the original work on ionization by collision and had done it by the method which involved using relatively high pressures and always having a very large number of collisions involved. In effect what he got was always statistical. I think it’s an interesting example of the very great importance of trying to get rid of statistics if you possibly can. The Franck and Hertz thing was really practically Townsend’s experiment, only done under conditions in which you might hope to get an effect from a single collision, and led to a remarkable discovery. Human nature being what it is, Townsend was pretty sick at this, having missed it, you see; so he denied it. And the same thing with the Ramsauer; he really missed the Ramsauer. Although I don’t think he denied the Ramsauer effect, I think he was inclined to defend himself and say that the Ramsauer effect was in some way contained in his experiment, as it was.

Heilbron:

Yes, well I should think he would have an easier time with the Ramsauer effect because after all the Franck and Hertz experiments were undertaken in part just to disprove Townsend’s notions about ionization, that in every collision the colliding electron must give up its energy so that the energy to ionize had to be acquired in the course of one mean free path.

Thomson:

I remember Townsend’s elaborate argument of what is meant by an elastic collision, a rather silly one. Of course it was obvious that in a collision between two things with very different masses, there will be a transfer of energy in any case. This had gotten rather mixed up, and I think that Townsend was rather naughty about it, saying that the thing wasn’t elastic because it transferred energy. It’s a triviality, just the use of words.

Heilbron:

I can certainly see how he could be unhappy about the Franck-Hertz business.

Thomson:

Yes, though he also hated Ramsauer. He said Ramsauer got the credit for something he turned him to discover. I think it was more that way around.

Heilbron:

What was the significance thought to be, or was this considered a puzzle?

Thomson:

It was considered a puzzle.

Heilbron:

And the quantum effect?

Thomson:

Respected. …

Heilbron:

It’s quite curious that Elsasser should have put those two things together.

Thomson:

…I would like to know, whether Elsasser influenced physics or not, whether he really was an originator of Davisson’s interpretations.

Heilbron:

I think not, because after all the Schrodinger equation had intervened, and one did have a way. I think quite likely that it came directly from the Schrodinger picture independent of Elsasser.

Thomson:

Well I don’t know. I was aware of de Broglie. When did Schrodinger’s things come out?

Heilbron:

In the early part of 1926. The first two or three of them must have been out by the time of the Oxford meeting.

Thomson:

I don’t think I’d read them.

Heilbron:

Wasn’t Born or Franck at the Oxford meeting?

Thomson:

Well yes, I may have got them by second hand. I can’t remember if I met them at the Oxford meeting. I remember younger Perrin curled up on the ground wearing knickerbockers. I did meet quite a lot of people because I was a great friend of Lindemann. I think I stayed at Christ Church.

Heilbron:

It’s Franck’s part in this that’s so curious to me.

Thomson:

Very curious. Of course if Elsasser was going to try and do Davisson’s experiment he’d almost certainly have failed.

Heilbron:

What kind of an impact on physics at Cambridge did Rutherford’s coming have? Did it change much?

Thomson:

Well it did in a way, but whether it was Rutherford’s coming or the war I don’t know. My father never really got into his stride again after the war. He had a gap during the war during which he mostly spent his time on quasi-governmental work. He was head of this committee of Board of Invention and Research. He was a chairman of the committee. He spent a lot of time on that. Then of course he became master of Trinity half way through, and that naturally upset everything. And then I think a small thing but actually important was that he never renewed subscriptions to a German journal. Whether this was —. He wasn’t violently anti-German. In all the arguments as to whether Germans should be admitted to things, he always sided with admitting them. Maybe he didn’t feel —. Mind you, the feeling was very intense, quite as intense in the first war as in the second, though for less good reason. He was getting old and could not go on as before — taking in all the papers and reading them.

Heilbron:

Had he quite a command of the literature?

Thomson:

He had quite a command of it. He got more and more behind. Many courses he hadn’t had for five years, so he ought to try to get up with the back stuff. … Starting on a new job, which took quite a bit. (???) Cavendish professor, excused himself so to speak.

Heilbron:

Was there more continuity at Manchester during the war, do you know?

Thomson:

Of course Rutherford you see didn’t take any war work, or very little. No I think probably I did mean of course a very complete changeover. You can see it in my father’s life. But it was made perfectly clear that Rutherford was head of the Cavendish, and he proceeded to be so, but this was all perfectly all right. My father had a few people working under him.

Heilbron:

Who were some of his later students?

Thomson:

Ditchburn; Appleton, was really neither, but I think that Appleton certainly worked in the Cavendish and had the room opposite mine. I think that this was after the war.

Heilbron:

And you continued on your old positive rays?

Thomson:

The positive ray was a big thing you see because it was the Aston isotopes. This was a very big thing in the Cavendish. I was in Cambridge for three years after the war and during that period my father was very decidedly active again in doing positive rays. He came out to the laboratory every day, there were photographs and so on which he measured up. He was very finishing off the work; in fact he was writing the second edition of Rays of Positive Electricity. The first edition, if I remember right, was pre-war. [Verifies this.] Yes, the first edition was October 1913, the second 1921. He was working on that and this was a very active thing — the isotope work was of course comparable in interest with even what Rutherford was doing on nuclear disintegrations. … Oh, he did change, it was at the time I was there, those three years.

Heilbron:

Somewhere there’s some correspondence published.

Thomson:

Yes it’s in the Recollections.

Heilbron:

I wonder if it made any difference in the relations between, say, the applied mathematicians and the physicists to have this change?

Thomson:

I don’t think so. Well perhaps it did, a little. In so far as it did, that was due to Fowler. In my father’s day there wasn’t really much applied mathematics in Cambridge, which was a little odd. It was partly because he was a mathematician to start with, he did his own mathematics as it were. There was no trained mathematician around the laboratory. He did most of what was being done. The only other person who was doing anything really was Larmor and Larmor never really had a school. He was not the only person, there was Cunningham, but he didn’t do very much. He published about it, but he never had a school. Larmor probably didn’t have a school on personal grounds; he was the sort of person who would just never have a school. Schools were not so much a matter of course in those days; nor are they now so much a matter of course in the mathematical side of Cambridge simply I think for geographical reasons. There was no real center. They’ve just quite recently, in the last two or three years, had a department of Applied Mathematics. There wasn’t one in Cambridge before that, just professors. Then Rutherford came, I think really it was Fowler — he was in many respects a very great man — and Fowler it was who introduced mathematics. This was for two reasons. Rutherford was no mathematician, and he knew it perfectly well. His highest achievement in mathematics really was looking up the (???). This made the need rather glaring I think. I think Rutherford had enough sense to realize that, he was used to having, first Bohr, and then Darwin and so on. … He came back to Cambridge 1920-22, and that was when he worked with Fowler. I think that Rutherford introduced Fowler as a more or less —. I don’t think that Fowler was paid quite yet, though I’m not quite sure. I think he had a normal Cambridge post as a fellow of Trinity and by that time a college lecturer and so on, very much the same as I was. Indeed, I was supposed to have been teaching mathematics. Fowler became the leader of applied mathematics in Cambridge, in close contact with the Cavendish. He, for example, did a good deal on the design of Aston’s spectrograph, which is all duly recorded, I think.

Heilbron:

So this really replaced nothing then. Your own mathematical training was really in pure mathematics?

Thomson:

No, it was in mathematics. The Cambridge mathematics were the Tripos. … Always did a lot of applied mathematics, about half and half.

Heilbron:

But without distinction among mathematicians?

Thomson:

Yes, you and to take the whole lot, you couldn’t take pure mathematics in Cambridge. I don’t think you can now, and a very good thing too, in my opinion, both for the pure mathematician and the applied.

Heilbron:

That makes things quite clear and reasonable. One does get a greater density of people one would like to call applied mathematicians after Rutherford’s coming — Darwin, Fowler, Hartree. …

Thomson:

One thing that I remember is a paper that [K. T.] Compton gave in a colloquium on ring electrons. It must have been between 1919 and 1922. This was not entirely I think Compton’s idea, he hadn’t invented it, I don’t think, but somebody had. I’ve forgotten what the ring electron did. This was in the days before Goudsmit and Uhlenbeck. The ring electron was really the electron with a magnetic moment. That was the point really. A good deal of the things you can do with magnetic moment, like having another degree of freedom you see. I don’t think it entirely convinced people. It did quite a lot.

Heilbron:

Do you know if he mentioned any extension to spectroscopic problems, because he does have a suggestion which was to explain some anomalous magnetic behavior.

Thomson:

I think the evidence was largely on spectroscopic grounds in Compton. There’s nothing written about that, I had only heard it discussed and I (???) think very much of it.

Heilbron:

Well there is a paper in which Compton proposes some sort of a spinning electron, but as I remember it didn’t make an application to spectroscopy.

Thomson:

Well I don’t remember for certain about this, it was a long time ago.

Heilbron:

How was, the Goudsmit-Uhlenbeck suggestion received in England?

Thomson:

This did remove one of the major objections to the original theory — there weren’t enough degrees of freedom — there were too many kinds of spectral lines. Roughly speaking you couldn’t explain the sodium doublet.

Heilbron:

Of course in the Goudsmit-Uhlenbeck scheme you can’t explain the relativistic fine structure either until one has the later Thomas’s readjustment.

Thomson:

I read an awful lot of that stuff and there was a time I could tell you about the Hamilton-Jacobi equation and all that kind of thing. The number of quantum theories I’ve learned in my time! I’ve never done anything with them.

Heilbron:

Would you say that the wave mechanics was perhaps the easiest of all to assimilate?

Thomson:

Yes.

Heilbron:

Born says, I guess in the obituary notice of your father, that your work went far to persuade him that the quantum theory wasn’t so reprehensible.

Thomson:

He was never “anti” quantum theory, he just didn’t like the form in which it was. He wanted to have it —. Well the form was pretty. He always wanted to make a mechanical explanation of these things. In a way it was mechanical, after all there were waves. Therefore it was, so to speak, respectable.

Heilbron:

Did he like de Broglie’s first attempt and Schrodinger’s notion that the wave was a real wave, the anti-Copenhagen interpretation?

Thomson:

Well, I don’t know, I wouldn’t care to say. I’m not sure how much he knew about Schrodinger really, as I say he wasn’t reading much. I don’t suppose he knew much about Schrodinger, indeed I don’t think I did myself until my experiments.

Heilbron:

What about your own views of the Copenhagen interpretation at first? The statistical business and the uncertainty.

Thomson:

I remember reading Heisenberg’s first paper on the uncertainty thing, with Ellis as a matter of fact. We happened to be in Cambridge at the time. I was at Aberdeen by then, but we met and we went into (???) and one of us said (???), and one of us read it over the other’s shoulder, so to speak.

Heilbron:

You were sympathetic to the Schrodinger and Einstein’s reluctance to accent the uncertainty, the complementarity.

Thomson:

Well I think complementarity a very fine thing. I’d really learned about most of those things from Darwin who came to stay with me when he’d just come from Copenhagen. We had long talks about all this, and really began to get an idea about it, I don’t remember just when; I think after I’d started doing work on diffractions.

Heilbron:

Yes, those ideas were worked out in 1927.

Thomson:

Yes, I got them pretty fresh. I dare say I didn’t read them but I got them by word of mouth with greater explanations, which helped a lot. Darwin was keen on them you see. I thought they were good. I never had the slightest objection to things being probabilistic rather than deterministic. I think that even now people don’t go far enough. I think they ought to start from the other end. I think that the remarkable fact is that some things in the world, starting with planets and so on, are reversed, are deterministic, in this sense, that you have in the past been able to predict correctly with a high degree of accuracy in certain results. This is what I mean by deterministic. … To a lot of people it’s quasi-religious. Then Newton came in, this was hailed as a remarkable instance of divine providence, divine law working on the planets, and this was highly satisfactory to religiously minded people. Now apparently religiously minded people on the whole would prefer it not to be all that deterministic, and the people who take a rationalist point of view say that the universe must at bottom be deterministic, but I can’t see either of these holding very strongly. They don’t move me at all. I’m very much interested in the things that Born was doing a few years ago. I’ve got some of his papers but I’m not sure I read them all, I’ll have to look into it more. He points out that if you have a gas and you really start to work to make it deterministic the thing becomes really quite impossible, not merely only because of the number of particles and the number of collisions, but because even such a very small error in your original observations results in the path of any one marked particle being indeterminate. And this lack of determinism does not depend on the magnitude of h.

This is my interpretation of Born. It’s easy to have a marked particle, it’s quite realistic. You have one radio-active atom, you see, and you allow this to bump about. … Perfectly good experiment, a little difficult, but this is genuine, not like one of these imaginary experiments which you really can’t do. You ought to be awfully careful about imaginary experiments. The quantum theory business was one of the examples of how frightfully careful you’ve got to be about thought experiments which you can’t really do. … [A discussion of how Born’s view of indeterminism might affect the stability of laminar flow or the ability to predict the place of origin of a cyclone is omitted.] Oh I think for practical purposes, in nature at any rate, determinism is the great exception. I’m interested in those questions now, I don’t know why, it’s the sort of thing you do when you get older. I’ve (never) been impressed by Einstein. I met Einstein (at) Princeton. I won’t say I argued with him, but I listened to views on why he couldn’t accept the probability view of quantum theory, and I wasn’t impressed. [An extended discussion of the paradox of reducing the wave packet by an observation is omitted.] You were asking questions about Rutherford’s first few years in the Cavendish. There were very exciting times there. I wasn’t in any way directly connected with this business about (transmutation) and so on. I remember him, talking about the chemists, he was always talking about the chemists. I remember one thing, it must be quite early on, when he didn’t quite believe the chemists. He kept on saying that the chemists told him that the nitrogen compounds he was using were devoid of hydrogen, but that didn’t seem to be the case because you’ve got these particles (knocked off) which must be hydrogen, really. Against his own conclusions he didn’t really trust these blasted chemists; he wasn’t sure. And he was convinced there was hydrogen there all the time.

Heilbron:

I guess he was suspicious. I remember that statement he made getting his Nobel prize that the greatest transmutation he ever made was himself.

Thomson:

It served him right I think.

Heilbron:

How were things at Aberdeen when you were there? Were there reasonable facilities?

Thomson:

No, there wasn’t. But I succeeded a very old man who stayed on during the war. He would have retired at the beginning of the war, he ought to have retired, really. In those days there was no compulsory retiring age. I came there and I never even saw him, in spite of the fact that his brother was, or had been, my (grand)father. William (Niven) was one time head of the College of Greenwich Naval College. He certainly was professor of mathematics there. I rather think he (??). I remember going to Greenwich; I must have been about four at the time; my father and mother were staying at (Niven’s) house. There was a little girl there whom I met in a bath — so you see I was quite small — and she was one of the daughters of this (Niven) in Aberdeen. Well I succeeded him, and never even saw him. Nothing had been done in the place, he’d rather prided himself on not spending any money at all during the war, and then by the time the war ended he was too old. So there was very little there, very little research too. There was only a staff of about four, three besides myself. As a matter of fact two of them were working on wireless, so research was not entirely dead. But I know (???) and there as very little in the way of apparatus. Fortunately they gave me what I considered a very generous grant of 1600 pounds to buy apparatus for the whole laboratory, which I spent. A large slice of it I spent on a device for making liquid air — you couldn’t get liquid air, you couldn’t buy it. Sending away for it would be difficult. This I used entirely for vacuum purposes. Charcoal vacuum. I think one of the most interesting things in the history since my father’s time, and even after, is the extent to which physics has depended on the method of getting a vacuum. Undoubtedly all that positive ray work really depended on — as it was done at any rate — Dewor’s discovery of charcoal as a means of getting a high vacuum.

Heilbron:

Yes, these are points one misses.

Thomson:

(???) my father’s apparatus for positive rays. But you had to get the positive rays by means of a discharge and then send them through a very fine tube to collimate them. You then have to receive them in a sort of camera, a photographic plate, after they pass through electric and magnetic fields. These positive rays very readily exchange charges, always what my father called “secondaries,” were due to the fact that the particles interchange their charges while actually in the deflecting fields. A lot of the things you got were really quite clearly due to things which had changed their charge half-way through — not in the magnetic field but before the region of the magnetic field. There were always quite a lot of- negatives; they all must have been accelerated as positives, and hence they must have had a double change of charge. The mean free path of these ions was rather short, or rather they had a large cross-section. The difficulty was that you couldn’t produce the positive rays unless you had a reasonable pressure of gas, you wouldn’t get a discharge to pass. On the other hand if you had a reasonable pressure of gas, they would change their electrical state, probably many times, in the region in which you are examining them. Hence it was essential that the pressure difference on the two sides of the fine tube, which was made as fine as possible for this reason, should be considerable. Now, with the pumps available at the time, this could not be done.

There was an early Gaede by that time, that is after the war. We had the early Gaede diffusion pumps. In the earlier days of the experiments, I think my father had a Gaede rotary which was a “slow” pump, and I doubt if any others would have done that. It had a speed of about three or four lasers which was really prodigious. He didn’t have this at the earlier stages, and his earlier photographs are really fuzzy; they are just good enough to show that you were getting definite parabolae, and therefore he just tried to show parabolae, very fuzzy parabolae. The technique was gradually improved, partly I think owing to Aston. Aston came half-way through this and worked with my father as his paid assistant, being paid with money my father got from the Royal Institution. In order to get anything like the sort of beautifully sharp parabolae which you’ve seen, you had to have the pressure on the camera side much lower than on the other. This was done by using this result of Dewar’s which my father got quite quickly. Of course it wouldn’t last for very long. … Using your Gaede pump you pumped down as low as you reasonably could. Then you heated up the charcoal, and kept it up; then you shut some taps so that the camera side was cut off from the discharge tube side except for the fine tube. Then you could run off a little gas into the discharge tube side and get it discharged. Then you put your liquid air on the charcoal as long as the capacity of the charcoal lasted. This gave already a very good vacuum. I don’t know really what that vacuum was. It was pretty good.

Heilbron:

How long would the charcoal last?

Thomson:

Perhaps an hour. Well, it depended on how much you had though. It would depend on whether it was a small leak. There usually were small leaks. Everything was done with wax of course. I don’t know, but unless you had a gross leak, it would last for an hour or two, and this was time enough to make an experiment.

Heilbron:

There is one other question. You wrote once that you were in the process of going through papers or had some you intended to look through at some point, correspondence or something, relative to this period.

Thomson:

In writing this book that I was speaking to you about, I looked through the collection of letters which they had at the Cavendish. These of course are not letters by my father, but to my father. They’re letters I gave to Cavendish.

Heilbron:

Your father didn’t have copies of his?

Thomson:

No. [amusement] Well, my sister was his secretary for a time, but —. My father was one of the most disorderly people in the world. And she just tried to order his papers and answer the things she could. In later years my father didn’t have much scientific correspondence. He rather disliked, especially in his later years, controversy. Except he conducted a colloquium, very much the normal type of colloquium with research students. One or two research students met every fortnight, if I remember right, would give a paper which might be their own work but quite often was not. It might be a topic suggested by my father, or an outside paper of interest. I’d say that about every other time there would be an outside paper and one of their own and this would be discussed. My father took part in some of the discussions. But he was not awfully keen on controversy. I think perhaps he wasn’t a very good arguer, I don’t know. He really didn’t like it very much. I think he arrived at his results rather intuitively and had a lot of opinions which he really couldn’t very well defend but on the whole, without being very dogmatic about it, was inclined to believe. Up to World War I he read a lot; he maintained his contacts by reading. He didn’t go to many meetings except more or less when he had to go; for example, the Royal Society, he went to those. He went to the British Association sometimes. He was president of the meeting in Winnipeg in 1909. He had been perhaps more in his younger days, but he didn’t go more than occasionally. The times that I remember he was asked for some special reason, to give a paper or a special lecture.

There weren’t of course nearly so many as there are now. But he did read, and he kept himself up-to-date. I remember his saying how different people were in this matter. He said Lord Kelvin depended entirely on talking to people, that he practically never read; he didn’t feel that he got the thing by reading so well as by talking to people. Other people, including himself, that is my father, got most of it from reading the thing. He liked to read it and think about it rather than absorbing it by talk. Also incidentally, although he read the foreign languages, at least French and German, the scientific ones, with great ease, he made no attempt to talk in any foreign language. Consequently conversations with foreigners depended on whether they were tolerably good at English. I think it’s true to say that in that period there was something seriously wrong with British science in that there were not nearly enough opportunities for people to meet in this way. There was the Royal Society, which perhaps gave more papers in those days than it does now. But it wasn’t terribly well attended, as indeed it is not now. It is noticeable — a great tragedy really — the number of people who are really of first rate intelligence who collapsed because they were too isolated. The two outstanding examples — but there were others — were Townsend and Barkla. Barkla with his J-radiation and Townsend with all this business about the quantum theory.

Heilbron:

It was quite early that Townsend retired then from further —.

Thomson:

He did. He didn’t get enough criticism; this was the real point. In those days you might have a colloquium but of course criticism of a professor’s ideas by the students; or even by the staff — the staff was not numerous in those days — was liable to be rather poor. Both he and Barkla at a comparatively early age, really ceased to promote science. Indeed you might almost say he retarded it, led people on to things which were not worth doing. This was perhaps less true of Townsend than of Barkla. Townsend’s pupils did do a considerable amount of work on this ionization by collision, … which after all do exist, which still have a value. If you want to consider what is happening in a discharge at the pressure of a few millimeters or something of that sort, well then these things are entirely relevant. So it wasn’t exactly wasted work, but still —. Then there was Barkla with his J-radiation, which was worse. And I think some of the other people —. Of course it’s natural enough; a lot of people cease to be productive at 35 who have done well as young men. But I think that this would have happened less if there had been more conferences more or less forced on people.

Heilbron:

At the Cavendish, besides this one colloquium you’ve just mentioned, were there others?

Thomson:

There was this colloquium, then there were meetings of the Cambridge Philosophical Society, which really was a colloquium. The Cambridge Philosophical Society deals with all sorts of sciences, so it really was a colloquium, a rather grander version of the colloquium, and they would meet in the Cavendish once a term or something of that sort. … It’s a society still in existence, and I don’t think it meets more than three or four times a term. It meets in the evenings and covers all sciences so that it doesn’t get to physics much. Once a term was about what it was. But it didn’t make very much difference; there really were much the same people, although rather more people came from outside.

Heilbron:

The colloquium which formed its nucleus met weekly?

Thomson:

Once a week I think. I’m not quite sure. You’ll probably find it in Rayleigh’s life of my father. But, looking back, there weren’t so many people in physics. Well I have a sort of feeling that a man like Schuster might have done more. And then there were the people like Clifton at Oxford who completely lost ties. He was a predecessor of Lindeman. There were two professors at Oxford, one was supposed to do a great deal with light and heat and the other was supposed to do with electricity and magnetism. Clifton was the light one. When Lindeman went to the laboratory, there was no electricity there, he said. There may have been electric light, I don’t know about that, I think there probably must have been. But Clifton was interested in light and he proceeded as follows. I think I got this from Lindeman, but I think I knew about it before more or less. He used to order stuff from Hilger or wherever. When the box arrived, he would go down in the evening when everyone had gone away, he would get this out, and would spend one or two evenings or whatever it was getting the thing to work, satisfying himself that it did what they said. Then he put it away in his box and there it stayed.

Heilbron:

Sort of a private bureau of standards.

Thomson:

I think this really was a quite serious fault. Maybe we have our faults nowadays, but I don’t think we’re in danger of having that sort of thing happening.

Heilbron:

Would the mathematicians attend these colloquiums at the Cavendish? I suppose Fowler would.

Thomson:

Yes Fowler would have gone, certainly. Rutherford kept on very much the same arrangement. It was done in very much the same fashion too. Mathematicians weren’t at all closely connected except like Fowler, Hartree and so on — they went. Not very many of the full college dons were mathematicians.