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Interview of Vivian Johnson by Paul Henriksen on 1981 July 13, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4694
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Born in Oregon 1912, entered Purdue University, 1932, studying solid state physics, teaching assistant work with Lothar Nordheim on crystal structure, 1937; Ph.D. thesis, 1937 (published 1940); physics department under Karl Lark-Horovitz grows in the 1930s, visiting lecturers (refugees from Germany and Europe: Lothar Nordheim, Hans Bethe, Edward Teller, Eugene Wigner). First cyclotron (homemade), 1935. War work: basic research in germanium, rectification of crystals (Bethe), close connections with Massachusetts Institute of Technology, Columbia University, University of Pennsylvania; Lark-Horovitz chose solid state physics as less sensitive field with respect to clearance; showed silicon-germanium intrinsic semiconductors, 1942; General Electric’s germanium interest; success interpreting resistivity and thermoelectric behavior in germanium, 1944. American Physical Society meeting intense interest in Purdue presentations, January 1946; the transistor, 1948 (William Shockley, Ralph Bray); how to grow germanium crystals, 1949; Esther Conwell’s thesis (Victor Weisskopf). Also prominently mentioned are: John Backus, Seymour Benzer, Hubert Maxwell James, A. A. Knowlton, K. W. Meissner, E. P. Miller, Ronald Smith, Herbert J. Yearian; and Purdue University Department of Physics.
We have already discussed a little bit about what we want to cover in the interview and so we will just start with a little bit of background first. You were born in Portland, Oregon.
On July 1, 1912.
That’s right. I had my high school education there and also went to college there at Reed College. It was a bachelor of arts degree in 1932. 1 started my graduate work in physics at Purdue University in the fall of 1932 and got the masters in 1934, the PhD in 1937.
Let’s go back just a little bit further. Tell me about your parents. What did they do for a living?
Well, my father had studied both law and engineering at Harvard University. He was a Massachusetts man, Worcester, Massachusetts was his original home, but he moved to Portland, Oregon because of the opportunity in a new city, one that was rapidly growing. He spent his time both in city service in the department of public works and also in private business in property development and so on. So he did make combined use of his training in the various fields. My mother was a native of Portland and, in fact, was born before the city was completely unified in its east and west portions. She taught public school there from roughly 1901 to 1908.
Was that the early grades then or high school?
She taught the early grades or elementary school. My parents were married in 1908 in Portland. My father continued in the type of thing I mentioned. He was a director for the state of Oregon of the manpower situation during the war and died in 1944 of a heart attack. My mother lived on to 1970, to the age of 87.
And you went to high school in Portland?
Yes, Washington High School.
Was that a specialized high school then?
No, they did not have specialization per se, but since they did have quite a large enrollment, a couple thousand or more. They divided major subjects like mathematics and English and so on according to academic ability; so we had fast sections as well as medium and slow ones. We got a lot of extra education that way in various fields.
Did you become interested in science at that time?
Well, I was particularly interested in mathematics at that time and I liked chemistry very well. I did not care for biology. Looking at the time, I decided that I was more interested in fields applying mathematics than in mathematics per se. So when I went to Reed, I was well aware that they had an unusually gifted professor of physics when it came to turning out majors who were very successful, A. A. Knowlton, who was one of the winners of the Oersted Medal from the American Association of Physics Teachers. So, because of this and interest in mathematics, I decided to major in physics and did stick with it.
Did your parents encourage you to pursue that degree or that field?
They left it up to me. It was perfectly all right with them. I don’t know that they would have tried to get me out of any field unless they thought it was one that was not sufficiently demanding.
Let’s go to Reed then. Was Reed a technical school or a usual college or university?
It was not a usual school because it turns out more PhDs per hundred undergraduates than any other school in the country.
Oh yes, it is one of the most highly rated ones, in the class with places like Oberlin, you probably know of them, so while it is liberal arts and sciences it is not an engineering school. It is fairly small, I think its current enrollment is around 1100 to 1200.
So you chose Reed mainly for the —
I chose it because I could afford it.
That was an important thing in those days. I went to Reed between 1928 and 1932.
Definitely, financial considerations were important then.
They were very important and then of course Reed had a good reputation, so there was no reason to try to scrabble around for funds to go to, say Oregon State.
So your degree was in physics then from Reed?
Well it was a Bachelor of Arts degree mainly because I qualified in two foreign languages and had courses in literature, history, philosophy. I could have had a bachelor of science but would have had to insist on it.
How did you go about learning physics at the time? What were some of the textbooks that you used? There weren’t a lot of them around.
Yes, I know what we used. In the freshman course, general physics, there was one written by Professor Knowlton. It was rather well received over the country because it had a rather new approach, giving more attention to the origin of ideas, brought in some of the new ideas and so on rather than being a cookbook of formulas. In my sophomore year we had mechanics. We used a book by a Canadian by the name of Campbell. It was a rather tough little mechanics book. Again, I think we used it because it made quite ample use of calculus, for instance. In electricity, we used a book, lets see, we used a couple of books, we had an easy one by Zeleny, (he was at the University of Minnesota) and then we used one of the Cambridge books, I believe it was Pidduck and our senior course in theoretical physics was by Page. Page was a man at Yale, I believe and that was a standard theoretical text for over a period of maybe 15 years. It was a rather general book and it made use of fairly advanced mathematics. In addition, we wrote theses — of course bachelors theses are a little on the simple side — but we had that too to cope with.
Did you have specific advisors at that time or a specific advisor?
There were two physics professors at the time I was there. There was a total enrollment of 450 in the college and there were two professors of physics, two of mathematics and we of course had ample opportunity to talk to any of them.
So you knew the whole department then?
Yes, it wasn’t a very formal counseling procedure.
Then, in 1932 you went to Purdue.
Right, I had a graduate assistantship here, so I could finance it.
Why did you choose Purdue? Was there something special about the school that you —
You know, 1932 was a tough time to get jobs, so you took what you could get, although Lark-Horovitz was in the position of building a department where there had been no graduate work and he knew Knowlton’s reputation for turning out well trained undergraduates who would succeed in graduate school, so he offered teaching assistantships to anyone Knowlton would send. He sent two of us. I was one and the other was a man by the name of John Backus who did not stay here very long but he did eventually complete his graduate work in physics and the last I heard he was professor of physics at the University of Southern California.
That’s interesting. I was wondering why you chose Purdue since it was such a long way from home. That is a good reason.
Well, in 1932, you didn’t have too big a choice. I did have another offer but this was much better than that with the caliber of the institution.
When did you decide on your field of specialization? Did you decide early to go into solid state?
I went into solid state before the field was recognized by that name.
That was another one of my questions here. I was wondering if they even called it solid state then.
I decided I was more interested in theoretical work than laboratory and the story is something like this. My masters thesis was done with Professor Akeley and it was essentially a story of applying Minkowskian mechanics to a problem of relativistic motion of electrons using some of the data that was relatively recent in 1933, let’s say, so that was not really solid state. Then we had, of course, rather limited personnel here at Purdue in those days, I think we had something like six or seven professors of physics. There weren’t too many actually in the research field at that time. So something like this happened, there was one year, 1934, I finished my masters, but there was a mathematics visiting professor here, a German who was interested in some problems having to do with mathematical expressions used in analyzing crystal structure and we just worked together and turned in a little paper that was printed in the Zeitschrift fur Kristallographie, it dates to 1937. It happened to be the first one I published and it really was just solid state. Then in 1935 a refugee from Germany by the name of Lothar Nordheim came here. Nordheim has had quite a distinguished career here in the United States. He spent quite a bit of time at Duke University and I think something like American Men of Science would give you something more on him, but this was his first appointment in the United States and he was here a matter of two to three years. He was a theoretician. Lark-Horovitz had started an x-ray study on zinc oxide by his students. Yearian wrote his PhD thesis on an analysis of zinc oxide by electron diffraction. A man who got his degree in 1939, Charles Ehrhardt, wrote on x-ray analysis of zinc oxide and what Yearian’s work had shown was the preferred orientation. Ideally, in zinc oxide the zinc would be at the center of a tetrahedron with the oxygens at the base and the one up above. Well the distance from the zinc to oxygen in one direction is greater than to the three in the base and the question was: could this be explained? Well this was the topic that Nordheim and I set about solving. We had a very distinguished consultant in that you might say. During the thirties —
[Shows her a paper] Was it that one perhaps? Was that part of the result?
Yes this is an earlier part and then a longer one came in 1940 sometime. Well anyway, Hans Bethe came here rather frequently; he was a visitor to give lectures. And he put in some suggestions about what he thought would be a viable approach. Well anyway we started work on this in 1935 and completed the work, I might just say off the record that the computers that we used in those days were simply the electric calculators, the Marchand and the Monroe which were excellent really, considering that you had to hand punch everything. They did work reasonably well at multiplication and division and of course addition but of course we had to devise techniques for solving differential equations you might say on an approach minimizing the errors step by step and so on. We solved all kinds of equations using numerical potential functions for the zinc atom that had been developed, the so called Thomas-Fermi potential, and so this is of course the technique of the mid 30s we were using and the paper you just showed me, the letter with Hubert James, is a modification I actually made working with James after I had finished the thesis and it looked like it had some bearing on the problem. Incidentally this is a paper that is referred to by Seitz in his classic The Modern Theory of Solids.
Let me just identify this. This is a letter to the editor in Physical Review 56, p. 119 on the Electron Distribution in Zinc Oxide Crystals, 1939. He does cite that. I saw it.
Yes, incidentally I’ve known Seitz for a long time.
Yes, you know when we did get solid state started right after the war, maybe 40 or 50 of us were talking to each other and then of course the field grew very rapidly. So that gets us through the work through the PhD level.
That was your thesis then?
That was my thesis and I’m sure you have the bibliographic quotation for the thesis which would be in a Physical Review article in 1940.
Was that the Effect of Valence Electrons on the Electron Cloud Distribution?
Yes that is the one. Now you may wonder why a 37 thesis was published in 1940. The reason was that while I was working on my thesis, Ehrhardt was doing this x-ray work and Lark-Horovitz wanted that paper, the work was finished in 1939 by Ehrhardt, but he wanted that experimental paper and my theoretical paper based on the thesis published together in the same issue of the Physical Review, so that is why it came out in 1940.
Yes, this is the Effect of Valence Electrons on Electron Cloud Distortion Upon Intensities in Electron and X-Ray Scattering, Physical Review 57, 613, (1940). So did you have one advisor for your PhD?
Well Nordheim was called the major professor yes, and he left Purdue pretty shortly after I finished up and then I did considerable work with Hubert James. First was a little extension of the thesis that led to the letter that we have quoted here a few minutes ago. Then we went ahead with another study that was not in solid state and this was a study of the H2+ ion. Hubert James had some ideas on that which I used very freely in the work. I actually believe I had to finish it up on my own because he had gone on to the Rad Lab at MIT before I had the work ready for publication.
That led to this paper, Correction for Nuclear Motion in H2+, Phys Rev 60, 373, (1941).
Yes that is right. And it wasn’t long after that that you might say everything became classified that we did.
I’d like to ask you a couple more questions about Purdue before we start on the war work. I’m trying to get a feel for what the department was like then. About how many graduate students were there lets say before 1940 let’s say mostly in the 30s. Did it grow?
I’d say that in 1932 when I came there must have been about 10 students. This would be a pretty good guess. The number decreased then for a little while. In 1937 there was a fairly substantial recovery from the Depression that was no longer as deep and widespread as it had been. That led to two things, first of all there was quite a group of people who finished their PhDs and went out and got industrial jobs. For example, H.T. Clark who went with Jones and Laughlin Steel and there was a man by the name of W.I. Caldwell. He went with Taylor Instrument, this sort of thing. But at the same time the University had money, so they took in several young PhDs as instructors which they did in the pre-war period and also a rather substantial number of new graduate students. So I would say in 1937 that the number of graduate students was more like 25. There were some who had come in in 1935 and 1936, there was always the flux of people who decided either they didn’t like it or couldn’t make the grade, it was a little tough for them, so we had a flow in and out quite a bit during the mid thirties. Then the numbers did not change drastically between the fall of 1937 and let’s say the 1940-1941 period. In 1940 we began to lose a few people because of the war, for instance some Canadians who went back home or over to Britain to help either in laboratories or in the military, and then by spring of 1941 we had people committed to the Rad Lab. Some of them left at the end of the Spring Semester in 1941.
Were there a lot of students here from other countries at that time?
Oh, not like there have been recently. We did have Canadians, but there were not as a rule too many from other countries. If you took the whole population of Purdue, undergraduates and everything there would be a scattering of people from China and Latin America and maybe European countries but it was quite a small fraction.
About how many faculty did the department have then? Was it on the order of the number of graduate students?
At what time?
During the late 30s.
During the late 30s I’d say maybe they got up to 15 or 18 something like that. Part of that increase had been in visiting lecturers, people like Nordheim who came, stayed a year or two and then went to some other place. Actually you probably would find that Dr. Tendam, who is the associate head of the department, has records which would show the personnel of the department year by year and I’m sure that he would be glad to talk to you about that.
Fine, for now I’m just trying to get a feel for the size of the department.
Well, between 1932 and 1940, probably we had increases of a factor of maybe two and a half, something like that.
Was there a very close interaction between the faculty and the graduate students?
Well of course there was at that time a given major professor with whom the students would have a great deal of contact. In this initial period Lark-Horovitz was a major professor for about half or more of the graduate students The only ones who were not were ones in theory and so we did not really have very many people who were interested in or were able to direct PhD work.
Was it very common for students to travel to meetings at that time?
Yes, but at their own expense. During that period before the war I probably went to maybe three meetings in Washington, but I think one time the University was able to supply 25 dollars to buy gasoline for a car that five of us rode in, but all the rest — it was just one of those things that you pay and of course you have to look at two things. First of all, expenses were low, we could go to a YMCA or a YWCA or something like that in Washington, get a hotel room for a dollar and a half or two dollars a night. You could buy anywhere in the country a darn good meal for 75 cents, that would be your big meal of the day. You could get lunches and breakfasts for 30 and 35 cents. On the other hand, we were being paid 700 dollars a year, no tax at least, so well we got by, but then during the war of course I was a bit surprised to find if you went to one of the meetings of the OSRD or the NDRC that you rode a pullman and had a per diem and a hotel paid and all the rest.
Quite a switch I’ll bet.
Oh, yes. Well I also went to meetings up in Chicago. The APS meets there, still does rather frequently. I recall they even had a meeting in Indianapolis one year, that I went to. So we went to meetings all right. We just had to figure out how to pay for it.
You had a lot of visitors to the department at that time, is that right?
Mainly due to Lark-Horovitz?
There were two reasons for it. First Lark-Horovitz wanted to give us a much broader view of what was going on in physics than we could get out of two or three people on his regular staff, that was part of it. We needed this kind of information. The other part is that there was really a flood of good physicists coming from Germany for reasons you know and these people didn’t have jobs when they got in and they had to get started you might say by going around and picking up honoraria as they presented material and of course they all got connections pretty well because of their ability. Bethe rather early obtained an assistant professorship at Cornell, Teller took a little longer, but a lot of these people are ones who later obtained the Nobel Prize. They were good people. Then there were the ones who just barely missed on it. Like the cosmic ray man, Marcel Schein who was at the University of Chicago so long.
How long would they stay when they came to visit? Would it be just to give a couple of talks?
It could be anywhere from a couple of days to maybe two or three weeks. Partly it would depend on how much money was in the budget around then. I think Oppenheimer when he was here was here about three weeks and gave maybe a dozen lectures in sequence and on the other hand Bethe would often stop here on his way somewhere to the West and maybe give a talk on Friday and Saturday something like that.
Which of the professors that visited stand out in your mind as the best? The ones that you found the most interesting.
Well one I think always has to mention Bethe, because he is such an effective and clear lecturer but Schein was another very good lecturer and Teller was. Then we had some people like Eugen Guth, the rubber man, he was a wonderful lecturer no doubt but you couldn’t understand a darn thing he said. His English never did develop very well. So we had people that we listened to but wouldn’t get much out of and then we had ones who were quite effective.
What were the research facilities like in the late 30s?
Experimental particularly? With the theory all you needed was a library and we weren’t — Well when Lark-Horovitz accepted the assignment from President Elliot to head up the department and develop it as a graduate department giving a graduate program, he insisted that we get in the library the periodicals we needed; English, German and American. So we had a good library and of course we calculated with these electric monsters. Now when you come to the experimental part, we started in having machinists, a glass blower and a shop, so that the equipment could be built right here. The initial equipment in electron diffraction and x-ray was constructed right here on the site because it was much cheaper than buying. And we had rather limited kinds of equipment. We had the x-ray lab, we had the electron diffraction. There was some work done on glasses for a while. This was later dropped, but in the mid thirties there were a couple of men working on the properties, one who carried it through to a PhD is Clarence Babcock, who is on your list, who spent his career in the research labs of the glass companies.
That was one of Lark-Horovitz’s favorite areas wasn’t it?
Yes, he had been working on that before he came to Purdue. So it was mainly in those fields that they did the early experimental work in X-rays. It was basically structure work which put us in a good position for studying solids later. And there were theses, for instance in the work of Danielson on the extent to which selenium retains structure as it melts. This was an experimental X-ray investigation. Then starting about 1935, we constructed the first Purdue cyclotron, again that was homemade and they got an operating cyclotron with an outlay of about 10,000 dollars. Mainly because the labor all came from graduate students and our own machinists and they even got the steel for the pole pieces and the yoke as a gift. The story is that a couple of our graduate students hitch-hiked to the offices of a couple of the steel companies and talked to the head man to get the gift so that we got it just for paying the freight and unloaded it on the Purdue siding and got together all the healthy men in the department to wrestle the, whatever it was, 47 tons of steel down in to the basement. The cyclotron was actually working sometime in the ‘37 or ‘38 year. Quite a few theses came out of that. So that was our introduction, you might say, into the nuclear aspect of things. Several of the people who came in 1937, did go in either as young post-docs directing work with the cyclotron or students who majored in nuclear physics.
Were there a lot of classes to choose from at Purdue at that time or did you take that many courses after the masters.
We had a reasonably good sequence in classical physics, the usual things you would expect, concentration on mechanics and electricity and magnetism and then we had courses in quantum mechanics. The quantum mechanics became a very good set of courses when Professor James came, I believe that was either 1934 or 1935 and he did give an excellent course and continued to do so for quite some time. We also had courses that Lark-Horovitz gave on a kind of an overall introduction to what they called modern physics in those days. There was a lot of emphasis on the experimental aspects.
One thing the project is interested in are the early textbooks that were used in the colleges and graduate schools at that time, Can you recall any of the ones that you thought were important in your background?
Well a lot of our work was done with books in the library. There were not all that many graduate texts in physics and we used Joos, you’ve heard of that Introduction to Theoretical Physics. It was a German bible but it had been translated into English. And it has just about all the classical physics in it. We also of course referred to books like Page and other books, Slater and Frank and, for instance, in mechanics Goldstein didn’t come out until about 1950 or 1951. Well we’d be as likely to use Joos as anything. We would work from professors’ notes that they had mimeographed and so on. In electricity and magnetism we did refer quite a bit to Jeans book; of course that is an old fashioned approach. He didn’t even use vector analysis. Everything is done in component form, but nevertheless there is much very sound material in it. So we weren’t really short of material. We had to know German in those days because we did have to refer quite a bit to articles in German periodicals or German texts, like Abraham and Becker. Before the days when it was translated we would use electricity and magnetism texts in German.
What were the big physics journals in German at that time?
Oh, Zeitschrift fur Physik, Annalen der Physik and Physicalische Zeitschrift, those were the three big ones, and then there was Zeitschrift fur Kristallographie that was more specialized. Of course, the British journals were the Philosophical Magazine, Proceedings of the Royal Society and Proceedings of the Physical Society of London.
Can you describe your first contact with Lark-Horovitz. Where and when did you first meet him?
Well, I’ll tell you what happened. He had hay fever very badly and in the days before air conditioning he had an arrangement whereby he would spend August and September up on a lake in Ontario, fairly well north of the border where there was no pollen to speak of. So he would return here about the first of October, so that is when I met him and actually wasn’t anything very formal about it. I think I first ran into him on the steps to the physics building.
What year was that?
Well it would be 1932. I think I saw him either on the first or second day that he got back from his trip to Canada. He owned a summer home up there on Sand Lake in Ontario.
It seems to me in looking back through the history of the department that it grew, would you say it grew rapidly, during the 30s?
No, not rapidly really. Say it did have an enormous change in 1937, but it hadn’t I suppose it is a matter of your base of reference as to whether you say it is rapid. The doubling does imply a big change, but nothing compared to the extreme growth right after the war was over.
Would that be mainly due to the cyclotron? Was that why it grew in 1937?
No it was really due to more money. The state was making more money on its income tax. It could afford to give more to the university, so they could hire more assistants.
Is it possible to pin point a little bit when solid state research began at Purdue?
Well, in an organized way as I say, because Lark- Horovitz was greatly interested in X-rays, and X-rays are a particularly good tool for studying solids, from the time he came there was some work in solid state physics. The more formal approach as you think of it now was just about January 1942. You know when Pearl Harbor was. Immediately, Lark-Horovitz contacted MIT, James was there already and was in a fairly high executive position, and said give us an assignment that this department can work on effectively and so it was only a matter of two or three weeks before we got some projects to work on and actually there are two things I should mention as starting initially. First there was a known difficulty with explaining the rectification of crystals of any type. There is a theory as to how the current forward and back should depend on temperature and voltage differential; it is an exponential where the exponent does not correlate very well with the observed one. Since it is an exponent that is off, there is a rather extreme difference between the expected forward current and the observed one. So quite a few of us did our first work on problems of rectification and in fact there is a paper with Yearian and Ron Smith and myself and also one: I think Yearian and Smith have a paper there about 1950 maybe.
Right. Yearian, H.J., D.C. Characteristics of Silicon and Germanium Point Contact Crystal Rectifiers. Part 1. Experimental. J. App. Phys. 21, 214-221 (1950). V.A. Johnson, R.N. Smith and H.J. Yearian, Part 2 The Multicontact Theory, p. 283.
A little bit about the background again, Bethe was interested in this right at this time in early 1942 before he got involved with the Manhattan Project and so on. Prior to this period there was a feeling when they talked about rectifiers, cuprous oxide was a big one but it was known that silicon would rectify. MIT split it this way; the University of Pennsylvania would look at the rectifying properties of silicon and study silicon as a semiconductor and so on. Germanium being the next element down in that column, Lark Horovitz was quite willing — and I think he also kind of suggested that we work on germanium — so the first thing that had to be done was to figure out how to get some germanium. Some of the early work in 1942 was concerned primarily with getting germanium and then going ahead and figuring out how to find out what its properties were; studying its rectification and so on. Now this all expands hut this is sort of where we started from.
Can we pinpoint exactly why he chose it? Is that possible to do?
Well if silicon is a rectifier germanium should be.
And silicon had already been worked on by other groups, right?
There was some earlier literature on it, but not much. For instance, at that period they had not even recognized the fact that if you make a semi-log plot of resistivity versus reciprocal temperature that you get a straight line at high temperatures, in other words the intrinsic behavior. All you have to do is read the English work by Wilson and Harding and a few more in the mid 30s to see how they were stumbling around on the edges of semiconductor knowhow.
But there was even less work on germanium then right? That hadn’t been touched too much at that time.
A man by the name of Bidwell in this country in 1923 wrote a paper on the electrical properties of germanium. He had a sample of germanium, of very questionable purity of course, but he published basically two graphs one of resistivity vs. temperature and the other thermoelectric power versus temperature and that is all. That is really all that was known. Of course there is some chemistry you know. And of course the work on germanium turned out to be a very fortunate choice because it was very fruitful although many of the biggest things came later when better and better material could be produced.
Let me go back just a little bit and ask you one thing about Lark-Horovitz. He was doing a lot of nuclear physics work in the late 1930s. Is that right?
That is right. He was working with the people dealing with the cyclotron and —
And in your book on Lark-Horovitz in the Men of Physics series [Johnson, V.A., Karl Lark-Horovitz (Oxford: Pergamon Press, 1969)] you mention he was even doing some fission work.
Let’s see, he was doing some work with radioactive tracers. He was well aware of those early papers on fission.
Right at the top there on page 20. Did he take that [fission study] very far? It seems that if he was doing a little fission work right before the war, it is sort of interesting that he stopped that and went into solid state rather than work on fission.
There were people from this department who went into the fission work on the Manhattan project, quite a few, one of the best known is of course Raemer Schreiber, who I think just before he retired was director of the lab out there at Los Alamos, but remember Lark-Horovitz was of Austrian citizenship originally and I think for this reason he thought that the solid state work was a little less sensitive and there was no problem in his getting the clearance to work with that where there may have been a problem with the fission work.
Did he have problems with clearances later, even in solid state?
Not later, I think once he got it the question was really not raised. Fortunately no issue was made about it during the McCarthy period in the 50s. There were a couple of people here who they ran extra checks on because of something suspicious; like one man was born in Russia and left when he was three or four years old and so on. Actually I think Lark-Horovitz had no real problems so that was a matter… Well I think they figured Austrians had about as much against the Germans as we did. It may interest you, before Los Alamos was finished you know the laboratories there, the work was being done all over the country. We had a unit here that used the Purdue cyclotron and it was used only for the Manhattan Project and we had 24 hour armed guards out there to keep anyone out of there who was not part of the Manhattan group. There were I think four PhD level people and four or five graduate students and the whole unit went to Los Alamos later but they were working here on measurement of neutron cross sections for scattering and this sort of thing. So I believe it was sometime toward the end of 1943 probably that they moved out there. One of the graduate students who was either the first or one of the first killed in an accident at Los Alamos in a radiation accident was Harry Daghlian.
Let’s see. The next part I want to go into is the summary of the History of Germanium Development at Purdue by Lark-Horovitz. I do have just one question though, was that the entire piece that you have reprinted in there or is there more to that?
It is in a sort of an outline form, isn’t it. I have my copy back here. This particular piece is pretty much the way it was written up except that I think I tried to make it into complete sentences and so on, but —
So he basically wrote an outline to start with?
This is right. This was part of a report to trustees of Purdue or the President of just what we had accomplished.
We are continuing in the afternoon on the same day and we were just about to discuss a few things from Vivian Johnson’s book here on Karl Lark-Horovitz. This is the History of Germanium Development at Purdue Section. And that is the complete piece there?
This is not abridged from something else except… The part starting here is not abridged.
This is the last paragraph on page 32.
The preceding two paragraphs are quotations from the… We don’t call it a memorial service, but that was what it amounted to. Do you have a copy of all of the things that were said at that?
No, I don’t.
Well, remind me and we’ll see what we can do on that.
That would be great to get a copy of that. There are some specific points that I would like to talk about there. Under the January 1942 part it says “that because of my experience in this field,” this is referring to Dr. Lark-Horovitz talking about crystal rectifiers, I guess, or semiconductors in general perhaps. Do you know what Lark-Horovitz had done with crystal rectifiers or any semiconductors prior to 1942. I was looking through the list of publications —
Of his publications I guess there is not anything.
Didn’t seem to have anything at all.
No, I think he was referring to another thing. Originally he was trained as a chemist and not as a physicist and therefore he felt that he was well equipped to look for properties found in one element in another element in the periodic table in the same column or row, in other words work like a chemist in looking for properties of one element in another one.
So it was mainly due to his background rather than anything specific.
Background in chemistry.
One of the things I am trying to track down is exactly why Purdue got the research work to do in the first place, on germanium. Was it because of Lark-Horovitz’s reputation or the department’s reputation as a whole or is there even a reason?
Well I am sure in that time any place that had a concentration of physicists would have been gladly welcomed to do something. There weren’t that many physicists in the whole United States after all. But it is true that something was arranged rather quickly because of Hubert James being already in place in the Rad Lab and in a position of some authority. When you see him you might bring that up.
Yes, I plan to talk to him a lot about the OSRD and the NDRC and all those things.
I was very surprised to learn that he had been with the NDRC during the war, I knew he had been part of the Purdue physics department before and after but I was quite happy to see that he had been there.
Well I think they called it OSRD first and then the NDRC Section 14, is the one that we were in, Crystal Rectifiers.
Then in the Jan.-Feb. 1942 reference, I think I’ve already asked you this question. I was going to ask if you think Lark-Horovitz chose germanium because he felt silicon had already been sufficiently developed by the Germans or did he simply think germanium was better?
I think part of it was first of all it was a new field. It was an open field. Secondly, it may have been part in deference to the University of Pennsylvania which I think had a preference for working in silicon.
Did Lark-Horovitz mainly like to do basic research rather than applied?
Did that pervade everything he did?
It does with most physicists. On the other hand during a war he was hoping that basic discoveries would be in line with future developments. Also of course one would have to admit that many of the things found out by our group during World War II were put to real application later on. But that is always going to be true.
In May of 1942, it says “after an MIT meeting, H.Q. North of General Electric asked Lark-Horovitz’s permission to work on germanium at G.E.” Why did he have to ask permission?
Well, I think at that time considering there were rather few physicists in laboratories, they did not want too much duplication and so it was a case of establishing that North, and whoever was working with him would not duplicate the work and that was about it.
Did your group help G.E. do any of that or get them started or let them know what you were doing so they didn’t do it?
There were relatively frequent meetings either some were at MIT, but quite a few were held at Columbia University in New York where everything said was of course secret, but representatives of the contractors in as I was saying Division 14 would get together and exchange information. The point along this line… Of course we were concerned with radar and as you may know they started using radar equipment designed for wavelengths of ten centimeters and then they went to three and then to one. These numbers were never mentioned in print. They were represented by symbols like X, S, and K. K band is one centimeter. This information could not be transferred say from MIT to us via the mail. It had to be done by personal contact. So someone here would go to MIT and find out what a code word like K stood for.
At any of these meetings, I suppose since they were secret, no minutes or records were kept of them.
I don’t know. The answer to that would lie with whether the chairman of the meeting who would be selected by OSRD or NDRC had instructions to keep records. The man that I know of that presided is Torrey.
Oh, he was one of the editors of the Crystal Rectifier book. [Torrey, H.C. and C.A. Whitmer, Crystal Rectifiers (New York: McGraw Hill Book Co., 1948)]
That is right. That’s the one. And he was the chairman. You might want to contact him if he is still living and available. He would know whether he kept and filed any records.
I don’t know if he is still alive or not.
At most it might have been just the programs of the topics that were presented and about the length of them.
Were there different chairman at each meeting?
I don’t think so. I won’t swear to it, but I think the same person would keep on for continuity.
Was it usually a representative of MIT?
I think most of the organizers, the chairmen were from the MIT staff.
So the people were scientists that chaired these meetings rather than representatives of the government.
They were scientists, yes. I can not answer for all the various divisions, because of course I was only concerned with Division 14.
In August, 1942 at the meeting at Columbia it says Lark-Horovitz announced silicon and germanium to be intrinsic semiconductors. Were you at the meeting?
I was not at that meeting but I know what he was doing. He was able to show by plotting resistivity on a semilog plot versus reciprocal temperature that the straight line exponential was obtainable with both of them and that they had a definite characteristic energy gap and that this would be true of a wide range of samples. I think by this time for instance we had, oh let’s say a family of four germanium samples doped with aluminum in various amounts. They would be widely different at room temperature. As you extended the temperature of the sample continuing to measure resistivity and or Hall effect it would come into a common family curve, one for all germanium and the other for all silicon samples. So that is what he meant and then it refers to the fact that people at the University of Pennsylvania had information on silicon whereas our group had it on germanium.
Did they come up with the idea that silicon was intrinsic? Or was that Lark-Horovitz doing both?
I think Lark-Horovitz He made a point of it but the actual proof was from Pennsylvania and I’m sure they understood what they had. But previous to this time everything in the literature was very limited in amount and no attempt to compare one sample to another to show an intrinsic behavior either on a…
So he mainly pulled everything together and then made the final step to get the final idea.
Were these meetings generally attended by all the people working in Division 14 then?
No, but representatives of all the different groups who had contracts. And how many? I think there was some conscious attempt to hold down the numbers, not just cost, but when people went to the meetings they were pulled off the work. We frequently had two or three people present what maybe six people had been doing.
Was there a lot of interaction between the different groups other than at these meetings? Did you ever really talk much back and forth?
You mean of the people working on crystal rectifiers?
Yes. Like the people working at Penn or G.E. or Bell?
In some cases arrangements were made for perhaps a person from here such as Ron Smith to go to Sperry Gyroscope and look at some of the experimental set ups they were working on and of course there would be conversation on the state of the art. Then of course we were putting out confidential reports that were mailed throughout to the members of that particular section.
I was just trying to figure out whether there was much informal discussion between the groups. But you said people did travel back and forth. Did anyone ever travel to Purdue?
Oh, yes. There were people here fairly regularly from Pennsylvania or General Electric and so on. You see of course you had to be a little careful. You could only talk to someone that you knew had a right to be told the information. This is why for instance, the first meeting that I went to, Professor Torrey had been out here to meet all the people who would be coming in the next few months or so to meetings so that he could identify them on sight, not relying on badges or passes.
Did the secrecy or just the fact that it had to be confidential during the War hamper you at all in the research?
Not really, of course you aren’t looking for things to stop you when it is important and we were working in a field that was basically a life and death situation, so you didn’t waste your time criticizing the government and so on.
Just did what you…
They didn’t hassle people like they do nowadays either.
In September of 1942 it mentions the germanium group. Now were you in the group right from the beginning?
Lets see, must have been just about, I think, the only difference in time was how rapidly different people had their clearances come through.
So basically it was early 1942?
I think it would have been sometime during the spring or late spring that we were started. After all it was roughly the first of January when we were getting started. Oh, the clearance business took a little time and so on.
What was your actual role in the group? Did you have specific assigned duties or was it just mainly an area that you worked in?
Basically what I was doing was trying to reconcile experimental results with theory. There was a limited amount of semiconductor theory and of course we had to develop some for ourselves as we got beyond what had already been worked out. We had more of an experimental information, and so on. One of the things we spent a great deal of time on was reconciling the experimental behavior of doped samples, that is ones that were impure, with the theory of the scattering of conduction electrons by impurity ions. And that overlapped some work done… There was some work done on that by Victor Weisskopf and his student Esther Conwell. I am sure you will run into her work in solid state.
I came across a reference to her paper that seems to be cited quite often in the papers from the fifties that I have of yours. Was that delivered at that same meeting after the War in 1946?
Yes that is when it was first published in abstract form. And it was actually her masters thesis along about 1943 from the University of Rochester. This was sort of an awkward situation in which neither Weisskopf or Conwell had the clearance to work in this field and they were just solving the hypothetical problem. If you have ionized impurities in a regular lattice what are they going to do to the resistivity and the modified Rutherford scattering, basic idea of Rutherford Scattering, to get usable results? Another thing that I spent a good bit of time on was the theory of thermoelectric power of semiconductors.
Did you do much work with measuring Hall coefficients?
There was a great deal of that. It was done regularly on all of our samples. Whenever possible they ran all three electrical properties, Hall coefficient, resistivity and thermoelectric power. Of course at this time our lowest temperature would be liquid air temperature or liquid oxygen. In later times, why the introduction, the availability of liquid helium they could go down about as low as they wanted to.
Did Purdue have one of the first Collins machines, liquid helium liquifiers after the war?
I think we had ours sometime in 1948 and primarily we got off to a good start because Pieter Keesom came here from Holland. His father was one of the first to liquefy helium and work with it over at Leyden. I guess actually Kammerlingh Onnes did the liquefaction, but Hendrick Keesom was one of the first to study properties of helium in the liquid phase and other related problems.
One of the things I want to try to figure Out is who was in the group specifically. I’ve got a few names here. Would you be able to tell me if these people were in the group or not?
Well of course Lark-Horovitz was here and you are correct, Whaley’s main job was purification and production and this involved finding out where to get germanium. At that time the best source was the Eagle Picher Company of Joplin, Missouri because germanium could be separated by them fairly easily from the lead, zinc and so on alloys they worked with. Of course at this period all of the germanium was polycrystalline. It was in the period of 1949 to 1950 before any single crystal germanium was grown. All right, now Sachs left here very early in the period. About all that he did was a little bit on this rectification theory material in 1942 and before the end of that year he had gone on I believe to Wisconsin at that time. Now Benzer came in just about the time we started the project in 1942 as a graduate student and he did all of his work in the solid state laboratory and was very much involved with the high back voltage rectifiers. E.P. Miller was an experienced physicist. He got his PhD here in 1936 and he and Walerstein were the first to make measurements on the germanium resistivity and Hall coefficient. Miller left however as of July 1, 1943 so he wasn’t with us very long. On the other hand Walerstein continued here until he retired back in 1971. Middleton came into the group a little later but he stayed through until July or August of 1945 right at the end of the War when he, you might say, got the permission of his draft board to go on over to Batelle Labs in Columbus. Scanlon was here and had been working in spectroscopy and then came in to working with the group and he did a lot of measurements particularly of the thermoelectric power. His presence in the group probably extended to something about like 1948 or so. Yearian was here the whole time. Smith, I think was here until just about the end of 1945 or very early 1946 when he left Purdue to go to Boeing. Yearian and Smith because they were both rather experienced people had their hands in a little bit of everything. They started in on the DC characteristics of crystal rectifiers and anything pertaining to that that came up they kept along with. They also dabbled in a little bit of everything else, measuring properties or working with the younger people who were trying to. K.W. Meissner was a spectroscopist. (He has been dead twenty odd years now.) And his involvement would just come up when they had some question on optical properties and this came up much more after the end of the war than during it. He probably made a few runs on infrared reflectivity of germanium perhaps earlier.
Would that be the Meissner of the Meissner Effect?
No that is Willy Meissner. Let’s see I guess I gave you that list with the addresses. I might be able to check that. You have Benzer. Boyarsky was here during the war. He was a brand new graduate student. Actually Boyarsky, A.W. McDonald and Paul Pickar were junior students that is junior personnel, graduate students who worked under the direction primarily of Yearian and Smith in making measurements. Now Ralph Bray, you don’t have him down on this list and he was really involved in this pretty much from the beginning, but he also started as a graduate student. Bray and Benzer came about the same time and they probably got here just about the start of 1942 and did their graduate work in solid state laboratory. I think most of the rest of these now joined either at the end of the war or even later, or were not here. C.N. Parshall I notice you have him down here. He left before this work started. I think he left about the summer of 1941, but he was one of these people who did an X-ray study as a PhD thesis under Lark-Horovitz so he was in solid state without having the name at the time. E.M. Purcell was a Purdue graduate but he was at Harvard as far as I know through the period. As you know he is a Nobel Laureate. Scanlon you have on your list. Julian Schwinger was here for a while at the start of the war and he did some very interesting and unique work on the diffraction of microwaves. Then he went on to Harvard sometime in 1943 for the duration of the war. Well I think that covers it now.
That is excellent. Another reference is to the summer of 1943 and the high back voltage of 150 volts. Did they actually have a rectifier that would withstand 150 volts in the wrong direction?
I believe this is true. Now remember these were hand made rectifiers, not production line so it is not surprising that they were lucky and got one that went that high. By that they mean that there was a very significant ratio at the given voltage between forward and back direction and break down would be defined when the current curve would make a sharp rise or break to larger values in the back direction, so there is no reason to doubt that and in fact it is probably contained in one of those research report books you mentioned.
I hope it is.
Now I came across… There are several of Benzer’s and a few from Bray and a few from Yearian and a few from the fifties.
Well there is a lot of material there all right.
I sure was glad to see that.
Now I think some of the things that were done after the secrecy provisions were lifted, you could get a pretty good grip on it by reading the quarterly and so on reports. Oh they went to the Signal Corps and then to one outfit and then another. The government kept changing names but it was the same thing. Oh I might mention that at our meetings we generally would have a representative of the armed services. I think there was a Major Anderson for instance who was sufficiently knowledgeable to be able to understand what was being said and to be in attendance to see what was going on.
In October of 1943 apparently that 150 volt high back voltage was reported at one of the Radiation Lab Meetings. Do you know if there is any record of that meeting? Or was that one of —
Of the conference like this I don’t know. That is just one of those things that no one knows whether there was something scrawled on the back of an envelope or actually filed.
Well that is something for me to dig up then. In October of 1943 it also mentions a conference with H.Q. North on future germanium development. What exactly were they talking about there?
At that time they were thinking of the high back voltage rectifier and its potential use in computers.
So that was mainly to just develop a working diode then.
That is right. Now already there were working diodes that went into the radar equipment including those that were actually at use in the field in Europe. In fact from time to time Benzer and Smith would just hand-make a certain number of diodes for incorporation into pieces of equipment that they needed right away before they could get factory lines set up for that.
Now those were the point contact rectifiers?
It seemed to be quite an art to make those and get them to work.
Before they could run them through a line there were quite a few that had to be made by hand. I think there was a British group. What was it? British Thompson Houston, BTH, that was trying to do some of the same sort of thing over in England.
Were those produced at Bell then? I noticed that Purdue was supposed to supervise development of the rectifiers at Bell.
I don’t know whether they did get into production or not. I can’t answer that.
And then the Spring of 1944 it mentions your success in interpreting the resistivity and the thermoelectric behavior of germanium. Is there anything more to that story that we haven’t discussed yet?
Well that is a thing we have been referring to several times. What Lark-Horovitz was referring to there is accounting for the wide difference in behavior of samples of different purity contents and by that time we had plenty of data and had a great deal of information on germanium samples and even some on silicon. Much later, outside probably even of the period you are concerned with, it was extended to materials like tellurium.
Where was this first published? It mentions an NDRC report from November 1945. It might not be in that reference but in some of the other ones. I’ve got NDRC report Number 14-585, November 1945. Would that be the first?
That was a kind of a wrap up report so I think it had a good bit more detail in it. It was pretty well put together I think right after the war. Well I think we had time to get it out after the war was actually over or we knew it was going to be over. During the war so many things were in fairly abbreviated form because we weren’t taking any more time than necessary for the write up and there was a problem of limited distribution and so on, but I think your answer to something like that is to get the set of reports and look at them. I suppose you can divide them into two groups. The ones up to and including this November 1945 which was essentially a wrap up of the war work and then the extension from then on. I think you would find quarterly reports and so on running all through the Fifties anyway.
This is also mentioned in a Phys. Rev, abstract from 1946, that would have been from the January meeting.
That January meeting there were five related papers from Purdue and the one Esther Conwell gave and we had a fantastic audience. You know before the war if ten people sat around and listened to papers at the Physical Society it was quite a few. Here we had a lecture room at Columbia and people were standing.
Were those most of the papers right there?
We started in here with the conductivity and Hall Effect measurements and then the work that Conwell and Weisskopf did on Rutherford scattering to get some theory of impurity scattering, and then Lark-Horovitz and myself. I gave the paper on the theory of resistivity and then we have the experimental work on thermoelectric power and finally the theory of thermoelectric power. Yes those S 1 through 5 are the story there.
So these are listed in Phys Rev 69, pp. 258, 259 from 1946. What kind of reaction did the papers draw?
Intense interest. This may interest you. You know when the war was over in August, some time toward the end of October, Bill Shockley and another gentleman by the name of Morgan, an engineer from Bell Labs, came here to spend some time finding out everything we knew because this was really the start in solid state work for Bell Labs. They had probably been pretty well tied up during the war with equipment and so on. And then at this point Shockley and some of the others there could see that solid state looked like it would have a big future for them so they made trips around to places. Not only Purdue but certainly Pennsylvania and others to make sure they werenÆt missing out on anything that they hadnÆt heard. RCA was very much interested particularly because they were interested primarily in the high back voltage rectifier and making it for application in computers and General Electric was interested in communications so it really did start something going throughout the electronics industry.
And now this research during the war you developed it to a little higher degree in the early 1950s?
Yes and also extended it to other materials. In 1948 we did have the invention of the transistor by the three men from Bell Labs who got credit for it and we also got into not only here but other places the effect of for instance nuclear bombardment or other types of bombardment on the properties of semiconductors. Then another development of oh about 1952 or 1953 was the introduction of manufactured semiconductors made from the periodic system here. You have your semiconductors down here, germanium and silicon, graphite and so on by taking two elements. Okay so in that period you had the development of new materials that were manufactured in the lab such as gallium arsenide, gallium antimonide, indium arsenide, indium antimonide and so on. Since there were three elements on each side you could combine them to make nine possibilities and so that was the development at that particular time and it has been very fruitful. Another thing we should mention is along about 1949 or 1950, they finally figured out how to grow germanium crystals so that you not only would get single crystals but before long single crystals of very substantial size and this led to not only cheaper transistors but much better ones, more predictable, so you have just a natural extension. Every discovery would suggest something else and the number of people involved mushroomed from a number maybe on the order of 40 or 50 to ten times that and then probably to something measurable in a few thousand.
Did they get single crystals of silicon at the same time as germanium? Was that sort of just a generic process?
About the same time, yes. It was the use of what is known as the Czochralski technique that has been used for other materials. It is a case of having a melt. Put a seed crystal in and withdraw it slowly and rotate all the time and as necessary you might be keeping the system reduced by blowing hydrogen across the material depending on what it is and so on. But the main idea of the Czochralski is the pulling of a seed at a steady rate with a rotation that seems to be important in getting the single crystal to grow.
Was Purdue at the forefront of growing single crystals?
We had a man (W. E. Taylor) who got his degree in metallurgical engineering a joint sponsorship by Lark-Horovitz and Professor Bray from metallurgical engineering. He was one of the early developers of a method for growing single crystal germanium samples. He later went to Motorola and was instrumental there in getting them set up on growing practical germanium crystals for transistors.
Some of the other interviews I have listened to that we have done, specifically the one with Hellmut Fritzsche, mentions times when Bell Labs people came out and talked to people here and got information but never really gave the proper credit. Do you know anything about this?
This is a matter of personality and so on. I think John Bardeen for instance was always very generous about giving credit to us and anyone else with whom he had talked. Shockley had a different type of personality and I won’t say that he denied getting help but it would not be natural for him to give credit to anybody else in the world for anything. I think there is a little more to it than that too. Some of our publishing was delayed more than it should have been and that gave, for instance, Gerry Pearson and Bardeen a chance to publish a good paper on resistivity of germanium before we were able to do anything on that. Now that isn’t their fault; they had it and we knew they had it, because they told us. Lark-Horovitz always hated to publish anything because he kept thinking we will get one more thing to add to it. He was one of those.
It must have been frustrating.
It was, it was. But that is where some of the problems arose on who should give credit for what, but I would say that Bardeen was a very decent man about that. He is just naturally that way. You probably know him because of his connection with the University of Illinois.
I’ve met him once very briefly. In 1946 or about there I guess Lark-Horovitz decided to keep up the basic research and concentrate on that rather than begin working on applications.
It is the best thing to do at a University anyway. And of course at that time he began getting new people in. There were some physicists who had completed bachelor’s work before or during the war who had been in service and then were released. One that I can think of is Norman Pearlman and he worked then with Pieter Keesom in the low temperature field, particularly on studies of specific heat. One of the things that we did before 1950 was get an operating source of liquid helium. Then John MacKay and Everett Klontz also came in from service (MacKay from the Canadian forces), and then they did some of the early work on bombardment, particularly, Klontz did electron bombardment of germanium. At that time the best source of high energy electrons was the Van De Graaf up at Notre Dame. So he would get his samples prepared here and go up to Notre Dame and run a while and then come back to do his analysis. Actually MacKay was working with the synchrotron group quite a while before he switched over to working in solid state. After he made the switch he stuck with solid state from then on.
Did anyone do any applied work at all here?
Many of the people went into applied work after getting degrees here, such as Milton Becker who with Dr. Fan discovered the infrared transparency of germanium. Milton Becker went out to Hughes and the Los Angeles area, and then eventually he founded a consulting firm of his own. A few years ago he retired. We have many many people who did go to Bell Labs, General Electric, IBM and so on, but the theses themselves were just about one hundred percent basic research.
I guess that is only natural.
That is just about the standard approach in the university.
I was reading in a book called Revolution in Miniature by Braun and McDonald. Was MacDonald the one who interviewed you a few years ago?
This was the man from over in England, yes that is right I am sure he is the one. He wrote a little article on what he obtained in the United States, maybe some in Great Britain too, that appeared in the American Journal of Physics four or five years ago. Probably if you go through author indices you will find it pretty quickly.
I will check up on that. He mentions in the book that there were two people who came close to discovering the transistor at Purdue, Would that have been Bray and Benzer?
Certainly Bray was one, he was getting the effects that are used in making a transistor but he had not worked out the reason for it and therefore he did not have a controllable effect. What he would want to add to that I don’t know, but you probably will be talking to him and I suppose to Bill Fan. Dr. Fan and Benzer of course probably in connection with some of his other work, he would touch on it you might say. I don’t think he was as close as Bray, but at the end of the war he rather quickly switched over into biophysics. He went from here to Oak Ridge and made a deal with them that initially they would pay him as a physicist for what he knew in that field while he was learning the biophysics biological aspects, which he did rather quickly. Then he was over in the Pasteur Institute in Paris for two or three years then he came back and worked as a biophysicist at Purdue for a while before going on to the Salk Institute in California.
Well that takes care of all the questions I had up through that period.
Now in my own research notebooks I have a very pertinent section on the Conwell thesis that gives her basic derivation. You see the problem with Rutherford scattering is that you need to adopt a cutoff or you run into an infinity in your integration. Therefore it is basically that if an electron say goes between two scattering ions you decide which one you are going to count as doing the scattering. If it is nearer A than B then it is scattered by A and neglected for B and this sort of this thing. And this was Conwell’s contribution in getting a resistivity. Now this problem of impurity scattering was tackled ten years later by a couple of other people, Dingle was one; I think Harvey Brooks also had something to say about it. And then there were papers again in the fifties on the effect of scattering by neutral atoms and by electrons scattered by other electrons.
Well I think that just about does it then. I really appreciate this. This is very good. Thank you very much.
Well, quite welcome to give you some information. Very glad to help you out.
I might be back to talk to you later on to ask you some more specific questions after I start writing things up.