James Koehler

Hoddeson:

Do you have any questions before we begin?

Koehler:

Do you want anything about my education?

Hoddeson:

Yes, I would like to go through it chronologically.

Koehler:

Fine.

Hoddeson:

On these notes I see you were born on November 10, 1914, but it doesn't say where you were born.

Koehler:

Oshkosh, Wisconsin.

Hoddeson:

I see. Could you tell me a bit about your parents, siblings and so on?

Koehler:

Yes, my father was an MD and he was educated at Hahneman Medical School in Philadelphia. He was born in Plymouth, Wisconsin, so the Midwest was his home. And he married my mother who was a nurse in Philadelphia at the time he met her. And they came back to the Midwest and he started a practice in Oshkosh.

Hoddeson:

I see. Was he a general practitioner?

Koehler:

He started out as a general practitioner, but about the mid 30s he decided to specialize in heart disease, so he went off to Chicago and then to Massachusetts General and became a heart specialist.

Hoddeson:

And did you have any brothers or sisters?

Koehler:

I have one brother who is five years younger than I am.

Hoddeson:

And what direction did he take?

Koehler:

Well he is now a lawyer, he is employed by Crum and Forster in Freeport, Illinois. Crum and Forster is an insurance company. He got a law degree, from the University of Wisconsin.

Hoddeson:

Now before we go to Oshkosh State Teachers College, could you tell me whether there were any important influences in your early years that were relevant to your decision eventually to go into physics?

Koehler:

Well, I think my father would have preferred I go into medicine. Medicine was interesting to me, but I was a little scared of the responsibilities involved. Being responsible for somebody's life, I was a little frightened of that. And I enjoyed and liked science from high school on.

Hoddeson:

Were there any outstanding teachers that you remember?

Koehler:

Yes, there was a man by the name of Clark. He did very fine demonstrations in high school physics and that certainly awakened my interest. And then when I got in to the Teachers College, there was a good man there who had gone to the University of Michigan. So that to some extent is why I went to the University of Michigan. It's basically his influence that led me to go to Michigan. Dr. James Duncan.

Hoddeson:

And you got there about 1935?

Koehler:

1935.

Hoddeson:

— and spent four years there?

Koehler:

Five.

Hoddeson:

Who were some of the people you were associated with most closely? I know of course about that paper with Dennison, so he must have been —

Koehler:

— well he was my thesis advisor. And I knew Goudsmit and Uhlenbeck quite well. And I knew Randall who was head of the laboratory. Randall was the man who started those summer sessions at Michigan and I went a great deal.

Hoddeson:

Did you attend many of them?

Koehler:

Oh yes. All of the ones I could.

Hoddeson:

Were there any lecturers that particularly impressed you at that time?

Koehler:

Well there were a lot. I heard Heisenberg and Fermi disagree politely.

Hoddeson:

On what?

Koehler:

On large cosmic ray showers. That was very important. Schwinger had an office next to me for one summer and I asked him some questions about scattering theory whereupon he gave me a beautiful lecture right off the top of his head. That was of course impressive. And then I can remember Randall saying that the summer sessions are supposed to be a good thing not only for the graduate students but also for the staff. And I can believe it. I saw Wigner and Seitz and Kramers and Bethe and Fermi. You name them they were all there, the top.

Hoddeson:

Do you have any photographs left over from that period?

Koehler:

Not from that period. Gee, I can remember Bethe lecturing on the lawn.

Hoddeson:

On what subject?

Koehler:

I don't remember what the subject was but I remember his appearance. He had a sleezy sort of rayon shirt on with short sleeves and a big pocketbook sticking out of his back pocket. And then there was a wonderful session when a nuclear scientist, an American nuclear scientist got up and gave a lecture on some nuclear problem, and some Frenchmen got up and proceeded to take him apart because of some French data they were eager to communicate. And then Bethe got up and said, well he knew all about that French evidence but it was all wrong and he went down 1, 2, 3 and devastated them. That was Inglis who gave the talk. So you can imagine those talks were educational. I can remember that they were wonderful and that some people were wonderful lecturers. Fermi was a wonderful lecturer. And then Pauli came and gave a couple of talks on spinors and the first talk the room was jammed to the rafters. The second talk there were 16 people there including me and I didn't understand anything. Pauli gave a terrible talk. He wrote it all on the blackboard in little letters and then sat and said "And now in the well known way we have...," and pointed to the next line. Fermi was wonderful in that he gave something to everyone, but the more you had the more you could understand from it, the less you had, well he gave you a little bit.

Hoddeson:

Was that the first time you met Seitz?

Koehler:

Yes, I didn't really get to know him, at those sessions.

Hoddeson:

He must have spoken about solid state.

Koehler:

He spoke about — you see. Wigner and Seitz had published their theory of sodium in 1935, was it?

Hoddeson:

Well they had written the first one in 1933 and the second one I was think was late 33 or early 1934.

Koehler:

Well, he spoke about band theory of solids.

Hoddeson:

Did you study solid state already at that time?

Koehler:

There wasn't any solid state at Michigan at that time. I wanted to. I started reading papers on dislocations at that time.

Hoddeson:

I see. Who did you read?

Koehler:

Taylor, G.I. Taylor. And then as soon as it came out I read J. M. Burgers. (Pause) Well we were talking, you said, why didn't I do solid state at Michigan?

Hoddeson:

And you were saying that there was no solid state —

Koehler:

— there was no solid state at Michigan.

Hoddeson:

How did you get interested in solid state to begin with? Do you remember? Were you immediately aware of the Wigner Seitz work when it came out?

Koehler:

Yes, I was aware of the work and I read the dislocation papers and I was interested in them. I wasn't interested in nuclear theory. Sort of had the feeling it was somewhat impractical. That was erroneous. But anyway, that was my idea. Now Michigan had done a lot of impressive work in infrared and in fact I did my thesis with Dennison simply because he was a good theorist in infrared.

Hoddeson:

I noticed in your papers that you started out in theory.

Koehler:

Well, I have always done theory.

Hoddeson:

But then you began to do experimental too after a while.

Koehler:

I have a very strong feeling that theory and experiment should not be separated, but that they should be as closely connected as possible.

Hoddeson:

That shows up in your papers.

Koehler:

Well, I believe it. Theory suggests experiments to do. Experiments that can't be understood suggest new theory. So the two are and should be inextricably connected.

Hoddeson:

Did you feel that this was not something that was generally felt by physicists you knew in the years you were a student? I think of theory and experiment as having been closer in the United States in that period, than it was for example in Germany and France. In Britain I suppose it was closer again but perhaps not as close as in the United States.

Koehler:

I think that physicists of those days did believe that theory and experiment should, at least in this country, be close together. For example, Fermi.

Hoddeson:

Fermi, certainly. What about people like Uhlenbeck and Goudsmit? They were important theorists.

Koehler:

I think Goudmsit should have after all, he did a lot of early work on spectroscopy and used all the spectroscopic data. Uhlenbeck was an interesting person. I did some work with him after my thesis when I was looking for a job.

Hoddeson:

What did you do?

Koehler:

I worked on trying to calculate internal conversion coefficients and he had an interesting way of training theorists. What he would do was pick a field, not just one problem, a whole field and then he and his student would go over this field and study it together and I like that as a mode of approach. I've never been able to do it, but I think it was a nice way to go at things. For instance, he trained Konopinski who has been a beta ray theorist from then on. But the beautiful thing about that approach was that it made the student when he went off an authority in this one field, so if you picked a good field, why it was a valid approach.

Hoddeson:

Well I guess it is also a valid approach in general. Even if the student changed to another field he would have this approach to learning and becoming an expert.

Koehler:

Right.

Hoddeson:

You started to mention the papers you read in dislocation theory.

Koehler:

Well I had read Taylor and then there was the one who did the beautiful work in Dutch, Burgers.

Hoddeson:

Oh, Burgers.

Koehler:

You see he published a couple of very nice papers early on the Burgers vectors.

Hoddeson:

At that time was anybody in the United States interested in dislocations? This was well before Seitz began to be.

Koehler:

Yes that is right. No, nobody was interested. As a matter of fact, long after that I went to Carnegie Tech and took a job in 1941, I think.

Hoddeson:

I can let you know in a minute. 1942 it says here. You were at Westinghouse between 1941 and 1942 according to this.

Koehler:

Right Carnegie Tech had a distinguished metallurgy department at that time. They had C.S. Barrett. But they didn't believe in dislocations at the time. And when I first got to Carnegie I would go down and try to talk to them and I remember Barrett was a very nice person and he would listen to me. But you could tell his interest was elsewhere. And really in order to get someone else who was interested I had to go and talk with Seitz. Seitz was interested. Seitz and Read published a series of papers early on.

Hoddeson:

I forgot to ask Seitz how he got interested in dislocations.

Koehler:

I imagine it was by reading the literature, the Taylor and Burgers papers and so forth. The other thing — and this is what is so astonishing about the metallurgists — is that the concept is so immediately geometrically obvious that it's incredible that anybody wouldn't get interested. Anyway, they weren't. And the thing that turned the metallurgists on was when they saw the Bragg bubble results? Have you seen those?

Hoddeson:

No, what are those?

Koehler:

Well, you mean you have never seen the Bragg bubble movies?

Hoddeson:

Please tell me about them. When were they made?

Koehler:

They were made in 1949 and by Bragg.

Hoddeson:

By W. Bragg? Which Bragg?

Koehler:

I am not sure. What they are is they float a lot of little bubbles on top of a soapy liquid and the bubbles try to make a two dimensional solid. The surface tension pulls them in to make a two dimensional solid and then if you shear the bubble rack, you get dislocations running across it. You know like this (motions with hands).

Hoddeson:

Like a wave?

Koehler:

Like a slip and that convinced the metallurgists that dislocations were real. Now that's not a real solid or crystal but that showed them that such a device was topologically possible. And then people began to see them of course.

Hoddeson:

So you had no one to talk to about dislocations for a while?

Koehler:

For a while no.

Hoddeson:

For at least five or six years.

Koehler:

That's right.

Hoddeson:

And what did you do along those lines? It took a little while before you started writing papers in that area.

Koehler:

Well you see there is an interim because of the war I was lucky. I went and spent a year with Seitz at the University of Pennsylvania on a post doc. And money was supplied by the University of Michigan. That was a Rackham fellowship.

Hoddeson:

This was 1940 or 1941?

Koehler:

That's right.

Hoddeson:

So the war was just beginning.

Koehler:

And then I got a Westinghouse fellowship and went to Westinghouse and I was there about six months. And then the United States got involved in the war and they were at that time starting work on radar. And so I took a job at Carnegie Tech and about a year later Seitz came to Carnegie Tech. He took the job as the head of the physics department, so as I say I was very lucky. Carnegie Tech was a good institution, but his coming certainly helped a lot.

Hoddeson:

He was able to build it up in the solid state area and especially in the modern sense. That fit in with your interests. Before we go into this period I want to ask you one more question about your education in solid state. You were reading the dislocation papers largely because you were interested and happened to come across them. Do you remember how you came across them?

Koehler:

I don't remember. Taylor's papers were nice.

Hoddeson:

Were many people reading Taylor's papers?

Koehler:

I don't know how other people happened to find them. I know I did.

Hoddeson:

Had you also educated yourself in the more classical work that was summarized for example in the Sommerfeld-Bethe Handbuch article? Did everybody read that in that period?

Koehler:

I think most people did.

Hoddeson:

You were not taking a solid state course, but you read it at the time?

Koehler:

I had to. My thesis was all about band theory in one dimension. So I read all the band theory stuff.

Hoddeson:

I see.

Koehler:

That was again a bit of luck. Dennison picked a nice problem.

Hoddeson:

So was it perhaps through your work on your thesis on methyl alcohol (Koehler and Dennison, Hindered rotation in methyl alcohol. Phys. Rev. 57, 1006-1021 (1940)) that you got to read some —

Koehler:

No, I think I read the dislocation stuff before that. Let's see if there are any references to it.

Hoddeson:

I didn't notice any, but I might have missed it.

Koehler:

I was interested in solid state and I read band theory. I knew I wanted to go into solid state even before I got involved in the thesis. So this was just a lucky thing.

Hoddeson:

A lucky thesis.

Koehler:

Yes.

Hoddeson:

Well your next paper is on the dislocation theory of plastic deformation. (Koehler, J.S., On the Dislocation Theory of Plastic Deformation, Phys. Rev. 60, 397-410 (1941)) That of course is the beginning of years of work in this general area. And now I understand why you submitted it from the University of Pennsylvania, but were at that time at Michigan.

Koehler:

No, this was done at Pennsylvania.

Hoddeson:

But you were still a fellow —

Koehler:

— post doc.

Hoddeson:

— post doc at Michigan.

Koehler:

Right.

Hoddeson:

And you refer to the series by Seitz and Read in the Journal of Applied Physics and thank Seitz. So I was wondering how closely you worked with Seitz on this — whether there were lots of discussions.

Koehler:

There were lots of discussions, you know, because he was deeply interested in plastic deformation and the behavior of dislocations.

Hoddeson:

Was the Seitz-Read series widely read? It is written, as I recall, in a style not so different from his text book.

Koehler:

Well, he published a little book.

Hoddeson:

— on the Theory of Metals. (Seitz and R. P. Johnson, The Physics of Metals, (McGraw-Hill, New York, 1943))

Koehler:

He's got a lot of that material in it. But I think there is a little difference between what Seitz did and what I was trying to do. I took a sort of a limited area in this field and tried to dig a little. And some parts of that worked and some didn't. For instance this idea of trying to calculate the stress-strain curve. I think that didn't work. And I don't think anybody even today can calculate for you a stress-strain curve. And I tried to calculate the energy stored in work hardening sort of a rough — the crude numbers are about right but the details are all wrong, I think. On the other hand, some small details were done. I don't know whether it was this one where it was the image force I was worried about. When you have a dislocation near the surface there is an image force that tends to suck it up and I think that was done early on. I think it was done in here. And then the calculations if you've got two dislocations on the slip plane, how much force is needed to push them by one another? That kind of thing, that turned out all right, but the more complicated things did not turn out so well.

Hoddeson:

Now we're just tracing the growth of this field in the United States. By 1941, when you were working, did you yet have a number of colleagues, other than say Seitz and Read?

Koehler:

Well in this country there was Seitz and Read. In Europe, there were lots. Mott was beginning to get interested. There weren't too many. In England there was Cottrell.

Hoddeson:

I don't know when he got started, but I can look that up.

Koehler:

And then Read and Shockley got worried about grain boundaries.

Hoddeson:

Didn't they start later, after the war?

Koehler:

We just about got started and then the war interrupted and Seitz very early got involved in war work and when he came to Carnegie Tech, he got us involved in some OSRD work on armor penetration and that kind of thing.

Hoddeson:

What does that mean?

Koehler:

Oh, on how a bullet penetrates into armor. And then later everybody got involved in the Manhattan project.

Hoddeson:

Right, let’s see the book you mentioned by Seitz was based on a series of lectures he gave in the evening to the metallurgists. [Background discussed in Seitz interview by L. HODDESON, 1981. LH] It's an interesting book. Let's see, in 1941 to 42 you were at Westinghouse.

Koehler:

That's right, six months.

Hoddeson:

Now how did you make this transition, why for example did you choose Westinghouse?

Koehler:

Westinghouse was doing some solid state research at the time. They had done some nice work on producing oriented iron crystals for transformer iron. They had done some nice work in that area and Condon was at Westinghouse at that time. There were some good people at Westinghouse at the time, so I thought it would be a good place.

Hoddeson:

And at Westinghouse, did you work in a group? How was it different? Did you work alone?

Koehler:

Basically I worked alone.

Hoddeson:

And did you continue the work that you were starting then on the dislocations?

Koehler:

Sure I was working on dislocations at the time. Yes. And then the war broke out and then Condon and the people at Westinghouse began to get interested in radar. And I wasn't so sure I wanted to work in radar. Carnegie needed some people to teach so I took a job at Carnegie. Emerson Pugh, the theorist, was head of the department at the time and so he made a job offer. And I took the job, and I also knew about the metallurgy department I knew it was a good department. So I thought, okay I'll try that.

Hoddeson:

That was a major concern of yours to have a good metallurgy department nearby. Was it because they were more likely to share your interests than the physicists?

Koehler:

Well anyway having two good departments is better than having one.

Hoddeson:

Let’s see, Westinghouse on pg 283 of this paper. (On Dislocation Theory and the Physical Changes Produced by Plastic Deformation, Amer. Journ. of Phys. 10, 275-285 (1942)) So you worked on dislocation theory and the physical changes produced by plastic deformation and this is a continuation of earlier work. Well by this time Zener is working, now is he in the U.S. yet?

Koehler:

Yes, let's see where was Clarence at the time?

Hoddeson:

I'm not sure. I can look it up. [Zener was a senior physicist at the Watertown Arsenal from 1942-1945 and then went to the University of Chicago as a professor until 1951. He returned from England in 1934. P. H.] Was his name beginning to be important in the field yet?

Koehler:

Well, Clarence was a very bright guy. I think he was at Westinghouse at this time.

Hoddeson:

He must have been interested in dislocations, No?

Koehler:

I'm not sure exactly what his interests were at that time. Clarence had a keen thermodynamic way of understanding the decrease in density that occurs when you cold work a solid and that's what this was from.

Hoddeson:

I see a reference on page 283.

Koehler:

And so now I tried to give a more detailed sort of atomic idea of what was happening.

Hoddeson:

I see. Did any of the work you carried on during the war feed into your main research?

Koehler:

No, not really. I was a little disappointed in my own achievements during the war. I tried to look back on it after the war and try to understand it and I think the major difficulty with it was that I worked on problems that originated with other people not with me. And so in a sense, though I worked long hours and I worked hard, my heart wasn't in it really. I think I do better if it is a problem I turn up myself. That's fun, finding problems, and I would rather work on something I find.

Hoddeson:

I see. What are some of the other things besides bullet penetration that you examined during the war?

Koehler:

For example, we got onto the Manhattan District and among other things we were measuring Young's modulus and the damping of graphite rods that were taken out of the reactor.

Hoddeson:

But here you were able to bring in some of your research experiences.

Koehler:

You're right and also we made some mechanical tests on the uranium metal and there again it connected up with what I was doing.

Hoddeson:

There again you are talking about the Manhattan Project. You're talking about the damage due to the uranium.

Koehler:

Well no, to graphite really. Well there was a worry. Do you know the story about the Wigner disease?

Hoddeson:

No.

Koehler:

Well it is pretty simple. The graphite, if you irradiate it, knocks atoms out of the lattice.

Hoddeson:

Yes.

Koehler:

And now if you anneal it, you warm it up, these atoms can go back to their holes and release some energy. There was concern for a while that you could get an uncontrolled release of energy in that graphite and this would have disastrous consequences on the reactor. You know the temperature of the reactor would go way up, so there was a worry and that was one of the reasons we were measuring the changes in the properties of the graphite.

Hoddeson:

Now was this connected with the Met Lab in Chicago, which Wigner was head of during the war? I know he became interested in this problem and that Seitz spent some time on it.

Koehler:

Seitz and Maurer moved from Carnegie and spent a year or two at Chicago.

Hoddeson:

I see and worked on this problem?

Koehler:

Well this and other problems. Even when we were working on armor penetration and heard that Wigner and Bethe and several others were going to Chicago, we knew what they were working on. It could only be one thing.

Hoddeson:

How did you put it together?

Koehler:

Well there had been a few publications on nuclear fission before the clamp of secrecy descended so we knew who was involved and could tell what the project meant. You could infer what it meant. Did you ever hear the wonderful story about Leonard Schiff?

Hoddeson:

No.

Koehler:

Schiff was at Pennsylvania when I was there and Schiff and Stevens were past graduates of Carnegie. Schiff and Stevens wrote a book about the atom bomb using only information that was in the newspapers. And then of course the Manhattan District didn't want them to publish the book. And the book was never published.

Hoddeson:

It would be interesting to look at the book if it is available now.

Koehler:

I don't know whether it is available.

Hoddeson:

It must be sitting someplace.

Koehler:

I'm sure it is.

Hoddeson:

Did you get a look at the book?

Koehler:

No.

Hoddeson:

How did you know? Did you just put it together from what was available, publicly and hints?

Koehler:

You knew the people.

Hoddeson:

I see you knew who would be likely to work on it and that they were moving to a certain place. The question that I was interested in before was the relationship between your work on this damage and the work that Seitz and Wigner and Maurer were doing at the Met Lab. Were you in communication, and in a sense were you working on it together?

Koehler:

Oh yes.

Hoddeson:

Even though you were at different institutions for a while.

Koehler:

Yes I would get out to Chicago once a month.

Hoddeson:

So you were sort of in the same group.

Koehler:

I was under the direction of Seitz and he would say it would be nice to measure the modulus of these things and so we at first put them in a reactor that was at what is now Argonne and then I'd haul the samples back to Pittsburgh and make measurements on them, communicate the results and then late in the war we began worrying about the mechanical properties of the uranium.

Hoddeson:

Do tell me about them. But before that I want to to ask one more question about the graphite. A colleague of mine, who is also an historian of science, Alan Needell, did some archival work at Argonne and told me that he had come across a bunch of papers on, that looked like solid state work on damage and I was wondering if these might be the same papers that were left over from the work you did there. I don't know what they look like, if they are copies or originals or what.

Koehler:

I don't know. There are some wonderful stories about Szilard.

Hoddeson:

Oh, tell me a Szilard story.

Koehler:

Let's see, in the one I like the best Szilard wanted lots of boxes of paraffin. So he called up the ordering department and ordered 70 boxes of paraffin and so forth.

Hoddeson:

This is at which institution?

Koehler:

He was at Chicago.

Hoddeson:

Chicago.

Koehler:

Yes, he was at the University of Chicago. He kept calling every day for the next two weeks and finally the fellow called back and said, "Professor Szilard your paraffin is in." And Szilard said "Yes, I know. I've already used it. And the man said, "But it is still in the boxes." And Szilard said," but I've already used it. You can have it."

Hoddeson:

Really?

Koehler:

No, no, he had used it. He put his source down in between all these boxes and put his detectors on the far outside of all these boxes and did his experiments and that was it. He didn't take the paraffin out of the boxes. And of course you can imagine all the trouble he had convincing the fellow in the ordering department that he had already done it.

Hoddeson:

Let's see you were going to tell me now about uranium, unless there is some more to say about the graphite.

Koehler:

Well, one interesting thing was that Seitz performed a service in a sense that he travelled around enough to get to all the places that work was being done on the Manhattan District. For example, he discovered that they were doing the same experiments at Oak Ridge that we were doing at Carnegie Tech and so they finally settled who should do the experiments. But communication was a problem because of the secrecy. Several of the groups would see a need to do an experiment and they would start out. Oh there was another wonderful Szilard story. I would get out there once a month and the people out at Chicago would tell me, "Oh Szilard's got another horrible thing that might go wrong." And I'd say, "Oh, what is it?" And they'd tell me. And the next month I'd come out and they'd tell me Szilard's got another terrible thing that might happen and we were working hard to find this out. And I'd say, "Well what happened with that disaster he had last month?" They'd say, "Oh he found a way to get around that, that was easy." And so month by month you'd hear a new Szilard story.

Hoddeson:

Each one worse than the other?

Koehler:

Yes.

Hoddeson:

I guess he played a role.

Koehler:

Yes he played a role.

Hoddeson:

One more question, before we move on to the uranium, but this applies also to the uranium. Can one talk about a specific advance in the understanding of solid state physics that came out of these war-time studies of the damage and the changes in these materials due to neutrons and fission fragments and so on?

Koehler:

Certainly. You must realize that our feeling for point defects and radiation damage in solids had its origin in those days. The fact that Wigner was aware that there were knockons that might have deleterious consequences, certainly.

Hoddeson:

That started those studies. Do you think it would have started anyway if the war hadn't forced you to start it at that time or would it probably have started in a different way?

Koehler:

It might have started in a different way. It probably would have been slower.

Hoddeson:

So this is another one of many examples of the war pushing the field ahead.

Koehler:

I don't know, the older I grow the less I think of war. I used to be a Civil War buff and read all those books about the Civil War. It's a real tragedy that the North didn't finish the War in 1862. Think of all the damage it did to the South really.

Hoddeson:

Yes. Now, uranium.

Koehler:

Uranium.

Hoddeson:

Yes.

Koehler:

Well we did mechanical tests on little tiny pieces of uranium, but I don't think we ever did them on damaged uranium. Just on uranium itself. Uranium is a very strange material. Anisotropic. And so mostly we just did bend tests on little pieces of uranium just to measure the strength of it. Just why they wanted that I don't know.

Hoddeson:

You were not informed of it.

Koehler:

I was not told why.

Hoddeson:

Some time during the war you moved over to Carnegie and most of the work you are talking about now was done at Carnegie as an instructor. So you were still doing the war work. Were you doing work on dislocations?

Koehler:

No.

Hoddeson:

That you then picked up just after the war and went at it with full speed ahead. By that time there were other people working on it in the U.S. as well.

Koehler:

Not very many up till about 1949. Bragg moved into it as I told you. It was sort of the beginning in this country. The metallurgists didn't grab it right away.

Hoddeson:

At what point did you start doing experiments?

Koehler:

Well, I got involved in experiments at Carnegie with a couple of people who came back from the war. These were good people. John Marx and Tom Blewitt and they were graduate students at Carnegie Tech and they did experimental theses. John Marx did experiments on damping and modulus and of copper and single crystals and Tom Blewitt did experiments of tensile tests of copper single crystals so I got involved with experimental work at that time. Basically, they educated me. Both John Marx and Tom Blewitt are very good experimental people. I'll tell you two things. John Marx is the only person I know who can write down on the blackboard a list of steps to do an experiment and then carry it through and have it come out right. And Tom Blewitt? He can work and do beautiful experiments in a lab so dirty that if I tried to do any experimental work in that lab I would blow it up probably. But he does it and succeeds and does beautiful work. So both of those people are good experimentalists.

Hoddeson:

Were you in communications with the groups in Britain who were working by then?

Koehler:

By 1950, yes. We had the Pittsburgh conferences and —

Hoddeson:

— well I guess we are coming to that pretty soon.

Koehler:

Yes, we communicated with Mott.

Hoddeson:

Was that the first time at Pittsburgh?

Koehler:

Well a little bit before but not much.

Hoddeson:

So when was this Pittsburgh conference?

Koehler:

May, 1950.

Hoddeson:

Was that the first time the field came together in your memory?

Koehler:

No, the English I think had a conference before that.

Hoddeson:

When?

Koehler:

1941 or 1942.

Hoddeson:

I see.

Koehler:

I think the English had one conference on dislocations before that, but it was early on.

Hoddeson:

Conferences are one of the social indicators that a field has emerged. If enough people are working in the same area you need to have a topical conference.

Koehler:

Well that was an interesting conference because —

Hoddeson:

Which, the 1950?

Koehler:

1950. Because, well I wanted to make sure that the discussion was recorded. Because I think after all, aside from social things, one of the valuable things is the areas of agreement and disagreement. And so we made a great effort to record the discussion and include it in the published version. Secondly, if anybody had an idea during the conference — and they did — we arranged to get it published in the conference. Frank-Read source was conceived during the conference.

Hoddeson:

Really.

Koehler:

Dislocation source, and so we got that. One trouble with our attempts to record the discussion was we didn't arrange to photograph the blackboard and Sir Charles Frank would usually leap to the blackboard and draw us some pretty pictures of what the dislocations were doing and all of us were trying to scribble it down in our notebooks. And I think we missed a few things because we didn't photograph the blackboard. But it was fun.

Hoddeson:

That happens to me sometimes when I'm doing an interview. If it's on a piece of paper I have the paper. I wonder if there is anything we need to say about the work you did on cold work theory. (A Calculation of the Changes in the 899 Conductivity of Metals Produced by Cold-Work. Phys. Rev. 75, 106-117 (1949))

Koehler:

No that's best forgotten.

Hoddeson:

Let's see one paper with Blewitt is on the lamellar nature of slip and its implications (T. H. Blewitt and J. S. Koehler, The Lamellar Nature of Slip and Its Implications. Pittsburgh Symposium on Plastic Deformation of Crystalline Solids pp. 77-88. (1950)). The other is on Young's modulus measurements on single crystals of copper and lead. (J. Marx and J. S. Koehler, Decrement and Young's Modulus Measurements on Single Crystals of Copper and Lead. Pittsburgh Symposium on Plastic Deformation of Crystalline Solids. pp. 171-184 (1950))

Koehler:

Cottrell was there and Charles Frank was there.

Hoddeson:

The experiments you discuss were the Heidenreich and Shockley and Brown experiments. Were those very central to the theory at the time? Many of the papers here are discussions of those experiments.

Koehler:

Certainly they were important experiments at the time and we were trying to understand—I'm not sure—where did cross slip come in? I don't know about other people, but I think in terms of pictures. No I don't think there is any cross slip in there. I thought there was a statement about cross slip in here.

Hoddeson:

Was it the paper with Marx?

Koehler:

Marx, right. That was a nice way of getting lots of easy and quick and useful data by doing these measurements using a quartz oscillator. And that worked very well and Marx did those.

Hoddeson:

Did Marx stay in the field?

Koehler:

Marx stayed in the field and then he came here with me. Let's see Blewitt got a job at Oak Ridge and Marx came here with me and stayed a couple of years and then went to Phillips Petroleum Company for a year.

Hoddeson:

There's a paper with Peach (M. Peach and J. S. Koehler, The Forces Exerted on Dislocations and the Stress Fields Produced by Them. Phys. Rev. 80, 436 (1950))

Koehler:

That's an interesting story.

Hoddeson:

Oh, tell me the story.

Koehler:

Well Peach came to me and he was in some engineering dept at Carnegie Tech. And he had a wife and a couple of children and he was somewhat discouraged and wanted to get out of Carnegie Tech. He wanted to get to some other job. He said what I'd like to do is get a doctor's thesis. Can you suggest anything? What I suggested led to this, the force on a dislocation. He had his own teaching duties. He had a family at the time, so he was under the gun. I said you're going to have to work like a dog if you are going to get a Ph.D.

Hoddeson:

It looks like a very nice paper.

Koehler:

It is; he did it. He deserves a lot of credit. He got his Ph.D. all right.

Hoddeson:

It looks like something on electrodynamics of dislocations.

Koehler:

This is where the idea came from. That is the idea behind it. But then he went to Houghten Michigan and I don't think he's done a lot since then.

Hoddeson:

There's a paper on the quenching in of lattice vacancies in pure gold. (J. W. Kauffman and J. S. Koehler, The Quenching-in of Lattice Vacancies in Pure Gold. Phys. Rev. 88 149-150 (1952)) Oh, that's later, it's out of order. I don't want to talk about that yet.

Koehler:

That's a nice paper.

Hoddeson:

Okay, there's a paper with Neurath on plastic deformation of single crystals of lead and copper. (P. W. Neurath and James S. Koehler, The Plastic Deformation of Pure Single Crystals of Lead and Copper. Journal of Applied Physics. 22, pp. 621-626, (1951))

Koehler:

Not too wonderful. Neurath was an interesting guy. He could get the best grades of anybody, but as far as being an experimenter he wasn't too —

Hoddeson:

too good?

Koehler:

No. He came into us one day with some wonderful data which he couldn't understand and Tom Blewitt and I explained it for him. He was using a creep test microscope and the microscope was slipping on the sample. To make a long story short, his data was not any good. He got some data eventually. There is a nice story about J. Weertman. You've probably got a paper there by J. Weertman.

Hoddeson:

No. I don't remember. Anyway why don't you tell me about the paper or the story.

Koehler:

Tom Blewitt was very disturbed with me…

Hoddeson:

Oh here it is, Internal Friction in Young’s Modulus. (J. Weertman and J. S. Koehler, Internal Friction and Young's Modulus of Cold-Worked Copper Single Crystals. Journal of Applied Physics. 24, pp. 624-631 (1951))

Koehler:

Now Weertman's had a distinguished career since then. He's now at Northwestern and has done very nice work in geology, on glacier movement and all that sort of thing. And he's a very bright sort of person. And let me tell you how I discovered him. I had a class and we were worrying about modern physics and we were talking about relativity. So I gave the class a problem. Can you concoct a theory in which relativity works for large distances and long times and where you have discrete intervals for short distances and short times? Well most of the class bombed out, but Weertman did come up with something nice. He's a very intelligent man and a good guy and his life since then has demonstrated it.

Hoddeson:

Where did he go since?

Koehler:

Well, I think he spent most of his career at Northwestern but he has done a lot of work on ice flow and ice motion and geology. He is a distinguished scientist.

Hoddeson:

Well we need to talk about the transition to the University of Illinois and that took place in 1950.

Koehler:

Yes, well the instigator was Wheeler Loomis. Loomis wanted to build a strong solid state group at the University of Illinois.

Hoddeson:

Why did he get that idea? Solid state was just beginning to come into its own then. It was just shortly after the solid state division of the APS was established. Was it a natural thing for a department head to do?

Koehler:

Well interest in semiconductors was building. I think it was a natural. The school built a Metallurgy Department about the same time. Tom Read became head of the Metallurgy Department when I came here and he had been associated with Seitz in writing those papers. But Wheeler wanted to build a solid state department. I think it may be that the dean of the graduate college had a role in that too, Louis Ridenour. Now I'm not sure he was here before Seitz came or if he came afterwards. Anyway he may have had a role, certainly Wheeler did. And Wheeler sought out Seitz and said I'd like to build a solid state department. I'd like to build a strong group in solid state. Will you do it? So he more or less gave Seitz a carte blanche.

Hoddeson:

Which was just what Seitz wanted.

Koehler:

Just what Seitz wanted. So Seitz originally brought Dillon Mapother and Maurer and himself and Dave Lazarus. That was in 1949 and a year later I came.

Hoddeson:

Were these all people from Carnegie?

Koehler:

No, Lazarus came from Chicago, the University of Chicago. But Seitz knew Andy Lawson who was at Chicago at the time.

Hoddeson:

Is Andy Lawson still alive?

Koehler:

I think he has passed away. But anyway Loomis wanted to build, so he gave Seitz the go ahead. So Seitz did. And I think those first years that Seitz was here, when Loomis was head of the department and Seitz didn’t have too many administrative responsibilities, I think they were some of the happiest years of his life really. He published a lot of stuff about vacancies, lattice vacancies and so forth in those years, a lot of little letters. And I think he had a good time in those years he wasn't running a department. You know, one of the nice things about Seitz was that he could do anything. He could be a good administrator. He could do good science. He could even sell if he had to. We would not have MRL had not Seitz sold some people in Washington. Moreover he was farsighted. He could see further into the future than certainly I can.

Hoddeson:

What did he do in particular that you are thinking about now?

Koehler:

Well his reason for fighting to get an MRL was I think that he saw that the glory days that we had just after the war weren't going to last forever, and that if one had sort of a laboratory that Washington was committed to, it would be difficult to cut us off. You know it is much more difficult to cut off MRL than it is to cut off just one individual contract and I think that with his foresight he saw that would provide some stable support for solid state science at the University of Illinois.

Hoddeson:

That certainly happened and it is still standing strong.

Koehler:

Well, we need another Seitz.

Hoddeson:

The nature of work hardening you worked on quenching in a series of papers you did fairly early on at Illinois. (The Nature of Work Hardening. Phys. Rev. 86, 52 (1952))

Koehler:

Now there was a nice — (looks through papers) — This one is all wrong.

Hoddeson:

Which one, the quenching in of lattice vacancies in pure gold, the one with Kauffman? (J. W. Kauffman and J. S. Koehler, Quenching-In of Lattice Vacancies in Pure Gold. Phys. Rev. 97, 555 (1955))

Koehler:

The reason it is wrong is that gold was blasted out of its pristine perfect state.

Hoddeson:

I see you wrote a letter on the large tensile stresses of dislocations. (The Production of Large Tensile Stresses by Dislocations. Phys. Rev. p. 480-481 no vol. number)

Koehler:

Yes, that was all right. Basically, if you have a lot of dislocations their stresses will add up.

Hoddeson:

Were you working with the metallurgists here in the beginning?

Koehler:

That was the period when Read was head of the Metallurgy Department and yes there was a lot of communication back and forth between the Metallurgy Department and the physics people.

Hoddeson:

I see. Did you have common seminars once in a while?

Koehler:

Sure.

Hoddeson:

And meet for lunch and things like that?

Koehler:

Yes and (looking at papers) there is the cross slip. I wondered where that was. You can see how geometrical it is.

Hoddeson:

Well by this time, by 1950, the field is certainly coming into its own.

Koehler:

Right.

Hoddeson:

It is strange to look at this field of dislocations from an historical point of view. It seems as though it could have come into its own even before quantum mechanics.

Koehler:

Oh sure.

Hoddeson:

Because even though the papers are written with quantum mechanics in them, most of the basic concepts don't depend on it at all and one wonders why it didn't come out of the old areas like strength of materials which has been around forever.

Koehler:

Because.

Hoddeson:

Why it started when it did.

Koehler:

It started when Prandtl and Dehlinger had the first ideas back in 1927-1928. And then there was a gap and G. I. Taylor wrote in 1934, I think, and then there was a gap and Burgers wrote in 1939. So that early work was very slow and not of much interest.

Hoddeson:

It seems strange that it should not have been, but maybe there just weren't enough people working.

Koehler:

Well maybe. I have another prejudice. I feel that physics people and scientists of all kinds who probe to the atomic nature of things are not satisfied until they have some kind of a model that goes to the atomic nature of what is happening. I don't think metallurgists are like that. I don't think engineers are like that. They are happy to —. If you remember all the theories of cold work that existed back then. And they said all right. There was a book by Nadai. Have you ever seen it?

Hoddeson:

No.

Koehler:

They said if the shearing stress gets real high then you'll get slip. But it acted as though the material was a continuum and it will sort of just shear under the applied stress. Well there do exist atoms and atoms have to move somehow and a scientist worries about the atom — what the ultimate beasts are that are going to have to do the moving, how they take part in this process. I think many of the engineers never get to that. So I think that is part of the reason for the delay. Until the metallurgists saw dislocations, even if it was in a bubble raft, they couldn't conceive of such a beast, and I think for a scientist he always wants the ultimate explanation, as ultimate as he can make it.

Hoddeson:

I still, I'm a little confused because one could imagine that if scientists thought hard enough they could have satisfied themselves, not completely but could have at least have gotten to another level earlier, and yet they didn't choose to do it in this area. But it does explain why it didn't come out of strength of materials, which is an engineering discipline.

Koehler:

Yes.

Hoddeson:

Could it also have been that the materials weren't good enough for the experiments. Did you need to have very pure materials?

Koehler:

Well when was Schmid and Boas published? That was published back in 1930.

Hoddeson:

Well I can find it.

Koehler:

It was certainly.

Hoddeson:

It is in the footnotes of some of your papers.

Koehler:

That was published in 1930 (the German edition), I think and they did beautiful experiments on single crystals.

Hoddeson:

1935, the Springer book. There's probably some earlier version.

Koehler:

There are some earlier papers I'm sure.

Hoddeson:

That is when the book came out.

Koehler:

They did very beautiful work on deformation of single crystals of zinc and cadmium.

Hoddeson:

What tradition did they come out of? Were they physicists or were they something else?

Koehler:

Metallurgists I would say. In Germany.

Hoddeson:

Prandtl?

Koehler:

Prandtl was a — you know Prandtl and Teichens did all kinds of hydrodynamics.

Hoddeson:

So they were physicists?

Koehler:

Probably.

Hoddeson:

One of us will have to go into that and find out the background of the early people in this area.

Koehler:

Prandtl did work in hydrodynamics in airflow over airfoils.

Hoddeson:

Taylor was of course a physicist.

Koehler:

Taylor was a physicist.

Hoddeson:

Then there was Clinney whom you worked with and I don't know how important Clinney was. Let’s see work hardening in face centered substitutional crystals. (Work Hardening in Face-Centered Substitutional Alloys. Acta Metallurgica, 1 508, (1953)) That is something that is worth spending a few minutes on.

Koehler:

Well I think that cross slip idea that was broached there was useful.

Hoddeson:

This division of the three different kinds of hardening. Was that central to the field?

Koehler:

I think that was a good idea and I'm not sure whether I originated that or whether somebody else did. Let me put it this way. I think that would have occurred to various people.

Hoddeson:

There is a theory of initial stress/strain curves in face centered crystals. (Theory of Initial Stress-Strain Curves in Face-Centered Metals, Acta Met. 1, 377 (L) (1953))

Koehler:

In this period was a theory of damping and modulus changes, that was published in that book on Imperfections in Nearly Perfect Crystals, p. 197, Wiley (1952). Seitz has an article in that book where he tries to cover all imperfections.

Hoddeson:

Oh is that the article where he tries to make a synthesis of the study of imperfections?

Koehler:

Yes that is a nice article. In that same book I had an article on the influence of dislocations on the elastic constants of damping. That theory was useful.

Hoddeson:

I don't think I have that one here. There was a very ice article a bit later which I didn't bring along, that you wrote with Seitz in the Solid State Physics Series vol 2. (F. Seitz and J. S. Koehler. Displacement of Atoms During Irradiation (a chapter). Solid State Physics, Vol. 2, 308-448, Acad. Press (1956))

Koehler:

Oh was that on radiation damage?

Hoddeson:

Yes.

Koehler:

That was an interesting story about this. What is worthwhile in that article was provided by Seitz, and what is worthwhile there was the theoretical background. He insisted on doing that well, and that is the part of that that lives today because the experimental data at that stage was not very good. And so to describe the experimental data at that stage was not too helpful.

Hoddeson:

That is the article that draws on Bohr's theory heavily.

Koehler:

That is right.

Hoddeson:

It expands on and explains the theory that was already developed in 1948 and 1949.

Koehler:

That's right, yes.

Hoddeson:

And, yes, I was interested in several things in there. It has a lot to do with scattering.

Koehler:

Sure. Radiation damage is nothing but a scattering problem.

Hoddeson:

The focus is not on the fission fragments on the neutrons, but on the electrons.

Koehler:

Right, on the electrons. There is an interesting story about that. We went to a meeting somewhere, I think it was Pittsburgh, and Seitz talked about electrons and how they make displacement cascades and so forth. And after he finished he got up and said "Well now, what about neutrons? Aren't they useful?" Well, Seitz said, "I guess they are useful for frying eggs." Well he overplayed it but…

Hoddeson:

But nevertheless, this study of electrons in materials grew out of the wartime work on the neutrons.

Koehler:

Sure, I think Seitz' point is that the damage you get from electrons is simpler than the damage you get from a neutron. A neutron gives such a bang that you get such a huge cascade that it is practically impossible to try to understand everything that happens with a cascade. That is his point really.

Hoddeson:

I see, with electrons you can actually study it.

Koehler:

With the electrons you can change the energy and you can give just enough to knock one atom out or two atoms out and or even none. And so it is a finer scalpel to tear the problem apart.

Hoddeson:

Did that grow into a big field?

Koehler:

That has been a nice field. We are still doing some experiments over in MRL; we are irradiating magnesium for instance. But the field is dying down and I'm disappointed in a sense because my feeling is what is going to happen is some day they are going to make a fusion source and then they are going to have really severe problems with radiation damage. We know something about radiation damage, but I wouldn't say we know everything. And they're going to have radiation problems coming out of their ears, so I think the field will eventually revive. I don't know when, but now, right at the moment, all kinds of facilities are being shut down all over the place. For instance, CP-5 had been shut down at Argonne, last fall I think. The swimming pool at Oak Ridge has been shut down. Various radiation facilities are being shut down. By the way, DOE is no longer willing to support the Van de Graaff over at MRL. So all kinds of facilities are being shut down. And I think they are over doing it.

Hoddeson:

Do you happen to know why Niels Bohr was working on that subject?

Koehler:

Well, Niels Bohr has always been interested in scattering. That is one of his earliest theories, scattering. Wasn't he the first person to give a good theory of Rutherford scattering?

Hoddeson:

Why yes, of course.

Koehler:

Do you have a J. D. Jackson? Now if he doesn't give the reference, I am going to be upset. I think my hunch is that it goes back to 1912 or 1914 or something. It is an old topic even then. And I think he reexamined — you know the name Lindhardt. He has always had an interest in scattering properties. Lindhardt of the screening, the Lindhardt screening. The Danes have always had an interest in this area.

Hoddeson:

Let's see we were up to —

Koehler:

Salkovitz.

Hoddeson:

Yes, you don't want to talk about this.

Koehler:

No, it was all right, but not too exciting.

Hoddeson:

Well what about the kink formation. (E. I. Salkovitz and J. S. Koehler, Energy Absorption and X-Ray Studies of Kink Formation in Zinc Single Crystals.)

Koehler:

That was not important.

Hoddeson:

Okay, this paper has lots of names in it and maybe they are worth talking about. For instance, Orowan, Liftig, and Hesselbach.

Koehler:

Well C. S. Barret did some nice work.

Hoddeson:

Chalmers is beginning to be mentioned a lot.

Koehler:

I've got a story about Chalmers.

Hoddeson:

What is the story? I like to hear funny stories.

Koehler:

I sent in a paper, I forget which paper to Acta Metallurgica. And Chalmers was the editor and he at the time he did all the reviewing. He just read the papers and decided whether they were to be published or not. This was a paper on plastic deformation and we had done some reasonable experiments on plastic deformation and slip bands, Tom Noggle. Anyway he sent back the paper and said, "The observations are very interesting when you have a theory to go with this, I'll publish it." And I didn't think of it at the time but I should have written back and said, Kamerlingh-Onnes would have had a terrible time with you when he discovered superconductivity.

Hoddeson:

Sure, I don't know how he got away with that. What about the paper on the irradiation effects in copper. (H. G. Cooper, J. S. Koehler and J. W. Marx, Irradiation Effects in Cu, Ag, and Au near 10 degrees K. Phys. Rev. 97, 599, (1955))

Koehler:

That was a nice paper.

Hoddeson:

Why don't you tell me about the background?

Koehler:

Well, we wanted to find out first of all what kind of damage you could put in at low temperatures and how the annealing went. As far as I know we were the first to do low temperature irradiation. That is below liquid nitrogen. No, I beg your pardon. But anyway we tried to get as pure copper, silver and gold as we could get so we could irradiate it at low temperature and see what happened. We discovered annealing down there in the range 10k to 50k. You know you look in a new region and you find some new things.

Hoddeson:

Now what about the Pauli principle scattering and resistivity. (Pauli Principle Scattering and the Resistivity Minimum. Phys. Rev, 94, 1071 (1954))

Koehler:

That is something that I didn't make enough of.

Hoddeson:

Nice little paper.

Koehler:

Well, the problem is I didn't finish.

Hoddeson:

It is a discussion of certain experiments by Blewitt and Coltman and Redman on the relation of scattering theory at grain boundaries.

Koehler:

There should be some interesting effects of grain boundaries on resistivity and I don't think even today that people have actually tackled that very well. The basic idea is that suppose you have a scattering event and the scattering event takes you to a place where the K vector would lie on the Brillouin zone. Then you can't do that scattering. You have to do it some other way and that ought to have an influence on the resistivity, I would think. Anyway, that was the basic idea. I didn't follow it up and I didn't do it carefully enough.

Hoddeson:

Now I guess we are getting into a long series on radiation damage and we discussed this already a little bit. Now there is a Bristol conference on defects in 1955. Oh, here this refers to the Bohr work. (J. S. Koehler and F. Seitz, Radiation Disarrangement of Crystals, Bristol Conference on Defects in Solids. 222-231, 1955)

Koehler:

The Bohr work, right. Well Seitz and I were reading the Bohr paper on scattering and trying to understand how many defects are produced and what the physical effects would be. That is the basic thing that is being worried about here.

Hoddeson:

This is the radiation disarrangement of crystals. (J. S. Koehler and F. Seitz, Radiation Disarrangement of Crystals, Zeitschrift fur Physik. 138, 238-245, (1954)).

Koehler:

That is the same general subject.

Hoddeson:

Is that the same paper?

Koehler:

Could be.

Hoddeson:

No, no it is not the same paper.

Koehler:

But it is probably basically the same.

Hoddeson:

Very similar. All right, there are some other assorted papers. Oh there is this very interesting one on the velocity of dislocations that you delivered in Japan at a conference in Nikko. (Velocity of Dislocations, Journal of the Physics Society of Japan. 10. 669, (1955))

Koehler:

Right.

Hoddeson:

I was interested in that in Japan in 1955 that they would have a symposium on crystal plasticity and dislocations. I didn't realize the field had grown up enough in Japan to make that natural.

Koehler:

Well, one thing you have to realize and that is that, now I don't know when this started but there were many Japanese who came to the University of Illinois and spent some time here and then went back to Japan so certainly they may have been affected by the interest here in dislocations. I know at this conference they had three different sessions, one session on dislocations and another two sessions on point defects.

Hoddeson:

Who were the major people in Japan in this field?

Koehler:

At that time?

Hoddeson:

Yes, the names of people I should be aware of and keep my ears open for.

Koehler:

I think Yoshida. Well there were several Yoshidas, S. Yoshida was interested. Let me think.

Hoddeson:

Was there a single base in Japan for studies of this sort, or were they just scattered all over the country?

Koehler:

Let's see. There is another guy who went back from here, Suzuki. And then there was a man who went back to the University of Chicago, Takamura. He worked on metals under Clarence Zener and he went back to Japan.

Hoddeson:

By this time which countries were working on it?

Koehler:

There was a small effort in Japan and there was a fair amount in this country, and the English were of course early in. And so were the French. There is a beautiful book by Friedel on dislocations. And the Germans and also the Yugloslavs, so it was fairly wide spread.

Koehler:

As far as the Russians are concerned, I do not think they were started in 1955.

Hoddeson:

In this paper on the velocity of dislocations, you criticize the theory of Mott, Fisher, Hart and Pry on slip bands.

Koehler:

Probably because I thought the damping was too big. That is probably it. By the way, Weertman, the fellow I talked to you about, thinks that you can have very fast dislocations with a speed of roughly the speed of sound. So I am not sure. Andy Granato probably knows whether there are dislocations with the speed of sound. I don't. I am not sure I know.

Hoddeson:

Okay, crystal perfection in aluminum single crystals. (T. S. Noggle and J. S. Koehler, Crystal Perfection in Aluminum Single Crystals, Acta Met. 3. 260-267. (1955)) You use an x-ray technique to get an estimate of the dislocation density as a distribution. It looks very impressive.

Koehler:

Well, it is not a bad way.

Hoddeson:

Is this a new technique developed at this time?

Koehler:

I think the general idea had been used before. By the way Noggle, who is now at Oak Ridge, is an exceedingly clever experimentalist. Just to give you one example, he did his thesis on getting replicas off the surface of crystals and doing electron microscopy with it and he could get resolution I would say two or three times better than anybody else on the campus using that same instrument. There are some of these experiments that depend upon skill in the fingertips and Noggle is the guy that has got it. You know, I mean those delicate things if you jab it or twist it wrong you destroy the sample and that kind of thing. That is what I would do but Noggle always did it right.

Hoddeson:

The technique was apparently developed by the people in the first three references, Guinier, Lambot.

Koehler:

Yes this is the one I remember Gay, Hirsch and Kelly.

Hoddeson:

They were in Bristol?

Koehler:

Hirsch was probably of the Cavendish lab.

Hoddeson:

This is electrical resistivity tensor of —

Koehler:

For aluminum single crystals. Well that is using a method of Noggle to grow nice single crystals. It is called Soft Mold technique. The basic idea is this. You pull the crystal so you get some slip bands and then the idea was to put a lot of potential probes on the aluminum single crystal and try to measure the potential drops in all directions and from that infer what the resistivity tensor was. You pass a current through the sample.

Hoddeson:

How did it work?

Koehler:

Fair, fair. I wouldn't say we screened out all the questions about dislocations using the technique but I think many of the measurements were reasonable.

Hoddeson:

Is there anything to say about this paper on water quenched gold? (J. E. Bauerle, C. E. Klabunde, and J. S. Koehler. Resistivity Increase in Water-Quenched Gold, Phys. Rev. 102, 1182. (1956))

Koehler:

Yes that is the beginning of nice work. Bauerle was a great experimentalist. Both these guys were. I can tell you a nice story about Bauerle and Klabunde. I came into the lab and they were quenching gold wire and measuring the resistivity as a function of the quenching temperature. And I said, "Gee it would be nice if you were quenching in lattice vacancies. It would be nice to measure the increase in the volume of the sample." And I went away and three weeks later they had measured it to three parts in ten to the seventh and the measurements were very ingenious. They had a wire which sagged and a catenary and they measured the sag. And then they annealed it and measured how much it crept up from the sag, using a creep test microscope. Now the creep test microscope is accurate to about 2 x 10^-5 centimeters. And using the catenary gives you another magnification by a factor of 20. The gold wire was mounted on a massive copper frame. And they calibrated it by having all of this down in a water bucket and then changing the temperature of the water because they could calibrate things by the thermal expansion of the gold relative to copper. Beautiful method.

Hoddeson:

I have a paper here by Henderson on the low temperature release of stored energy in cold worked copper. (J. W. Henderson, J. S. Koehler, The Low Temperature Release of Stored Energy in Cold Worked Copper, Phys. Rev. 104. 626. (1956)). This also looks very interesting. They were seeing peaks and making a theory based on moving imperfections in that temperature range being divacancies.

Koehler:

I don't know whether this data is any good. So we better forget it. Henderson was an awfully nice decent fellow but he by no means was a Bauerle. An interesting story about Bauerle. He was a wonderful experimentalist but he wanted to become a theorist and I told him — I said you'll have a tough time ever doing theory as well as you are doing experiments. He sort of intuitively knew how to do experiments. He wouldn't have to sit down and make order of magnitude calculations. He did them right the first time. I don't know how.

Hoddeson:

My last question has to do with your sabbatical year at the University of Cambridge. That was 1956 to 1957 and is just at the tail end of the period that I am supposed to be focusing on for this project. I am curious about whom you worked with and what you worked on there.

Koehler:

Well I was interested in visiting with Mott, and Mott was most interesting.

Hoddeson:

Tell me a little about that and perhaps that can be the last little story on our interview.

Koehler:

You didn't have to go anywhere if you went to the Cavendish Lab because every few weeks Mott would come in and say, "Wouldn't it be nice to have a conference on this?" And so then he would sit down at the telephone and call up Friedel or somebody from Germany or somebody from somewhere and they would have a nice little conference and two weeks later he would say "Don't you think it would be nice to have a conference on this?" And then we'd have a conference on that. So you didn't have to move. You could just sit there and just go to Mott's little conferences where you got everything that was going on in Europe in solid state.

Hoddeson:

What was of most interest to you at that time, in that group?

Koehler:

Pete Hirsh was there at that time and he was doing electron microscopy on dislocations in metal crystals and I was very much interested in that. He was interested in what we were doing on quenching, so there was a mutual exchange. And Mott of course was interested in everything. So it was fun.

Hoddeson:

Did he work long hours, or did he have a normal schedule? He was so productive. I was curious about how he worked and how he got so much done over the years.

Koehler:

He was fantastic. We had a sabbatical house and we stayed in Coslett's house, which was on Long road, and Mott had a house sort of across one of the Commons. He would encourage us to come across the Commons and I don't think the College whose Commons that was thought so much of that idea. Anyhow that is my second story of Mott. He was fantastic. I don't know much about how Mott worked.

Hoddeson:

Well I'm sure one of my colleagues will be speaking with Mott and hopefully get that.

Koehler:

I thought that business about the conferences was wonderful because they were very enlightening.

Hoddeson:

That is in fact how our history of solid state project got started. Mott had a conference on it.