Herman Burger - Session I

Heilbron:

Well, I wonder if we can begin at the beginning with your early life and ho you became interested in science.

Burger:

Well, I can tell you something. I feel that I, personally, am not so important, but it gives you an idea of just what it was like, As for my family, there were, as far as I know, no physicists among my ancestors. They were rather people of art and technology. There was some interest in construction of machines and construction of little things, and that determined my life in part, but had nothing to do with what you want to know about me. Certainly there was a basis of interest in physical problems, and I learned a lot of physics from my father, who was in the Navy in his younger years as an engineer. So that in the early time. The first years of my life we lived in Utrecht and later in Zeist, a little town not far from here. There are some questions here in this outline you sent me which I can answer to some extent. But I don’t know how my interest in science was awakened; it was more a biological interest in the beginning, as a young boy. I lived in a little town among the woods and there was a garden with flowers, and this interest was primary in those years. But then, on the other hand, there were problems of magnetism. I remember that I asked my father, “How is it possible that you have a magnet here and a piece of iron there and nothing in between — how can one attract the other?” That was one of the problems that puzzled me when I was, say, 10 or 12 years old. I saw it happening, and my father, of course, he couldn’t answer me. Anyhow it gives you an idea of what was in my mind in those years. But then came the moment at the end of the high school when we had to decide what to study, and what was then the reason that I went to physics? That may be one of the things that you have interest in.

Well, I had some doubt about whether I would begin in medicine or in physics; both had me interested. These two interests determined my further work. Now interest is for both and for the combination of the two. And that I felt at that moment. There was some advantage to beginning the study of medicine because we had a very peculiar situation in those years here. There were two kinds of high schools, or rather, a high school and a gymnasium — a gymnasium with Greek and Latin and a high school without. Well, you could study medicine without the gymnasium, without Greek and Latin. So I was at the high school in those years, and I could study medicine without further preparation. Well, the (names) of the examinations were a little bit different in the two cases, but that was not serious. You could become a physician without any difficulties. But the study of physics required the gymnasium, and Greek and Latin. I didn’t like that, so that was the difficulty. There was a professor of physiology here, Zwaardemaker; I think you have never heard his name. He knew me because my brother studied medicine, and he advised me to study medicine; he was not very much interested in physics. But finally I decided, nevertheless, to study physics. But there was no prospect after that study other than becoming a teacher, and I didn’t want at all to become a teacher. Not that I had no interest in didactic problems, but a teacher had a class with many pupils and a very low salary, and so on.

Science attracted me more than this teaching alone. So a combination would be better. Yet the probability of perhaps 80 per cent or more of becoming a teacher didn’t prevent me from studying physics. But then I had first to study Greek and Latin, and I spent almost a year in studying those two languages; it was the most terrible year of my life. Because I knew that I could do nothing with it when I began to study physics; well, there are a few names that you can explain, but such a small fraction of what you must learn could be used that it is not worthwhile to speak about. I tried to do it in the shortest time possible. I had a good teacher and, I think, in 9 months or so I passed the examination in Greek and Latin. Then in 1912 the way was open to study physics, and I became a student at the University here in Utrecht. You see that I didn’t move so much; I was born in Utrecht, lived for some time in Utrecht and then in Zeist, about 6 or 7 miles from here, came back to Utrecht, and studied at the University here. Well, those were very happy years.

Heilbron:

What had you learned in the high school that was more helpful for studying physics? Did you learn much physics and mathematics?

Burger:

Well, we had physics. It was not so bad. We had mathematics; not calculus, but mathematics up to the “boundary” between the lower mathematics and calculus, as it was called then; we had a good deal. And I had teachers who were willing to help me. I had also an interest in chemistry in those later years at the school — 1910 and ‘11. I was helped by the teacher of chemistry; he gave me books and reprint and so on. There was still another point: there was a physician in our house. He was, for some time, studying in the laboratory of the physiologist Zwaardemaker. He was in our house for a year or so. Later he became a well-known physiologist in this country; (he’s a psychologist). He had a lot of books, and he allowed me to read his books. There were several books that made a great impression on me, they were not medical books; he had an interest in physical chemistry. There was the book of Nernst on physical chemistry, perhaps you know it; it’s a very thick book that I read in the last year of my school in 1910 or ‘11.

There was another book of (van Lear). He was a Dutchman, but he lived in Switzerland. He was a physical chemist, and he wrote a book on electrochemistry. It was rather the mathematical, and from that book I learned what an integral is. There was a formula with that sign which I didn’t know at that time, but then there was an answer. From that I understood that integration had something to do with an average, and that there was a factor which had to do with the interval. Anyhow, I was somewhat on the way to understanding what integration is. And I understood what thermodynamics is, not from a book of thermodynamics, but from this book. Then at the end of my school, I passed the final examination, and my grandmother wanted to get me a book. I bought the book of Planck, Thermodynamics, and that I read. And I studied thermodynamics on Sundays, and during the week Greek and Latin. So it was my life. There was then another book on calculus, an elementary book on calculus. Anyhow, I learned a lot of that before my study at the University. So when I came to the University, I knew what I had learned then at school the elementary mathematics and a lot of physics. It was elementary physics, of course, but a good basis. In that year I learned not only Greek and Latin, happily not only that, but also mathematics, physical chemistry and physics. There was a book of physics I don’t remember the author — designed for the high school.

It gave a lot more physics than necessary for the high school, and I saw that a year before my final examination. I visited a friend of mine in another part of the country, and he used that book in the school he was in, and so I learned things. What most impressed me then was, again, electro-chemical problems; for example, transportation of ions, and transport numbers, and that sort of thing. That made a. great impression on me; I don’t know why. Well, and then came the University. The University meant studying physics and mathematics; there was not a choice between the two with a tendency more in one or the other directions. There was also chemistry and mineralogy, and astronomy. That was all compulsory; you could not make a choice, Well, I’m happy I’ve learned a lot of all those things; it was quite right.

Heilbron:

How long did you pursue this course in which you learned everything? Was it for two or three years before you began to specialize?

Burger:

It was two years. Well, normally, it was three years; most students took three years for it. But I had lost that one year with Greek and Latin and so decided to do it in two years, and succeeded. I did my first University examination — that is what we call the (Commendance) examination after two years. It is more or less like the Bachelor examination, but I think a little bit less. There were all those different sciences, but with most emphasis on physics and mathematics. There was not so much experimental physics as I would have liked. In those times I had a little room upstairs in our home, and I worked there with very simple things. I made my own galvanometer by suspending a needle that was magnetized on a very thin fiber of silk, with a coil at the side, and so on. I had a friend who had interest in these things more for the construction than for the physical principles, and I had interest more for the physical principles than for the construction. We made an electro-motor and we made a galvanometer — it was all very primitive. We did a lot of chemistry, also. Before the final examination, even, we gave each other mixtures of different salts, for example, to analyze. So when I came to the University, there was not so much new in this — certainly not in chemistry. That was more or less a deception, [disappointment], but chemistry was almost the same as I had had in the high school. Physics, even, was also not so much — certainly not in the first year. The course of physics was designed for students who came from the gymnasium. And in those times there was no physics at all at the gymnasium. They only had mathematics and Greek and Latin and history.

Heilbron:

It was quite primitive?

Burger:

Yes, but you must think this was 50 years ago. Fifty years ago I came to the University, so that’s really long ago. But indeed I felt that it was rather primitive, but I couldn’t it alter this; I had to make the best of it. Therefore, I made some simple experiments in my house. But then, to some extent, the University was a [disappointment]. Not the mathematic; that was my greatest interest in those two years because it was so new for me. I had done some calculus for myself at home in that intermediate year of Greek and Latin, so I knew something, but I learned a lot more of the calculus. Anyhow then it was my duty to study it and not merely a Sunday recreation, and that made some difference. With physics my main interest was the experimental part because the lectures were not so much more than I had heard in the school. There were special topics in the second year, but, well, that didn’t appeal so much to me; I was less interested in the mathematics side than in the experimental physical side. The experiments in physics were very simple in the beginning, but we had fairly good instruments so that now I felt that I could begin to do real physics.

Also it was a disappointment that it was a regular course; in the first year you had to do an experiment in one afternoon and make a report. One week later you did the next experiment, and, of course, imagination couldn’t be used at all in that time. You had simply to do what was the instruction of these experiments. That was a disappointment too, because I had done experiments for myself quite freely and full of my own fancy. The second year it was a little bit better. Then came the examination, and then I had to choose to specialize. My choice was physics. But anyhow there was a lot of mathematics in those years. It was mathematics that is not so necessary for physics, but yet it was much more adapted to physics than mathematics is now. I didn't learn much about group theory, for example. There was a high school teacher, who was a privatdozent, and he like to give some mathematics for interested students. It was always a very small group. Our total first year group, with mathematics, physics, and astronomy all combined was seven. But this group was still smaller, perhaps in the beginning there were four or so, and at the end, in one of those two years, I was alone as the only student. But the lecture was proceeding, I learned about non-Euclidian geometry, for example, and group theory, and such subjects.

Heilbron:

You were taught group theory?

Burger:

Yes, yes. Yes, but that was in my third year, after the first examination. Yes. There was a little bit, the first principles, that was all. It was not a regular lecture; we didn’t have to know it for the final examination, what we called the doctoral. The lecture was quite free — one could follow it or not. It didn't help any with passing the examination, but it did help in getting broader view of mathematics. But the physics lectures in the second half of this study, in the third and fourth years, again, were not so much. There was a professor of physics here; he was the director of the laboratory. The laboratory was smaller than this, but was at this place. He had an interest in solar physics. That was his own interest, so he gave lectures on solar physics. So it was neither astronomy or astro-physics. Well, he also gave lectures on other subjects; for example, he gave lectures on the book of Planck, Warmestrahlung and this book made a great impression on (us). But he was an experimenter, and he didn’t know much about theoretical physics. He told what there was in this book, but he didn’t know more, I think, than what was in this book. Then there were the lectures of theoretical physics, and Ornstein. I must emphasize the importance of Ornstein in those days. Here he was the theoretical physicist. He came after Debye went to Zurich, I think, but I never knew Debye in those years. He left, I think, half a year or so before my last examination. Then Ornstein cane. I learned a lot from Ornstein, but it was more the classical physics, statistical physics, as he called it, and kinetic gas theory, and thermodynamics. But we didn't learn much about quantum problems; that was new, and was not his interest. He had worked with Lorentz, and was interested in other things.

Heilbron:

So he didn't discuss the quantum really at all?

Burger:

As far as I remember, very little in those times. Later, after my last examination, we talked a lot about those problems together. Then I became his assistant, and in this connection, of course, this was an interest of Ornstein, too, But in his lectures I think we heard only very little of quantum problems. I remember, in those days I was working experimentally with van Cittert. You know his name? He was a very good friend of mine; he died three years ago. He had a terrible life in the later years, lung cancer. I worked with van Cittert on optical problems. We worked with the echelon grating, simply to learn how the instrument works, but also with the idea that also the intensity, and not merely the position, of spectral lines means something. That became then the idea here in this paper.

Heilbron:

That was quite early; when did that work begin?

Burger:

Oh, perhaps already in 1916 or 1917. It mainly applied to spectral lines, but Ornstein worked on problems of light intensity with Moll. Moll was the experimenter; also a good friend of mine; I worked very much with him. Moll had constructed his apparatus to measure radiation. Ornstein, who became director of this laboratory in ‘19, had interest in those experimental matters, and his problem was what to do with those methods. So the possibility was primary, one had the experimental methods, and then came the question of what to do with it. So he was directed to those intensity problems.

Heilbron:

But there was already this interest in the intensity problems that you and van Cittert had…

Burger:

Moll had already made his thesis on the intensity of infrared lines in the spectra of the alkali metals. So that was in 1907, I think. And Julius, who was director of the institute before Ornstein, had an interest in those problems and made an (???) himself in 1900 or so. So it was a tradition in this laboratory to have interest in measurements of radiation. Gradually it was more and more directed from experimental matters to physical problems. That grew gradually during 20 years, or so, before it was entirely ready to do real measurements. I came in here during this development — I came in contact both with Moll and with Ornstein. They worked together in that time on liquid crystals. The magnetic field was acting on liquid crystals, and the effect was a change in transmittance of light. And one therefore came into intensity measurements. There was a light source and a layer of the liquid crystalline substance and a thermopile, and galvanometer and so forth. A magnetic field was put on and there was a change; it was put off and it took some time. You saw that the process was going, and they both worked, and I sometimes worked with them. So I was introduced into those problems of light measurement. It was a good means to know what were the shortcomings of this method, and that was the main point in those years. Well, now I must see that I don’t lose the line.

Heilbron:

When did you select your own thesis topic? When did you get involved in the crystals yourself?

Burger:

My final examination was in 1916, my doctoral; that was just before Christmas. Then came the moment when I had to choose, but then I fell ill, and I was at home for some months and had time to ponder about this. I think it was in the begriming of 1917, in January, that I came to Ornstein, and then I had three proposals for my thesis. Now, I must think about this because there were two others that I didn’t choose. One was the magnetic properties of atomic hydrogen. And the other was solid state physics (bending) of vibrations in solid state matter. The third was the dynamics of crystallization and solution. And I mentioned all three to Ornstein. He advised me to take the third one, and I have always been very happy that he gave me this good advice because I feel that both the other subjects would have been failures; they were too difficult. So I came to this problem. And, yes, why I did choose this problem? I think it was due partly to the influence of the book of Voigt you know, on crystal physics. By chance I had seen this book, and I read it, and it brought me into the direction of crystallography. Perhaps another point was that my first lecture in 1912 was a lecture on crystallography.

Heilbron:

By whom?

Burger:

(Wiersma)

Heilbron:

And this s probably before the von Laue experiment.

Burger:

It was 1912. There was nothing mentioned about it. He was a geologist — more a geographer. That was too far from physics. He gave a lecture on mineralogy and crystallography for physicists, chemists, and one or two people of his own direction, but there were not so many students in geology at that time, and even now there are only very few. So most people were from other fields of science. What we learned were the Miller indices and so on, and, well, we learned to know what system or what class it was when he showed us a block of wood of a certain shape. That’s what I learned. But, well, it was the first lecture — my very first — and I was very anxious to know what was in the University and what the subjects were. I think that had some influence, and the influence remained and was enhanced by the book of Voigt, so I came to this direction for my thesis. I saw that it was possible to apply some mathematics, and I was interested in the application of mathematics to experiments. And that I found in this subject because I saw there (a boundary problem with diffusion and so on) so that was after my last examination, and I had my time free. Almost all my time — say 90 per cent — was spent on work for my doctor's thesis.

Heilbron:

Do you recall when you heard about the quantum theory, Bohr's atom, and so on?

Burger:

I don't remember exactly when, but I remember one moment that van Cittert and I had the intention to measure the ratio of the intensities of the components of a mercury line — the green mercury line — the fine structure. And then there was a paper by a Japanese physicist, but I don’t know who it was, and van Cittert saw this paper. This physicist had the idea that it had something to do with the Bohr idea, and van Cittert told me that. He said, “Let us measure the intensity of those lines, and then perhaps it can be brought into connection with this idea of Bohr's.” And from him, I think, I heard of this idea first.

Heilbron:

What did you think of it? Do you recall what you thought of Bohr’s idea and the strangeness of the Bohr atom?

Burger:

Certainly it was a very strange idea, yes. The idea was, well, it may be true, but it may also be a fantasy. Bohr was not a man who stood with both feet on the floor as we call it. Yes, it was really with some skepticism that it was received; yes, that's certain. But it was also received with the idea that you cannot know whether there is some truth in it. So such a problem as the intensity of the fine structure components of the green mercury line was more attractive because the ideas of Bohr became known here. But the primary idea was to develop the method. And the secondary idea was that we also thought, “Well, you can't know whether it could be of some value to connect this intensity with those ideas of Bohr’s.” But it was — at that moment — rather far from what we did here. Now, you know I left the laboratory here in 1920. I think, that in those years between my promotion, my doctorate, in 1918 and when I left in 1920, I had an interest in quite different things. They were more the same line as my doctor’s thesis. And I measured the temperature at the boundary of a solidifying 1iquid. I had a good substance for it that could be supercooled… And I had in a little tube a supercooled liquid the point of fusion was, I think, 10 degrees, or so, and at room temperature it was quite stable.

You could bring in a little crystal at one side and you saw the boundary moving with a constant velocity. And that was the puzzle: what happens then at that boundary, that moving boundary? Certainly it had something to do with heat conduction in the solid and in the liquid. So I developed the mathematical theory of it. Heat conduction with a moving boundary was more or less related to the problem of (Stefan, “Die Forderung des Frostes”). Perhaps you know about this paper — it was a famous paper of the 19th century considering how the frost penetrates the soil. It was also a moving boundary, and I knew this, and therefore, well, I developed the mathematics. I made the measurements. The experimental method was possible because Moll had made very thin foils of two metals soldered together and then rolled out. So I had the experimental method from the side of Moll, and I had the interest in treating an experimental problem by mathematical methods — that was the influence of Ornstein a great deal. I had my own interest in crystallographic problems, and there I went. I did this work between the years '18 and '20. And I think at that time one set up that experiment on the green line of mercury, but that is more a secondary problem for me, and so those two years went by without our hearing much about Bohr's discovery and work. I got more of an impression from Planck; I don’t know why. Perhaps because his book, Warmestrahlung impressed me. I remember my sitting in the library, not at the lab, but the University Library, and reading Planck's papers (I think in Zeit. f. Phys.). And I was impressed with his ideas of phase space, and the cells in a phase space, and the meaning of “h” and so on. As for me, it came to me more by the Planck way than by the Bohr way; I don't know why.

Heilbron:

Well, Ornstein was perhaps interested in the statistical mechanical problems presented by Planck.

Burger:

Yes; and he worked in those years with Zernike; you know Zernike? He is still alive. And they worked on those little liquid crystals, but also on a lot of theoretical problems, but, as far as I know, not quantum problems. What did they do in those times? I remember a problem about porpoises in water. They had made a little trip together over the former Zuider Zee and they had seen the porpoises in the water behind a vessel, and that was the origin of the paper about flow in a liquid. I don’t know the special points, but you see, it was far from quantum problems. It was more classical problems. Also, they worked in relation to the liquid crystals. The direction of a molecule has some influence on the direction of another molecule; there is a correlation. And that is propagated indirectly from one molecule to the other, and that gives those oriented regions in a liquid crystal. That was a theoretical paper of theirs too.

Heilbron:

Our information has you teaching high school?

Burger:

That was, I think, in 1914. Yes, indeed. In 1914, I had just passed my first examination, and then the professor of mathematics came to our house and told me that a teacher in a high school in a little town in the neighborhood of Utrecht, in (???), had to go into military service because the war had then started. And he couldn't find another teacher. And he ask me whether I should like to be a teacher in that high school. You cannot exactly use this term high school you know. It’s more or less like the German Realschule. Well, I said, “Yes”. It was a financial necessity at that time. My father had died long ago, and it was very difficult financially. So I accepted this, also, because I thought that my only future would be to become a teacher. And I thought that beginning ear1ier then I would have some experience when my study was finished.

Heilbron:

Were you able to continue studies, or did preparing lectures, and so on, take most of your time then?

Burger:

No. I had only six lessons a week, then. And they were all the first lessons, so I went by train to [this little town] and then came back, missing only one lecture, and followed the rest of the lectures. So I studied and I taught at that high school and I also had private lessons. I taught pupils of high schools in Utrecht; I helped them in the higher classes of the school to prepare them to pass the final examination. So those were the three things I had to do. Well, it was possible; I was a young man. It could be done. No, I had not much to prepare. It was elementary mathematics that I had to teach; the beginnings of algebra and geometry. Well, perhaps I had had the organization to study more for those lessons, but I didn’t do so. I taught them those subjects as I had learned them in school, and I don't know whether they became better or worse by it. That was for one year. It was not very nice, but, well, it was possible, and it gave me some money; and I could study further. It didn’t cost me much time. Of course, the serious point is then to consider whether a later final examination is a financial loss. Is it more serious than what you gain by having a job? But still, I had taken this way, and it was possible to combine the things, so that I could pass my final examination after about two and one half years or less after my (Commendance), what we called the first examination.

Heilbron:

And were you Ornstein’s assistant after you obtained your degree? Was that your position?

Burger:

I must say that I don’t know when I became an assistant with Ornstein, Some time ago I had to send the papers to the curators of the University, and I couldn't find this paper. I don't know exactly what year it was. It may seem very strange, but, I think, was after my final examination after ‘16 and, I think, before '18. It must have been about '17, I think. Of course, it may also have been during ‘16 that I became his assistant. Perhaps it was a little bit earlier than my final examination. But then I had very much to do with him because I had not so much to do in those years between ‘18 and ‘20. I told that; I worked only for my thesis. And I had many talks with Ornstein; I learned a lot from him, and we together — I must tell you that — had interest in probability problems. One of my first ideas was to derive a differential equation for the chance that a particle suspended in a liquid showing Brownian movement, comes to another point. So now the particle is here — what is the probability at the time, that it is at a position with other coordinates. And you can say that this probability you can find from the probability that it comes here and the probability that, given this new position, it comes there. So now you can write down an integral equation for this unknown function. And this integral equation can be transcribed so that it gives a differential equation. And that is the equation of diffusion for equilibrium. That was one of my first papers, perhaps the very first one; I don’t remember exactly. This kind of work proceeded working together with Ornstein. We had an interest in Brownian movement, and I remember that in those years there came a new microscope, a Zeis microscope, and it was a very, very expansive instrument. It cost perhaps as much as 500 gilders or so, which was enormous for that time. And we looked through the instrument and saw all different kinds of particles moving there. I remember we saw a chain of particles of, I think, arsenic-sulfide. It was fixed at one of the cover glasses, and it was moving. And we thought of how to describe that quantitatively and theoretically. This gives you again the idea that it was all classical physics that had our interest.

Heilbron:

Where did the funds come from?

Burger:

Oh, it came from the state. In that time there was no other way. Now we have other possibilities,… but at that time there was no other possibility. There was an amount of money, and, when I came here as a student, the total amount for the whole laboratory was about the same as it is now for the telephone. Yes, but of course, the value of the money was different; you must multiply by a factor of 4 or so. But even then, well, that became still worse a few years later. But we try to follow a chronological line, and I think I can now tell you about the years ‘20 to ‘22.

Heilbron:

Your time at Phi1ips?

Burger:

Philips, yes. It was very difficult for me to decide to go to Philips. I liked very much working here. There were still plans for a rebuilding of this lab, and an enlargement, and so on. And I liked it very much; I liked Ornstein very much. Then I worked with Moll, and we formed a couple of people working together in good harmony. Then there was the problem of what to do. I had no prospects here. I could stay here as an assistant on a salary of, I think, 1800 gilders a year. If you say that in dollars, it is, of course, very astonishing, but it is not entirely correct. So I simply had to go. There were then, I think, six people in the Philips physical laboratory, two physicists, and a few engineers. They had seen my doctor’s thesis, and that was the basis of their request that I come. And I said, “Yes.”

Heilbron:

Were they interested in any particular application of that at the time?

Burger:

They were interested in what I had done because the physical properties of tungsten and the crystallization problems was one of their interests. On the other hand, there was the interest in quantum problems because they were busy with noble gases and currents through noble gases, and so on. And the man there who had this interest was the director Holst. I think you know his name, Holst. Well, there I began with the same line, the properties and crystallization of tungsten and the elastic properties of tungsten. I found a curious fact with single crystal wires. Then I had such a wire of tin which was a millimeter thick, or so, and then I felt that it yielded; and you could draw it out. You could draw it out to the fourfold of the length, almost without any resistance. And then it was flat; it was a kind of tape of tin. And I looked at it under the microscope, and I understood that the glide planes were in this direction. You could see the curved line on the surface of that tape under the microscope. That was one of my interests in those years. But Holst was interested in gases, and so on. And from him I heard a lot about quantum problems and Bohr’s work, and so on. Then the crystal problems brought me to analysis of crystals with Rontgen rays, and I worked about this problem of identifying oxides of tungsten.

Well, I remember I had a diamond as big as that. [demonstrates] It was fine for a demonstration. Once I had it so in my pocket on the train to Utrecht to an exhibition to show that experiment here, I didn’t feel quite so safe then. There was a man opposite me, and I thought, "Well, that man knows…" And then Hertz came, Gustav Hertz. Well, he was a very nice and fine man, and it was really very nice to have him there as a colleague during some time. From him I learned a lot about the excitation of gases. He made experiments, the extension of his experiments with Franck. Now whether I must call him “Frenk” or “Frahnk” — in that time, of course, he was called “Frahnk.” His mother was English, you know. He was a little bit older than Hertz, and he often came to Eindhoven. I met him several times. He was a good friend of Ornstein’s also, so I also met him here. And then I remember the moment that Holst came in my room and said, “Now they are entirely mad in the United States.” And that was the Compton effect. “Now you must see this paper," he said, “the collision of a quantum with an electron!” He thought they were entirely mad. He was the man of the practical physics and industry; he was an engineer. Well, he was a physicist too, but this went too far for him.

Heilbron:

What did you think? It’s very interesting because of your later work.

Burger:

Well, now that would come to those papers that you mentioned of the years '24, '25 and so on. That was later. But I don’t remember how the idea in these later papers occurred to me. I think it was through Ornstein; it was Ornstein’s idea. And I worked with him, and I thought it was very interesting, but it was not my initiative. So in those tines, I think, that nothing had any appeal in my mind other than that I heard this idea. I think I agreed with Holst 100 per cent.

Heilbron:

That it was mad.

Burger:

Well, mad is too much to say because now in those years we knew that (Bohr) wasn’t mad at all and that you could do a lot with those ideas. But, anyhow…

Heilbron:

Was Hertz there at the time?

Burger:

Not yet, no; I think it was just before Hertz came. But there was so much compressed into the yea and a half that I was there. There was the influence of Holst and Hertz. We went with Hertz to a Physikertag in (Jena). We were there and Hertz introduced us to several other people. Well, Holst knew those people perhaps, but I met a lot of people there. I knew we were there with Gerlach, I think. Now I don’t know exactly with whom, but he introduced us in the physical circles in Germany. There is a very important point to tell about that time. And that was the influence of Ehrenfest. Well, I had met Ehrenfest already here in Utrecht. He knew my doctor's thesis; I spoke with him for a very short time about it, but that was only for a short time. But then in Eindhoven he came regularly because Holst had connections with Leiden. He had studied first in Zurich and afterwards in Leiden, and therefore, he knew Ehrenfest quite well. And he invited Ehrenfest to come, I think every month for a colloquium. And Ehrenfest discussed with different people their problems, and Ehrenfest was the man who finally introduced the quantum ideas in my mind. That was his Person. He was a fantastically good teacher. I remember his fist talk; it was not a colloquium of physical people — it was not in the laboratory — but it was in a room in town. All different people in town came to hear about those new ideas.

They were not so extremely new at that moment, but, well, it was in ‘20. But Ehrenfest was a man who could tell the things so that everyone could understand what he meant. I remember a question after his lecture. Someone asked him, “How is it possible that the electron never combines with the positive nucleus? What would happen then?” And then Ehrenfest said, and he showed with his face how he fe1t, “Oh, but that would be terrible!”… Whether be had had a premonition of what would happen later, I don’t know, but that things like that could give a terrible effect, he realized. It was certain that he felt this. Now, this was only the first lecture, and he told about the Bohr model and so on quite simply. And then he came back every month. I think and then these colloquium were more scientific. Several people from the laboratory — we were 7 at the beginning and 10 when I left — gave a lecture and a chemist came there, too. And Ehrenfest had a very pronounced influence on the whole group. He gave us questions, and said now, “How is that?” I remember I asked him "Why, for example, is KOH basic and not acid?” And he said, “Well, you needn’t ask it of me; you can calculate that. You know the energy of binding of the different atoms, and now, simply calculate the energy necessary to break the molecule between K and O and between O and H, and tell me what you have found.” That was the way it went there. And it was entirely clear what the consequence of this was — that it wouldn’t break between K and O.

Heilbron:

But it was from these colloquia of Ehrenfest that you first learned, in any detail, about the Bohr theory?

Burger:

Yes.

Heilbron:

That was true of most of the others also at Philips? Except perhaps Holst?

Burger:

Yes, I think so, except Holst and Hertz, of course. And perhaps there were one or two others, but the main problems of the others were quite different. I remember, for example, the work of Cath; perhaps you have never heard his name. He was also one of the physicists there. And what he did was to pass current through a tungsten wire, and he measured the temperature of the wire as a function of time, and he calculated from this the specific heat of tungsten at those high temperatures. It s quite a practical, technical problem of interest for this industry. But on the whole we were left very free with our subjects. There was not so much directed research. Of course, there was some direction, but it was not so that we were forbidden to do this and that we were obliged to do another thing. There was a talk with Holst, and he had some suggestions, but to a certain degree, we were left free, and that was very nice for him to see it so broadly.

Heilbron:

Indeed. Had that been Philips’ policy all along?

Burger:

I think so, yes; I think it’s still about the same policy. Well, the number of physicists is now, I think, about 4 or 5 hundred — enormous. We worked then on the fourth floor, and there behind us was the factory with the machines going. So I remember one time I had put a thing on the table, and when I came back after a few hours, it was on the floor. There was a small inclination, and it went down and fell on the floor. For example, it was not possible to use a galvanometer. At that time a galvanometer was an important instrument. Otherwise, it must be done by electronic means, but that was too early for electronic means to be used as they are now. But I remember it was told that there was a room in the building and there on some evenings you could hear music that was made in the Hague. It was a very curious thing; that was all. It was the beginning of the radio industry, of course.

Heilbron:

How did you get back into the University again?

Burger:

This position of the laboratory, and the noise of the factory, and well, there were other things that were not so very nice. Perhaps there was a certain parsimony in this rich surrounding that was not so nice. And on the other hand, here in Utrecht I had been for so long a time. My family, my mother was here in Utrecht. So there were two reasons — the laboratory there amongst the industry was not so nice, and, on the other hand, I had my family here and I had friends here in this laboratory. And they were building a new part of this laboratory — where we are here — that was then a very old-fashioned loft, with the tiles where you could look through and see the sky, and so on. I knew they were building this; now and then I came to Utrecht, and I saw this, and it attracted me. So there was a repulsion and an attraction, and, therefore, I decided to go away. I told Holst, and he was very much upset. I told him why. I told him that the way of working there in some rooms amidst the industry was too bad for me. And then he said, “Well, I am very sorry that you will go, but we understand you. But please tell this Mr. (Fairer), as he was called then. There were two brothers Philips. The oldest one was the founder and he was the engineer; the younger one was the man of the economical problems; he died a few years ago and the older one died long ago. But then it was so snail that people who came and left went to Mr. (Fairer), as he was called. So I visited Mr. (Firer) and I told him that I was very sorry, but that I had to go and told him why. And he said, “Yes, I understand it quite well; I know it.” In those times now and then he came in the labs. Later on it was quite impossible, but then he came there. And he said, “I know it, but, well, it is so very expensive to build an extra building, but well we will see.” And a few years later the building was there, and I always feel a little bit proud that my leaving gave some impulse in a good direction. So I left, and I had learned a lot from Holst and Hertz and Ehrenfest about those quantum problems. I also learned a lot of technical problems, vacuum technique and so on.

Heilbron:

Do you recall any other episodes during Ehrenfest's visits?

Burger:

I remember something, but it has nothing to do with quantum mechanics again. It was the paper of, well, who was it? It was about the theory of the current between a hot cathode and an anode. It was very important for the problems in this industry. Langmuir had given a theory of this. And then there was a better theory. Richardson was the experimenter; he had found the effect. But the characteristic relation between current and voltage was not a proportionality, for the current is proportional to the [3 over 2] power of the voltage. And was explained by Langmuir, but he had neglected the thermal velocity of the emitted ions. And that was taken into account, I think, by Schrodinger. I remember that I was struck by this theory — I liked it, and spoke about it at the colloquium. But some time before another man had treated the same problem at the colloquium, and that was forgotten, and I had another view on it because I had learned so much from Ornstein about these problems. That was what I remember, but that had nothing to do with Ehrenfest. No, I’m very sorry, but I don't remember more about Ehrenfest than I have told you. I only remember that he came there, and he didn’t use the elevator to go to the fourth floor; he didn't like it and he went up all the stairs because he was afraid. But that’s not so very important to know that. But certainly Ehrenfest was a very nice man. Ehrenfest was a man who could listen to what others told him, the important things. But during such a colloquium, he listened, but he spoke too. He couldn’t listen for more than five minutes without speaking; after five minutes there came his interruption. And that was very usual; we all learned a lot in this way — by his interruptions.

Heilbron:

Was he aggressive about it?

Burger:

Oh, not at all. Oh, he was a very kind man. Oh, no, no he couldn’t be aggressive at all. I remember be had a little beard, and then one day be came and he was quite smooth. And he said that he felt quite different from the way he had felt with the beard. He felt so very high and that was not good; he had to have his beard again to be more humble. No, no, to be aggressive was not at all his character. He had a very difficult life… I know this from Ornstein; Ehrenfest did not speak with me about those problems. I was too young; he was a much older man. He was, I think, when he first came to Eindhoven, 38 or so. Compared to me he was very old One thing which made an impression on me which I must tell you about was the thesis of Kramers.

Heilbron:

On the spectral line? The Stark effect, and so on.

Burger:

And intensity of light, and it certainly influenced what happened here. I didn’t know Kramers at this time, but I saw his thesis, and also the thesis of Burgers on adiabatic invariance. I knew Burgers; I met him several times in our Dutch Physical Association, and I read his thesis. But it had no effect on my work. It was an interesting book to read and very curious to see how he could do this, but it was not that I would do something in that direction myself.

Heilbron:

When did you read Kramers’ Thesis? Do you remember? Was that after you had left Philips?

Burger:

Much later, much later; after I had left Philips, yes, yes, yes. Because then came quite a new development. Then I came here; they were building this part of this building, and Ornstein had become a director of the laboratory. Well, there was quite a new life here, and when this building was ready, then it was possible to start a lot of different experiments. And in those years came the idea of the measurement of spectral lines, and that was the beginning, as far as I was concerned, of the fine structure of the mercury lines. But then we understood that it was a much too complicated problem. Then in those years came the idea that there must be something question of intensity of spectral lines, and especially of the doublets of the alkali metals. Then came the work of Dorgelo. I don’t know exactly the year, but it was in the Twenties certainly.

Heilbron:

‘24.

Burger:

Dorgelo was my age, but as a student he was much younger because he had been a teacher in an elementary school. Then he studied here, and I met him, I think, at the end of his study; a little bit before or after his doctoral, his final examination. And then his plan was to work for his thesis. He worked and he worked, but he didn't succeed. His experimental possibilities then were not enough. It was too difficult for him; he had not much experience. Then I remember there came a day that van Cittert and I talked about his work, and we said, “This takes too long a time. It is too interesting a subject." And then came the impetus from the quantum side — "Now we must know how it is with this ratio.” "Is it really one to two for sodium, and how is it for the other alkalis? Is it one to two, three, four, five, and so on…" We were very anxious to know that. And we thought, "Now we must do something to promote this work."

Heilbron:

Had you any theoretical reasons for believing in integers?

Burger:

Not at all; it was all theology, as we called it. No, no. I did not meet Pauli before that. But I met Wentzel in ‘21, I think. And then Pauli was an assistant of Wentzel; and Wentzel told me about Pauli.

Heilbron:

What did he tell you?

Burger:

That he was so very active that he could not go with him in that (temperament), he said. Well, you know Wentzel died early. Well, he appreciated it very much, but it was a little bit too active for him. That is what I remember that he told me. Well, that was quite correct. You see how it has developed later, Van Cittert and I were just discussing Dorgelo’s thesis. And then we decided to do it ourselves. In one day we put up an arrangement to measure the intensity of spectral lines and measured the ratio, I think, for sodium. And then we called Dorgelo and said, “Here you have the arrangement, now go further. Measure it for other lines and improve this experimental method and so on.” And this was the stimulus for Dorgelo, I think, to do it himself, and he was able enough to do it. But the difficulties of the experimental method were too heavy for him in the beginning. He had to be helped to go over the threshold and find his way, and he found his way. Well, then came those rules, not only for the doublets, but for the higher lines. And in this time came then the connection with Sommerfeld. This was very important. I don’t know what was the beginning; perhaps it was the book of Sommerfeld, Atombau und Spectrallien.

Heilbron:

When did you start to read that book?

Burger:

I think almost immediately when I came back here, and, well, that made a great impression. I read that book and reread it, and discussed it with Ornstein. And by the means of this book we went further. I mean, we had interest; we knew something about the problems of analyzing multiplets, and we saw how the knowledge of the wave lengths, or wave numbers, could make it possible to analyze those multiplets. Then was the problem of how to do it now with the intensities, especially where you have more complicated cases than those simple alkali doublets. And the stimulus, for a greater part, came from Sommerfeld; he was here, I think, three times or so. And then for several days…

Heilbron:

After the measurements started?

Burger:

After the measurements started, yes. He knew of the measurements of Dorgelo.

Heilbron:

I think Ornstein had told him.

Burger:

Ornstein had told him, or he had seen papers before Dorgelo took his doctor’s. I think there was something published. And then we had talks in Ornstein’s house with Sommerfeld — Ornstein, Dorgelo, and me and perhaps others; I don’t remember. And that was of great importance for the further development because we saw that we were in a good direction and that this problem was a really worthwhile one. There were also contacts of Sommerfeld with Moll; this was very curious because Sommerfeld was the pure theorist and Moll was entirely an experimenter. But yet the two — they were of about the same age, Sommerfeld was a little bit older — they understood one another. Sommerfeld lived in the house of Moll for a week or so, and they became good friends. With Sommerfeld we made some trips in the neighborhood, and spoke about physics, and in this way we learned a lot more than from the book alone. We learned that there was something going, and we heard about the work of his pupils. There was Hönl. I think Hönl was here too. Well, Sommerfeld was a very nice man. Have you ever (met him?)

Heilbron:

No, no.

Burger:

He was also quite different, but he was a very nice man.

Heilbron:

When did you start to get interested in the more complicated multiplets, the complete doublets and triplets? And where did the idea of summing come from?

Burger:

Summing came from the idea to generalize that one to two ratio. Once we knew that it was one to two over the whole series, which was not true, but within the errors of measurement we thought at least that it was so, there came the idea of considering how it was in more complicated cases. I think this came rather early, ‘22, ‘23 or so, if I remember. We were curious to see how it was, and Dorgelo measured those more complicated cases. And you ask how did the idea of summing come about. It came; there it was. But you can't disentangle this and discover whose idea this was, and certainly not after almost forty years.

Heilbron:

But you and Dorgelo did publish the first paper?

Burger:

We published the first paper, and in this paper was the idea. It might have been his idea or mine. When I remember the whole situation, perhaps it might have been my idea, but his experiments. And in this way we published, but I’m not quite sure of this.

Heilbron:

But then Dorgelo took no further part.

Burger:

He went to Philips. I remember he came to me; he knew that I had been with Philips for some time, and he came to me and he said, “I have gotten a request from Philips to come there. I can have an appointment there, but I don't feel sure that this is my real place. Can I do the work there? Am I good enough in physics to accept this offer?” And I encouraged him, and I said, “Yes, you must do this; this is the way to develop your means, and to have a good milieu to work there; you can have all you want for your experiments, and you sit there amongst people who have interest in these problems, so I advise you to go." And he went, indeed, and he did a lot of good work there. Now and then he came here. We had a talk here now and then, but there was not so much contact in those years. He went his own way, and, of course, there was the influence of Holst. So there was not contact of any importance.

Then we went further here; experimentally the method was developed more and more with Moll. Then came the idea that we had the necessity to compare spectral lines of greatly different wave length. That was a very important point. You cannot restrict yourself to those narrow cases; it may be that we will find important things when the lines are further apart. Well, this idea was already developed n a very early state simply as an experimental problem; I think that was in ‘22 or so, or perhaps still earlier. The man who did that is now professor of radiology on the medical faculty here. He studied medicine later. When this was ready, we could make measurements of the ratio. Then came the idea to measure the ratio of lines going from one level to two different levels far apart. I don’t know whether I follow the line of history quite exactly, but I remember we wanted to compare two lines of hydrogen. I no longer remember which two lines. You can find it in the papers. But our idea was if quantum theory is far enough to describe the behavior of this simple hydrogen atom, it must be possible to calculate this ratio. And, indeed, it had been calculated by a Scandinavian, I don’t know the name. And we [???] I shall not tell all the experimental difficulties.

Heilbron:

Please do. Please do.

Burger:

Well, the difficulty was that to compare spectral lines of wave lengths very far apart we needed some standard. As a standard, normally, we used an incandescent lamp, but in this case an incandescent lamp needs a very constant current because the ratio of the intensities of wave lengths very far apart changes a lot if there is only a very slight change of current. Therefore, this was not a good means. Therefore, we developed another method; we used a helium lamp under standardized conditions, so that the ratio of two lines, or more lines, was within very small limits, constant. We could keep the current sufficiently constant so that it was a good standard, a relative standard, but anyhow much better than the incandescent lamp. The comparison of those intensities of lines very far apart would be done with a thermo-pile, since its sensitivity is independent of the wave length. Ornstein and I did that together. Say you have two lines and you have the ratio of deflections of the galvanometer without knowing the ratio in which the lines are attenuated. Then you could put in a filter which gives the attenuation of this line and that line in a known ratio.

If, with this filter in, you measure the ratio of the deflections again, then you can solve for the ratio of the intensities of the two lines. That was a method we developed, and once we had this standard line light source, we could compare this ratio with the ratio of lines in other spectra and so measure the ratio of lines wide apart. And in this way we did that for the hydrogen lines, I think one was infra-red and the other was in the visible spectrum. I think nobody had ever tried to measure the ratio of spectral lines so far apart, but it succeeded. But it didn't come into accordance with the theory. There was a great difference, and we thought it was not the fault of our experiment. Then I remember we had a talk about this with Pauli, about how it was possible. Is it possible that a ratio of the numbers of sub-levels for the same total quantum number in hydrogen is not the ratio of the statistical weights, but is different from that. And we discussed this with Pauli. I think this was his idea, and we were very glad that there was some explanation to explain this discrepancy. Then I remember that I expressed my feelings and said that we were so happy that he gave us an explanation, and we would say that in our paper. And then Pauli said, "Well, no, so far we can't go; don’t tell the whole world that Pauli has said that there is an explanation for this discrepancy. I only gave you a possibility, but I’m not sure that is the correct explanation." He was very prudent.

Heilbron:

Is that when you began to associate the Einstein coefficients with the theory?

Burger:

Yes; after that. Yes, certainly, because that was where we got the conviction that it was really an important problem — that it has something to do with the basis of quantum theory. Then later, I think, we measured also the ratio for helium lines far apart to be free from this difficulty of the different ratio of sub-levels…

Heilbron:

I wanted to ask you where you had heard about the Einstein explanation, or derivation, of Planck‘ s law, the introduction of the "A's" and the "B's" and so forth. How that got into the theory after your talk with Pauli.

Burger:

Yes. I know that we had a discussion with Einstein. You know, Einstein and Ornstein were good friends; they were both curators of the University in (Jerusalem); that was their connection. And now and then Einstein came to have a talk with Ornstein about these problems of that University. Of course, there was also a talk about physics. So I met Einstein, and it was, of course, very nice for so young a man as I was then, to meet a man of this importance. And he was quite willing to hear about the things we were doing. But I think this discussion was after we had seen his paper. We had seen his paper; we had discussed it, Ornstein and I, and we were very much impressed by it. Think of the work of Ornstein in former years, statistical mechanics, his interest for statistical problems in general, our common work on Brownian motion and statistical problems — also quite mathematical… This, all together, made it so that he had interest in this way of approach. I think that this way of approach said much more to us than the preceding ways of deriving Planck’s law; you saw how it happens. You saw how such an atom goes up and down and is in contact with the radiation; certainly we felt this as a very important step. Then, I think, we brought in a chemical process, too. Because we were influenced by the ideas of Perrin. You know those ideas of Perrin that in all chemical processes some radiation is concerned; the energy of a transition is Planck's constant times a frequency, and this frequency must come out in some way. I know that we spoke about the transformation of (saccharates) into (fructose) and (glucose), and then there must be an infrared radiation. But, anyhow, this idea brought us to the idea to bring into this Einstein treatment also a chemical reaction. I’m sorry that I have not looked up this paper… But I remember that we discussed this point with Einstein.

He had interest in this way of thinking, and I think he approved our ideas in general, and we felt, of course, quite happy that he agreed with us that this could be a way of describing a process in which we had a chemical reaction and radiation. Instead of considering two levels. of an atom we thought of a chemical transition in equilibrium with radiation. That was the Perrin idea combined with the Einstein idea… I think Ornstein and I published a paper on this. I have no bibliography or reprints, but you can look for a citation in that little memorial book on Ornstein. Well, I remember still one point, and I think that it is worthwhile to tell you. One day, and I don’t know the year, Heisenberg came here; as a young man he came to visit us here. It was in the summer, and we went by streetcar to Zeist one fine summer evening. And Heisenberg told about his new ideas; this was before publishing his first paper on quantum mechanics. Why he came here, I don’t know. But I think that it was Sommerfeld who said, “There is a young man, and I think that he will contribute to quantum theory appreciably, and I think it is nice if he could come to Utrecht to have a talk with you." I remember Heisenberg said, “Now I have a very strange idea.” And he told what it was. It was quite unorthodox, and we were very astonished by such an idea. “How is it possible that you deviate SO much from classical mechanics and have got ideas that are so mathematical and formal?" we asked him. And he said, "Well, you can explain it in this way: that I have never learned normal mechanics as you have in your preceding years, so I was quite free for new ideas…"

Heilbron:

What did Ornstein think?

Burger:

No; he believed nothing of it at all. No. I remember discussions about quantum mechanics between Ornstein and Kramers. You know Kramers came here as a theoretist. And they had many discussions. I remember that there was a lecture of Kramers. The Dutch Society for Science met once in two years, and there Kramers gave a lecture about quantum mechanics. Then Ornstein was very much upset. He said, “This goes too far. To speak about those ghost atoms!"

Heilbron:

Oh, of the Kramers Heisenberg dispersion theory?

Burger:

Yes, yes. And the conviction of Ornstein vas that there is a truth behind all this. And I think that he even gave this analogy which was later given by Kronig, that is, of a shadow of occurrences. And you see the shadow, but by knowing the shadow the whole thing is not determined. When you see two bodies approaching and colliding, you cannot predict how they will behave because with that shadow image there correspond many real configurations that give the shadow pattern. That was the way it was explained. And then Ornstein said, “Yes, but there is a truth about that, what occurs there. And so I think that it is so in real nature.” So he couldn’t believe quantum mechanics as an undetermined problem. I think he never believed it in his life. He thought there must be a complex of phenomena behind what we can perceive and when we once know what that is, then we can explain those shadows…

Heilbron:

How did you yourself feel, for instance?

Burger:

I certainly felt a little bit in between; of course that’s a very easy standpoint. You can say in between, and you can always switch to one side or the other, depending on how it goes later. But I felt for this idea of Ornstein’s; yes, yes, certainly. I had the idea, too, that in any way once there must come out a real truth of a quite determined physical process; it was quite wrong. I was just as wrong as Ornstein.

Heilbron:

In your article with Dorgelo, you begin by saying that these measurements might be of importance for atom models. Then you don’t say anything more about atomic models at all, and I was wondering if you could possibly recall what was in your mind when you wrote that.

Burger:

Perhaps I can explain a little bit of this to you when I tell you that one of the things I had to study for my examination was the book of Voigt on magneto and electro-optics. We had to study that for our examination — purely mechanical models, and I couldn’t set this aside. That remained, so I had the idea that a description in this way must be possible. I don’t know how, but —.

Heilbron:

But you had nothing specific in mind when you had written that then?

Burger:

No, no, certainly not. Well, I remember still one thing about what I had in mind. Before my doctoral, my final examination, I had much interest in radioactive processes. I don’t know why, but even as a boy I had tried to detect the radioactivity from the thorium in the Welsbach mantles. I had tried for myself in my little room upstairs to detect this radiation with an electrometer I had made myself, and so on. That made it so that I had an interest in radioactive processes. One of the problems I put to Ornstein, and I think that was about the year ‘16 — now we’ll go back in history — was whether it doesn’t make some sense that in radioactive processes helium is formed. Because helium is the substance which we know has the greatest entropy constant — the third law of thermodynamics is that there is an increase in entropy. I asked, “Have radioactive processes something to do with the third law of thermodynamics?”

Heilbron:

You had known of alpha ray emission, though, before?

Burger:

Yes, but the whole process had nothing to do with thermodynamics then, only with energy, I think. And that was what Ornstein said; in the expression E - TS, the E is so predominating that TS has nothing to do with it. Yet I think I didn’t believe him. And then I had to write what we called a description for my examination. We had to write something about a subject we cou1d choose ourselves; it was not for publication, not so deep, but something more or less like a publication. And mine was about radioactivity. I had chosen this myse1f; and they were all things I had read in the papers of Madame Curie and others of the French school. But I remember finally I had the idea of a model of the nucleus according to statistical mechanics. There was a point in the phase space moving around and approaching some dangerous region, and when it comes too near, then there is an explosion. That was my idea. I don’t remember what Ornstein said about it, but anyhow he thought it sufficient to have me pass my examination. But I had that combination of an interest in statistical mechanics and experimental things. But since then I’ve had no further interest in nuclear physics.

Heilbron:

But that was the origin of your interest in the models?

Burger:

Yes; this was an expression of this interest in models. Well, I liked to see the things so that you could apply fundamental physical laws on a model and see what comes out, and I tried to do that with a classical model and see what came out. I also remember the work of Niessen. He was a good friend of mine. When he was working on his thesis, the problem was — you remember perhaps — the binding of the two protons in the hydrogen molecule. It was in the early times, and nobody knew exactly how, but there was some idea, “Can you make a model applying the quantum ideas and Bohr’s ideas.” But what about those protons; I remember that the idea was that there is an average force: they are driven apart and driven together, and that then the average must be zero; it was a thing like this. But, also, the tendency was to have a model for the hydrogen molecule that is classical as far as possible. It may have been the influence of Planck because in Planck’s book, Warmestrahlung, you know he says very explicitly that he doesn’t want to go too far in the direction of the quantum hypothesis, but the idea is to stay as near as possible to classical theory and deviate only when it is absolutely necessary. That was also in my mind during many years.

Heilbron:

Where did Niessen get his degree?

Burger:

Here at Utrecht. He studied here; he got his degree here. His Promoter, as we call it, was Ornstein. Then he was Kramers’ assistant for several years. He worked with Sommerfeld for a year, I think. Sommerfeld also had an influence on his way of thinking. Finally he went to Eindhoven and Philips. He is now pensioned; he is still alive. He’s younger than I am.

Heilbron:

The connection with Munich was quite strong then?

Burger:

Yes.

Heilbron:

What did you people think of the work of Born, say? What did you think of what Born was trying to do at Gottingen?

Burger:

I think, on the whole, there was not so much interest in that work; but I, myself, was interested. I was not actively working in this direction, but was quite passively taking this up. And that, again, was connected with my interest for crystals. I read the book of Born, Dynamik der Kristallgitter. I was very much impressed, and I remember I gave a lecture for the student club. They invited me on an evening to give a lecture on a modern problem of physics, and I told them about crystals — the beginning of crystallography and von Laue, and then Born, and the way of explaining the compressibility of alkali-halogen combinations. I remember that that was a thing that impressed me very much, that you can give such a model with the eighth power of the interatomic distance, and so on. I never worked actively on this subject, but it was a thing that was in my mind and that interested me very much, but without my doing anything myself on this.

Heilbron:

Well, I want to ask you about your final paper with Ornstein on the sum rules, in which you arrive at a conclusion that apparently conflicts with the Correspondence Principle.

Burger:

Yes; I have thought about that, but now I don’t find the paper here.

Heilbron:

It’s paper No. 9 on the bibliography I sent you. I’ve got that with me, as a matter of fact. Here you make the statement.

Burger:

Yes; I have read it. I’m very sorry, but I don’t remember this.

Heilbron:

Well, the thing that interested me was not so much the details of how you arrived at the conclusion, but what you felt about that conclusion once you had arrived at it. You had put your finger on a very important difficulty. Was it comfortable to have arrived at such a conclusion?

Burger:

Yes, but I feel, at this moment, that I didn‘t think it was so important.

Heilbron:

You didn’t care about the Correspondence principle?

Burger:

Well, we were impressed by Kramer’s work, and I had the feeling that there must be something in it, but I think that at that time, as far as these problems were concerned, I was more interested in the effects than in the theoretical background. I knew that it was very important, but I saw my duty more in the way of finding effects than explaining them. And Ornstein was just the reverse. Although he was director of the laboratory, he remained to the last of his life the theoretical physicist. So I was always drawn to different sides — the side of Ornstein and the side of my own. Not that there was a real conflict; I think in this case I was more inclined to see all that work on multiplets as purely empirical. I had the idea then that there are theorists who can work on this problem and find an explanation, and my job was to build the bridge between purely experimental work and the theoretical world. Now those sum rules can be a first part of the bridge coming from the experimental side, giving the results in a shape that may be easier to grasp for the theoretical people.

Heilbron:

But did Ornstein feel a conflict here with Kramers?

Burger:

He felt this conflict, but he was the classical man in the first place, so there was a barrier for him there. And the second point was that at this time, his interest there was this laboratory. In addition, he was engaged in a lot of work about organization — not only here, but in physics in other lands. He was also interested in physics introduced in industry; that was one of the main points of Ornstein — to introduce physics in industry. When he came here, there was only Philips and well, even ten years afterwards it was almost only Philips. And it was his work to introduce physics into several other industries, and to see the importance. And it took a great part of his mind during those years — the 1920s and still more in the 1930s — his theoretical work was not his main problem. He was no longer the theorist, officially, and, therefore, he left it to others. Now, I think when we did some theoretical work, especially in the late Twenties and later, it was more that we both felt a support in the other, and that brought us, in this direction, together. But we were with our thoughts both in quite separate regions — he in his problems of organization, and I in experimental problems and later the medical direction. But the moment we came into contact with each other, we felt our interest in these problems and. that was enhanced by this interaction. I think that was the explanation.

Heilbron:

Well, do you remember how the theoreticians felt about this?

Burger:

No, no. That means Kramers?

Heilbron:

That means Kramers.

Burger:

We had not much contact with other theoreticians here.

Heilbron:

And there was Sommerfeld as well.

Burger:

Sommerfeld, yes, but that was — yes, yes. I don’t remember how they felt about it. No, I’m very sorry, but that was —.

Heilbron:

How did you get involved in extending these ideas to the Zeeman effect? You were about to mention that.

Burger:

That is easily told. That is because in the beginning of the measurement of intensity of spectral lines, in the beginning of the Twenties, we needed an object that was interesting. And spectroscopy was not so well equipped here in these years; it was later that we had good gratings and so on. Then it was very primitive here. Then Zeeman had a small plate — I see it still, it was as small as that (demonstrates) — with the two components of the sodium line. He gave it to Ornstein simply as an example, and. as an object on which to try and to see whether the resolving power of our method was enough to separate those lines. Perhaps, also, there was the idea to find, really, the ratio of those intensities, but there was no calibration, so we knew that that could not be done well enough. But our first point was, “How does this instrument work?” That was Moll’s instrument to measure the intensity as a function of wave lengths in such a complex of lines. One of our first objects of measuring intensity in spectral lines was this Zeeman effect. Then this was too (discerned) and we went to the multiplets and so on. But we always kept in mind that the Zeeman effect was an interesting problem, and we were curious about that ratio.

Heilbron:

So as soon as you had the opportunity you returned to it.

Burger:

Yes, yes. So it was the personal influence of Zeeman who was so near. And then Ornstein and I tried to photograph Zeeman effects, but the difficulty was that our grating was not as good as the gratings of other people; it was all very difficult financially, you know, in that time here. But we tried to find a ratio, and there came a better grating and better equipment, and so on, and then a doctoral student did the final measurements… Well, of course, then we tried to find the simple rules, but all as quite empirical rules, all in the idea of those theological methods, as Pauli called them. Really, we were purely theologists at that time, as far as these problems were concerned. Then there was another new idea — to combine a multiplet of the triplet system with a level of the single system and see what then comes out of the sum rules. I don’t remember whether we did it.

Heilbron:

Well, you certainly stirred up quite a subject with those papers. Everybody began to contribute after that.

Burger:

Yes, yes. Well, this feeling made it so that we then proceeded. We knew of the interest of important people, like Sommerfeld, and perhaps others, and then that was a support for us to go further in this line.

Heilbron:

Did Kronig come here at all to visit you during the early years?

Burger:

I don’t know exactly in what year Kronig came here, but it must have been in the 30s, I think. Yes, not in this time. I think I didn’t know him.

Heilbron:

And was there any contact with Russell who also came out with similar —?

Burger:

I have a vague remembrance of the name of Russell, but not more than that; I’m sorry, no, certainly not. There were some American contacts because Ornstein went to America and stayed there, for a few months. I remember the name Harrison…

Heilbron:

You’ve given me quite a good idea of how you reacted to Heisenberg’s work. How did you react to Schrödinger’s work here; do you remember?

Burger:

I think that’s too difficult for me.

Heilbron:

It’s too difficult? But was Ornstein interested in it? Or was he just too busy at this point?

Burger:

No, not much. He was much too busy in another direction. He saw it; I think he didn’t believe it. Of course, the formulas were correct, and so on, but he thought there must be a truth behind all this, and nature must be quite determined. That was his idea; that was his philosophy of life. And, therefore, he was not so willing to accept this way. Anyhow he was willing to do experiments and test the theory by these experiments, and therefore, he did measurement of the ratio of lines very far apart to see what the theory gives for it. So it was not entirely an unbelieving standpoint, but there was doubt. It goes so far that you can’t know whether there is something true in it — that was the standpoint. But it was worthwhile to do experiments to test it; that’s how he felt.