In this interview, Wade considers the influence of some of his most notable inventions and projects. Topics include traveling wave tubes, the parametric amplifier, the quadropole amplifier (Adler tube), ultrasonics, acoustic imaging, the acoustic microscope, ultrascopic imaging, and acoustic holography. Wade describes his experiences in teaching and management, and he describes the publication philosophies he implemented as an editor of IEEE publications.

## About the Interview

GLEN WADE:An Interview Conducted by David Morton, IEEE History Center, 4 June 2000

Interview #397 for the IEEE History Center, The Institute of Electrical and Electronics Engineers, Inc.

This manuscript is being made available for research purposes only. All literary rights in the manuscript, including the right to publish, are reserved to the IEEE History Center. No part of the manuscript may be quoted for publication without the written permission of the Director of IEEE History Center.

Request for permission to quote for publication should be addressed to the IEEE History Center Oral History Program, IEEE History Center, 445 Hoes Lane, Piscataway, NJ 08854 USA or ieee-history@ieee.org. It should include identification of the specific passages to be quoted, anticipated use of the passages, and identification of the user.

It is recommended that this oral history be cited as follows:

Glen Wade, an oral history conducted in 2000 by David Morton, IEEE History Center, Piscataway, NJ, USA.

## Interview

Morton: David Morton

Date: 4 June 2000

Place: Santa Barbara, California

### Childhood and education

Morton:

Would you tell me a little about your childhood and what led you in the direction of becoming an engineer?

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My wife tells it much better than I do. My father was a topnotch lawyer and Supreme Court Justice for the State of Utah who eventually became the Chief Justice. I was going to follow in his footsteps and get a law degree. But then Pearl Harbor happened. I had always enjoyed math, engineering, English, and particularly science in high school and college. With the war on, I gradually changed what I was doing including my program of study, deciding to get into engineering. I perceived it as being the best way I could contribute.

I was able to get into an engineering curriculum in early 1943 at the University of Utah. I was in it for about a year when our dean told us the war was escalating and study deferments would be problematic. This was in the spring of '44 when the invasion of Europe was the major effort.

### Naval service and training

There were forty-nine students in this study program. Dean Taylor advised us all to go down together to the U.S. Navy Recruiting Office and volunteer. We took his advice. It's almost unbelievable, but all of us passed the physical exam and were inducted at the same time. A few days later we were sent to the Great Lakes Naval Boot Camp in Chicago and became members of the same boot-camp company. It was a very good company, there being several good athletes and superior students in the group. All of us became electronic personnel in the Navy.

I applied for Officers Training and got sent to Notre Dame. In a few months I graduated, commissioned as an Ensign. After that I was assigned as an electronics officer to go to Bowdoin College in Brunswick, Maine, to continue in electronics training. Bowdoin was and still is a good school with excellent professors. There were also some good students in that group. As I remember, Cal Quate, eventually to come close to Nobel Laureate status, was one of them. He later invented a number of things including the Scanning Acoustic Microscope and the Atomic Force Microscope. He is presently at Stanford.

While in Brunswick, I got married to LaRee Bailey who flew out to Maine from Salt Lake City. A Navy Chaplain performed the wedding and we went down the aisle under crossed swords. It was a great experience.

While we were at Bowdoin, President Roosevelt died. I guess that was in April '45. Hope I am remembering all of this correctly. Following that, I went for further training in the Naval Air Corps. Cal Quate and I, along with others, went to Boston and got further training at Harvard and MIT. When I got out of all of that training I knew quite a bit about electronics. All these programs, starting at the University of Utah before enlistment, were fascinating to me. I didn't want to go back into pursuing a career in law.

Morton:

We had to pass what was called the Eddy Test in those days. It was a test designed to determine whether a student was qualified to learn more about these things. And the instruction was very good. The textbooks used were great, written by first-rate physicists and other technologists who really knew the material. But the training was specifically directed toward learning about naval electronic systems.

Morton:

Was the training similar to what one would get at engineering school?

Essentially, but with the specific orientation I have described. It was like taking a college course with only one complicated subject. After the war and after being discharged, I returned to the University of Utah in '46 and spent two years getting a bachelor's degree plus one more for a master's in electrical engineering.

### Naval Research Laboratory; traveling wave tube

Following that I went to the Naval Research Laboratory for a job as an electronic scientist that I really enjoyed.

Morton:

Was that a laboratory near Washington?

Yes, near Andrews Air Force Base. It was and is a very good place to work. Both of my bosses there - Walt Weedman and Hank Wiedemann - were fine collaborators to associate with as well as good scientists. My first daughter, Kathy, was born while we were there. We were actually living in Alexandria, Virginia, a very attractive town.

Morton:

How does it compare with Santa Barbara.

Well, Santa Barbara is my all-time favorite place. Kathy lives down the road a bit in San Diego where she is a real estate agent. Two other daughters live elsewhere in California and the fourth in Montana. But I still like Santa Barbara. I'm close to having lived the Biblical fourscore years. Each day beyond that is a bonus. I think that if I live another fifteen years I'll be very, very lucky. I want to do that, because every day is an enjoyable day for me. I've got two fine German Shepherds that I take down to the beach in the mornings. I usually jog about four and a half miles with them. Exercise gives me a high. I don't need drugs. When I come back from the beach, everything looks beautiful to me.

But Santa Barbara came late in the game. The move to the Naval Research Laboratory was first. That's where I started to learn about high-frequency tubes. At NRL, some of my work involved traveling-wave tubes. The research in that area at that time was hot and heavy. By now it has pretty much died down, though there are still a couple of places. Dick Grow, a former Stanford student presently a professor at the University of Utah, still has projects ongoing on high-frequency tubes. When I was there a couple of years ago to get an award, I said to Dick, "You are probably the only one in the world right now working on that, aren't you?" He said no, pointing out that there was still some work at the University of Michigan in that area. Joe Rowe used to do tube research at Michigan and was very good.

Morton:

Are you referring to traveling wave tubes or electron tubes in general?

High-frequency, high-power tubes in general. I guess the NSF picked a couple of places to continue that work. Stanford had been the leading place. MIT also did marvelous work in that area. Those were the two biggest places for it.

Morton:

It sounds like the traveling wave tube had already been invented when you were at the Naval Research Lab.

Yes. I think Rudy Kompfner was the inventor. I would guess he invented it around 1941. He was originally from Hungary and a great, marvelous guy. He was at Bell Labs and then later went to Stanford and became a professor there. I’ve lost track of these guys because I moved out of that area. He was at Stanford for quite a while with Cal Quade, who got a tenured position there. Chapin Cutler also went to Stanford for a while. Chap was Assistant Editor of the Transactions on Electron Devices during much of the time I was Editor.

### Employment history

During the 1950's after working at NRL, I went to Stanford first as student and then got a tenured associate professorship there. But I left in 1960 to work at Raytheon in the Boston area as Director of Engineering in its Tube Division and later as Assistant General Manager of its Research Division. Around 1963 or '64 I also was asked to teach a semester-long tube course at Harvard. For a few months, I spent half my time at Harvard teaching and the other half at Raytheon mostly in administration. Almost immediately I saw that I liked Harvard better. When I got an offer from Cornell - a distinguished professorship with an endowed chair plus head of my department - I decided to take it. But my family never liked Ithaca feeling that Boston and especially California had better climates. In addition, I didn't like the administration part of what I was doing at Cornell and I wasn't very good at it. However, Ithaca is very beautiful. We had a five-acre lot with a waterfall that was about 1,000 feet from Lake Cayuga. The scenery was gorgeous but the weather was not. It couldn't compete with California.

### Ph.D. studies, Stanford

Morton:

Let's back up to the Naval Research Lab days. What exactly were you doing there?

Among other things, my job as an electronic scientist involved dealing with traveling wave tubes and I became very interested in that and in knowing more about tubes. Lester M. Field, who was one of the shining lights in that area, was at Stanford. So I applied there for graduate school and got admitted.

Hank Weidemann was against my leaving NRL. He told me, "You're making a big salary here." I remember my salary. It was $3,737 per year, and to me it was indeed tremendous. I said, "That's true." Then later after we had our daughter Kathy, who was born in George Washington Hospital, Hank said, "You won't be able to support a family as a student at Stanford." I said, "Well, I'm going to try anyway." But I got scholarships and for my first year at Stanford I took home$5,200 - a big increase.

Morton:

That is a big increase.

I got several fellowships there, one an RCA Fellowship. Lester M. Field was my thesis advisor. He eventually went to Hughes Aircraft and became director of their Malibu laboratory. He was a great guy and an excellent teacher. He knew as much about traveling wave tubes as almost anyone else in the world including Rudy Kompfner.

### Tube applications; consultant work

Morton:

I understand these tubes were used for radar applications.

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Yes. Along with klystrons, they were high-frequency generators and amplifiers of electromagnetic energy at about 3,000 megahertz. Stanford was good in research involving both these tubes. The Varian brothers, from Stanford as students, were the inventors of the klystron and they founded Varian Associates. The magnetron, invented in England as I remember, was also a great microwave tube. It was not extremely well understood in those days and a lot of research had to be done. By now the researchers have tied up most of the knots and know about everything there is to know concerning these rather fundamental tubes. It was a very exciting field in the forties and fifties, and in our studies of the operation of these tubes, we learned a lot about waves and wave phenomena.

Consulting jobs in the tube area were available, even for students if at Stanford. I consulted for Zenith and Bob Adler in those days. Bob Adler is extraordinary. I always thought of him as the brightest guy I ever knew. And his lifetime research covered a number of areas including ultrasonics. In fact, for an article on human uses of ultrasound I got portrait pictures last year from the IEEE History Department of Bob Adler along with Lord Rayleigh, Pierre Curie, Paul Langevin and other superstars in this area. The article was the written version of a keynote address I gave at Ultrasonics International '99. The talk was 45 minutes long but they gave me only six pages for the write-up in the journal Ultrasonics. With that little space I could use only Lord Rayleigh's picture and had no room for Bob Adler's or the other pictures. Bob's certainly deserved to be in the paper, but Ultrasonics did not give me room. I could put in only one picture and one reference.

Bob Adler has a website and when I visited it I was pleased to see he mentioned me as the inventor of what was commonly called the Adler tube. I was the only person other than Bob mentioned in the site. The tube was very sensitive getting down to a noise temperature of 30° K from its cathode temperature of about 1100°. I called it the quadrupole amplifier.

I was at Cornell when the Arecibo Observatory in Puerto Rico was dedicated, and got to fly on Air Force One to represent Cornell at the ceremony. Lyndon B. Johnson was President. He didn't go, but other high-mucka-mucks in government as well as some scientists went. The Arecibo Observatory has a big dish that is about a mile across. They found a natural depression among the beautiful hills in that area where they could bulldoze out a mile in diameter and install a nice dish. The receiver sits above it right in the middle. Radio waves come in from all over space, hit the dish antenna and get directed up to the receiver.

### Evolution of tube research; commercial applications

Morton:

What happened with traveling wave tube development after that?

Stanford pretty much got out of that line of research. Cal Quate, who was great in traveling wave tubes, went into acoustics. As a graduate student he had gone to Stanford, just as I had - and before that as an undergrad, to the University of Utah, also as I had. However, I did not overlap him at either place. At Stanford I believe he did a thesis on noise in traveling wave tubes before I did the same thing. Cal, I think, was also under Professor L.M. Field. My thesis was entitled "Noise in Electron Tubes." It meant microwave tubes - because it was microwave noise that I was examining.

Morton:

Why did people get out of this?

They had already figured out the most interesting things and moved on to exciting new areas. NSF needed further work in that area, however, and I believe they picked two universities for continued support. Dick Grow, also a Stanford graduate, went to the University of Utah as a professor and organized a fine laboratory working in the same area. That laboratory still exists. Joe Rowe had already done the same thing at the University of Michigan.

Morton:

I'm trying to get a sense of where this went after it was out of the laboratory. After people moved on to other projects, did the tubes survive in the commercial realm?

Oh yes. TWTs and magnetrons are used to this day and klystrons were a big seller for Varian Associates for a long time - not just in the fifties, but also in the decades to follow. Magnetrons are frequently found in the kitchens of our homes providing power for microwave ovens. Many klystrons are needed, for example, to supply energy for particle acceleration over the two-mile stretch of Stanford's linear accelerator called SLAC.

Morton:

Were traveling wave tubes also commercially viable?

Definitely. They are still highly used today. That's amazing, isn't it?

Morton:

In what kind of systems are they currently used?

Generally they are generators of high power at high frequencies, the klystron and the magnetron, but I’m not very up-to-date on that.

Morton:

Did this field go into solid state after the fifties?

Solid state took over. There used to be what was called the Tube Conference, and J.R. Pierce, Rudy Kompfner and a few others were the gurus. Are you familiar with J.R. Pierce?

Morton:

Oh yes.

He got a Japanese award. I guess he never got a Nobel Prize, but was certainly worthy of it. That guy was good.

Morton:

Did semiconductor devices move quickly into the high-power, high-frequency area, or was there resistance to the move in that direction?

They moved fast but tubes have advantages in certain applications. For example, traveling wave tubes are prominently employed in synchronous-orbit satellites where high-energy cosmic rays abound. TWTs work well in that environment both as wide-band, relatively low-noise receivers and as high-power transmitters. Solid state devices can easily be put out of commission by cosmic rays, but not traveling-wave tubes. J. R. Pierce was not only a guru in traveling wave tube research but he was also the proposer and inventor of the communication satellite. Pierce was the preeminent worker in all that research.

The IEEE Transactions on Electron Devices in July, 1976 published a special Bicentennial Issue commemorating the country's 200th birthday. The issue dealt with "Historical Notes on Important Tubes and Semiconductor Devices." Gerald Pearson and Roland Haitz were the guest editors and the authors included Bill Shockley, Leo Esaki, Jack Kilby, Bob Hall, J. B. Gunn, Ed Ginzton, Rudolf Kompfner, Henry Boot, John Randall, Art Schawlow and George Heilmeier. The editors limited themselves to "devices which proved to have economic impact." Henry Boot and John Randall covered the invention of magnetrons; Ed Ginzton, that of klystrons; and Rudy Kompfner, TWTs. The other authors mentioned dealt with solid-state devices.

But the microwave tubes continued to be attractive for the high power, high frequency areas.

Morton:

My general sense is that for a while these were exactly the areas in which semiconductor devices really couldn't compete.

That's right; up to the mid fifties they were not particularly competitive.

Morton:

It's hard to gauge when they started competing.

I think they started in about 1960 plus or minus a couple of years - and after that they really began to compete. That's when the Stanford people essentially got out of the field - Tony Siegman and others. He had been a great star in tube research but switched and wrote a fine textbook dealing with ways of analyzing lasers. Is that name familiar to you, Anthony Siegman?

Morton:

No.

Really a superstar. But so many of the Stanford people were in that category. They were and still are wonderful people that we are talking about: hard workers, enjoying what they were doing and their work being first-rate. Others elsewhere were just as good: Heilmeier, Robert Hall, Shockley, Gordon Teal, Friedolph Smits, Esaki of Esaki diode fame. That was in the late fifties.

And especially Jack Kilby who invented the integrated circuit. Once they started integrating these circuits they had something very good going. That was in the late fifties also.

### Postdoctoral employment and research

Morton:

Exactly. I spoke to him recently. He seems to be doing quite well. You left the Naval Research Lab at the beginning of the fifties. Besides Stanford where did you go after that?

As I said, I spent one year at NRL and then went to Stanford getting a Ph.D. in '54. During the year after that I remained there for awhile as a Research Associate. The way I remember it, I transferred slowly over to GE in Silicon Valley at Palo Alto in '55, working part time at both places. GE had a fine lab not far from Stanford. I don't remember the name of my position there. It took me about five months to completely leave Stanford. Then when I finally got to be full time at GE, I think I worked there for only a month before I accepted an Associate Professorship with tenure back at Stanford. I returned to Stanford in '55 or '56 and stayed there for about four years.

I then went to work for Raytheon, first in their Tube Division as Director of Engineering and then in their Research Division as Assistant General Manager. Both were first-rate divisions and I really enjoyed the work. But it was largely administrative in both cases. As I have already said, I enjoyed teaching at Harvard even more when I was asked to do so as a visiting professor.

Morton:

What kind of things did you work on at Stanford?

I continued to work mainly in the tube area and taught the course on tubes. Marvin Chodorow went on leave for a year or so. I taught his course a few times at Stanford and then later at Harvard.

Morton:

We have interviewed him.

He is a talented guy.

Morton:

Was there pressure for people to drop courses on electron tubes?

No, I don't think there was any pressure to do that. We just saw that there were other areas that were equally or even more interesting. Cal Quate, for example, made the change. I got to know him well, having run into him at conferences. I was the chairman of a 1970 conference we held in Santa Barbara at the Biltmore Hotel. It was the Fourth International Symposium on Acoustical Holography. Cal Quate presented a paper at that conference. Plenum published a series of volumes on these conferences. There's an article in one of them by Cal Quate describing what is called the Scanning Acoustic Microscope. It is a wonderful instrument. It's hard to believe, but its resolution can easily be comparable to that from light microscopes. It's just amazing. That was Quate's invention.

### Management vs. teaching

Morton:

We're getting ahead of ourselves. You went from Stanford to Raytheon, which brought you to the East Coast. Did you work again on high-power, high-frequency tubes?

Yes, among others things. But it was mainly administrative work. Both the Tube Division and the Research Division were interested in high-power, high-frequency tubes in those days. However, the Research Division had much broader interests than merely tubes.

Morton:

So these were management positions?

Yes. Both were substantially management jobs. I didn't like that kind of work as much as teaching and research. Teaching is obviously a very good thing for keeping the mind on technical things, more so than being a manager. That was one of the things I realized while teaching at Harvard. I could spend nearly my entire Harvard time working on my teaching. I didn't even have to erase the blackboards. Janitors were always doing this between classes. Have you ever seen Harvard?

Morton:

Yes.

It was very nice. I had a nice clean board for every class I taught there, an adequate office on a scenic campus and first-rate students. Then when I got an offer to go to Cornell, I accepted it.

Morton:

About when was that?

I went to Cornell in '63 and stayed there three years, up to '66. Then I had an offer from UCSB. Cornell had a big School of Electrical Engineering. I think there were sixty-six professors in it. Some very good. However I didn't like being head of the School. It was sort of like Raytheon all over again. I was devoting more time to administrative activities than to teaching and research. Even at a university, the head of the department has to spend a good deal of time on purely administrative activities. Especially if the department is big. I was not a good administrator, but I did enjoy being a teacher and a researcher.

### Engineering projects

#### Parametric amplification

Morton:

Let's talk some more about the engineering projects you worked on over the years. We talked a little bit about your parametric amplifier.

Yes. Parametric amplification is applicable to both high-frequency tubes and to solid state. It does not necessarily require the use of electron beams. Harry Suhl in 1957 was the first, I believe, to suggest employing bound electrons which surround the atoms of material rather than free electrons in a beam.

#### Ultrasonics

Morton:

What other kinds of things did you work on after that?

I got into ultrasonics. I was very much influenced by Bob Adler. He was a vice president at Zenith, in charge of research. His research department had many fine workers. Bob himself was the main idea man. He was extremely good. He could administer and do research at the same time. He invented what in those days was called the space command for TV remote control. I don't know what they call it today. In those days it used ultrasound. Nowadays they use infrared for remote control.

Morton:

Yes, the Zenith Space Commander for remote control. I didn't realize he did that.

He was the first inventor of this sort of remote control. He invented a lot of things. He was both a good inventor and a good administrator. His interest extended to high-frequency tubes - hence his development of the quadrupole amplifier.

Nowadays Zenith doesn't even do research. In the late seventies they decided research wasn’t making very much money, and I presume that was true. Universities don’t have to make anybody money, so doing research at a university is wonderful. Students just have to be trained. It’s not quite as good at a company. Bell Labs may still be good, but neither Raytheon’s nor Zenith’s research areas were very good.

The space command got Bob Adler into the field of ultrasonics. I interacted very closely with his research group. A few years ago, one of my sons-in-law searched for my name on the web. He found it on Bob Adler's web page. The web page didn't mention any of the very good people he had in his research department. I was the only person he mentioned and it was in connection with the quadrupole amplifier. I think he never called it the Adler tube, but other people did because he made it work. I invented it, but he actually made it perform as it was supposed to.

Morton:

I think that sort of thing happens every once in a while.

He was good at everything he did. If someone invented something that was difficult to develop and Bob wanted to make it into something of value, it was going to get good - and fast.

Morton:

You got into ultrasonics then?

Yes.

#### Imaging

Morton:

Where did that lead you?

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It led me to imaging in general, including tomography and holography. I'm still in it. Acoustic imaging is essentially what I still know quite well. Last year I was the keynote speaker at Ultrasonics '99 joint with the 1999 World Congress on Ultrasonics in Copenhagen. Ultrasonic imaging in a broad sense was central to what I said.

I enjoyed preparing that talk immensely. LaRee tells me I'm getting too old to do this sort of thing. I tell her that I'm not; and that I'm still smarter than some of my students - though not all.

### Current professional activities

#### Grant evaluations

I started out last year planning to do three rather major things. One was to serve on the evaluation committee for the UC MEXUS-CONACYT Grants for Collaborative Projects. I'm doing the same thing this year and will be going to a meeting for that later in about a week. That was my first major effort last year.

This year, for example, we have 25 different projects we need to know everything about without making contact with the writers of the proposals. We have to rate the proposals for funding. The committee members need to know a lot in the particular areas of the proposals. In the physical area, for example, the proposals can be for research in physics, chemistry, engineering and other fields of physical study. I really had to broaden my knowledge. Last year they appointed me to serve one more year, so I am on the committee again this year. I determined to do my best to be useful and get to know those projects really well; and I did. I knew them about as well as anybody on the committee. In fact, last year when one guy left the committee meeting early to catch a plane the chair asked him, "How do you vote on the last three projects?" He said, "I vote the same way as Glen Wade." I felt pretty good about that.

#### International talks and teaching

The second thing I planned on doing was to prepare and give a good talk in Copenhagen. I called it "Human uses of ultrasound: ancient and modern." I had to do considerable studying for that.

And then the third thing was to go for another semester to the University of Guanajuato in Mexico. I have research students there and am invited to go for a semester whenever I want. So I went, teaching a graduate student class and carrying out research.

I think I accomplished all three of those tasks pretty well. I still have a research group at UCSB, but I haven't taught a formal UCSB class for a few years. I've taught quite a few, though, in Mexico. LaRee tells me I should retire. I tell her I'm having too much fun to retire. Last year was a lot of fun.

### Ultrasonics applications

Morton:

Getting back to ultrasonics, you mentioned the nice little invention of the space command. That's was a high profile invention because it was a consumer item. What other applications were people imagining for ultrasonics?

Ultrasonic microscopy for one thing. Acoustic microscopes are pretty good. I mentioned Cal Quate's Scanning Acoustic Microscope. Zenith invented the first acoustic microscope. It was called the Scanning Laser Acoustic Microscope. It came in the early '70s. It is still a commercial item currently being built by Sonoscan. That company started out small but seems to get bigger every year. By now it may be good-sized. It's based in the Chicago area and Larry Kessler, who used to work under Bob Adler, is the president. Zenith wasn't interested in developing and selling the microscope even though it was invented by Zenith and first built there. It is a very good microscope.

Morton:

Was the acoustic microscope tied in with your work?

Yes. I did work on that microscope with Bob Adler and others at Zenith.

### Zenith consulting work

Morton:

Did you have a relationship with Zenith at the time?

Yes, I was still a consultant. They still had a research department in those days.

Morton:

The consultancy started a few years earlier and continued?

I started consulting for Zenith as a student in the early fifties and consulted right up to the time they disbanded their research department in the mid-seventies. I have consulted for several other companies also, but Zenith's was the best consulting I ever did. It was a lot of fun. In ultrasonics, for example, I consulted for a company called InnoVision up until a few years ago. But it went broke.

### Quadrupole and parametric amplifiers

Morton:

We're skipping around a bit, but there are a few things I want to make sure we cover. Unless I'm mistaken, the IEEE singled out your work on the quadrupole amplifier when they gave you the Fellow award.

No, I think the award was for my work on parametric amplifiers in general. The quadrupole was specific - a tube operating on parametric principles. But I did some rather general work also on the parametric principle. It was in the late fifties along with Hugh Heffner - a greatly different person than the Hugh Hefner of Playboy fame. My colleague Hugh was a very talented Professor at Stanford. He took a leave to become Deputy Science Advisor for the Johnson Administration and afterwards came back to serve as Stanford's Provost. He and I had been students at Stanford at the same time and I knew him very well. We wrote a number of papers together. Several of my first papers were written with him.

Morton:

Would you summarize the history of the parametric amplifier and your place in the bigger picture?

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The principle of parametric amplification was by no means new at the time in the mid 1950's that it emerged for research scrutiny. Lord Rayleigh more than 50 years earlier showed that oscillations could be sustained in a mechanical resonant system by the energy extracted from a source driving an energy storage element. And several decades before that, F. Melde in 1860 became the first investigator to describe and analyze in detail a system in which oscillations were excited parametrically. It consisted of a vibrating string, one end of which was attached to a vibrating tuning fork. Under the right conditions a motionless string can easily be caused to build up from rest into vigorous oscillations, the energy for the buildup coming from the tuning fork. The principles were simply rediscovered in the 1950's.

What was my place in the bigger picture? Perhaps I can best tell you by referring to the papers I wrote. In 1958 I published seven papers with co-authors, all involving parametric amplification. The first with L. D. Buchmiller entitled "Pumping to Extend Traveling-Wave Tube Frequency Range" was in the Proceedings of the IRE (later to become the IEEE) in July, 1958. Five others were with Hugh Heffner and the remaining one was with Bob Adler and George Hrbek. The best paper of this group - certainly the one that got the most attention - was one with Heffner published in the Journal of Applied Physics and called "Gain, Band Width, and Noise Characteristics of the Variable-Parameter Amplifier." Perhaps that one got me the Fellow Award from the IEEE in '63.

Marvin Chodorow told me at a conference in '62, "I've got to apologize. You were proposed for a Fellow Award, and I didn't get my papers of support in on time. It will have to be next year." And the award did come the next year.

Morton:

Why did that particular paper get so much attention?

Well, as the title implies, Hugh and I worked out the general expressions for gain, band width and noise figure for these kinds of amplifiers and also for parametric frequency converters. Gain, band width and noise were important features for parametric amplification and frequency conversion. The paper was later included in a prominent reprint volume, Selected Papers on Semiconductor Microwave Electronics. Hugh and I were not the inventors of the two-tank equivalent circuit we were analyzing. I believe people at Bell Labs may have first proposed it.

Morton:

Almost everything was coming out of Bell Labs at that time.

Yes. They were good, weren't they? They were the best, with Stanford second and MIT third, in my opinion. Stanford was about as good as Bell Labs. We jumped into research on parametric amplification as soon as we heard about it - Hugh Heffner in particular.

A parametric amplifier is used with a high-frequency pump. The pump for example can be a traveling wave tube or other means for providing energy at a higher frequency than that of the signal being amplified. In the case of Melde's tuning fork and string, the tuning fork is the pump and the string's vibrations represent the signal being amplified. The tuning-fork frequency is just twice that of the frequency of the string's vibrations. Another example is that of pumping up in a playground swing. Children use parametric amplification all the time. They pump a swing up into vigorous motion by standing on the swing and pushing with their feet. Whenever the swing goes through its straight up-and-down position they push hard against it; and whenever the swing reaches either of its maximum excursions, they diminish to zero their pushing effort. We've all done it as children. The feet provide the pump energy for the swing and the pushing frequency is just twice that of the swing motion.

In our paper, Hugh and I considered a two-tank electronic circuit coupled together by a time-varying capacitor operated by the pump. Thus there were three frequencies involved: one each corresponding to the resonance of the two tanks and the third, that of the pump. Under the right conditions energy being furnished by the pump will amplify an input signal fed into one of the two tanks. The resonant frequency of the other tank is called the idler frequency and must be the difference between the pump frequency and the input signal frequency.

Morton:

It sounds a bit like a laser.

Yes, it is something like a laser. That might explain why it was scrutinized so closely at Bell Labs, because the laser people there were very good. But the principle of operation of the parametric amplifier does not involve quantum mechanical phenomena in any essential way and therefore the amplification process can be explained in terms of classical circuit concepts.

In the case of my particular tube, I don't use classical circuit concepts to explain it. Instead I use classical principles of electron motion. For experts on free electron beams, a few sentences are more or less sufficient to explain the whole thing. An electron gun produces a beam which flows through an input coupler, an amplifying region and an output coupler. The beam is immersed in a uniform magnetic field and flows parallel to the flux lines of the field. The frequency of the signal to be amplified is roughly the same as the cyclotron frequency of the electrons in the beam. The two couplers interact with the fast cyclotron wave only. The input coupler extracts the fast-wave noise in the beam and inserts the input signal at the same time. The beam then passes into the amplifying region which consists of a quadrupole structure fed by a pumping signal. The action of the quadrupole field is parametric in nature, modulating the beam at the idler frequency and amplifying the input signal. The output structure extracts only the amplified input signal. A cross section of the quadrupole structure shows two sets of parallel plates oriented at right angles with respect to each other. The pumping energy is supplied by a voltage placed across each of the two sets simultaneously. The electrons in the beam follow a spiral path which grows with distance of travel in the quadrupole structure. For the proper geometry, high gain and low noise for the signal to be amplified is rather easily obtained..

Morton:

A kind of corkscrew effect for the electrons.

Yes, a corkscrew trajectory for each electron. But eventually, at the end of the quadrupole structure, the surfaces generated by the motion of the electrons as they pass through the output coupler are simple cylinders produced by straight horizontal rotating lines of electrons.

What I figured out in my invention was how to quiet the beam. Theoretically the noise is cooled down by a factor that is equal to the ratio of the signal frequency to the pump frequency. Assume we start with a noise cathode temperature of 1000oK. We want to cool it down to 30oso we divide 30 into 1000 to get a factor of 33. Thus we make the signal frequency to be 1/33rd of the pump frequency. The beam will then be cooled down by that factor. It could, of course, be cooled down by a bigger factor by choosing a larger frequency ratio. But a factor of 33, I think, is what was chosen for the tube at the Arecibo Observatory - and that was pretty good. Thirty degrees was by far the lowest noise microwave tube ever built.

Morton:

Why are they called parametric amplifiers?

I'm not sure. In the 1958 paper with Heffner, referred to above, we called them "variable-parameter amplifiers." Perhaps that is a better name because, as we explained it, they operate through periodic variations of a circuit parameter such as a capacitor or inductor. The more common name though is "parametric amplifiers" and that is the name we mostly used in our other papers. A 1959 paper with Adler and Hrbek was entitled "The Quadrupole Amplifier, a Low-Noise Parametric Device" (Proceedings of the IRE). An earlier paper with my same two co-authors was "Low Noise Electron Beam Parametric Amplifier" (Proceedings of the IRE.) Another paper was "Gain, Bandwidth and Noise in a Cavity Type Parametric Amplifier Using the Electron Beam" by Hugh Heffner and myself (Journal of Electronics and Control, 1958).

Morton:

That was another tube embodiment.

Yes. I still had tubes on my mind, along with solid state, until about '65. It was then that I published "A Look at the Future of Tubes" (1965 IEEE International Convention Record). I had been assigned to examine this question and said some things that, surprisingly, are still more or less correct. For example, after pointing out the severe competition solid-state devices were then giving electron tubes, I said in effect that "inroads will continue" but that "where high power is needed, tubes may be expected to maintain dominance." I even mentioned continued usefulness in display, cooking, microwave amplification for satellites, and particle acceleration.

### Publications

Morton:

When did you finally abandon the tube area?

I think that my "look at the future" was my swan-song as far as tubes were concerned. I published a couple of hundred papers after that and am still publishing, but none on tubes. The paper was written while I was still at Cornell and I went to UCSB a year later. I did not get back into tubes at UCSB.

Morton:

Were you still working on devices after that time?

Oh yes. But my work was not in tubes nor did it always involve electrons. I was still editor of the IEEE Transactions on Electron Devices. The topics of some of my papers after that might have been suitable for that magazine, but I almost always submitted my papers elsewhere. As an editor I did not want to be processing one of my own papers.

Morton:

Were you working on new acoustic devices or were you more on the theoretical side of it?

What I mostly did was more practical than theoretical. My consulting for Zenith was a big factor in my choice of research subjects because I loved working with Bob Adler and his research department. Zenith had to be practical. Bob and others at Zenith influenced my thinking a lot. But it's true that some of my work was theoretical. For example, I published a paper on "Plane-Wave Approach to Fresnel & Fraunhofer Diffraction" (IEEE Transactions on Sonics and Ultrasonics, Jan.,1968).

### Bragg-diffraction imager; holography

Morton:

You mentioned the microscope project. That is more an application than a device in the sense of a tube.

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I look at it as both an application and a device. Maybe it could be called a system also. It has a number of separate parts, that's true. But so does the quadrupole amplifier with its electron gun, special input and output couplers, and the quadrupole structure. It had to be invented and thought of as a whole just as did the scanning laser acoustic microscope. This was true also of the so-called Bragg-diffraction imager. I think we can put all three of these pieces of equipment in the "device" category. Webster says a device is a mechanism designed to perform a special function.

I remember thinking of myself as the inventor of the Bragg-diffraction imager. But as it turned out, I was not the first to think of the idea. Do you know Art Korpel? He beat me to it. He is presently a Professor at Iowa. He was a very productive Zenith worker in their research department. From Holland, he went to Zenith in the 60's and when he finished writing his thesis he sent it back to Holland. It was on Bragg-diffraction imaging.

The first article I wrote on the subject was with two UCSB students, Landry and de Souza, in a '67 paper "Acoustic Transparencies for Optical Imaging and Ultrasonic Diffraction." The paper was given at the First International Symposium on Acoustical Holography. By then Art's paper had already come out on "Visualization of the Cross-section of a Sound Beam by Bragg-diffraction of Light" (in Appl. Phys. Lett.) But, as I recall, I had a Zenith patent notebook in which I wrote up the idea in August, 1966 and sent it off to Bob Adler. He showed it to Art Korpel, who wrote me back with photocopies of his invention. It was exactly the same as mine but done two or three months earlier.

Others also came up with much the same idea. H. V. Hance, J. K. Parks and C. S. Tsai of Lockheed Research Laboratories did closely related work publishing their paper in March, 1967 on "Optical Imaging of a Complex Ultrasonic Field by Diffraction of a Laser Beam" in Journal of Applied Physics. In addition, three or four decades earlier, S. J. Sokolov of Russia used a bulk system strikingly comparable in overall appearance to that of the Bragg systems at Zenith, UCSB and Lockheed. He used it to detect flaws in metal test pieces. Is it fair to draw the conclusion from all of this that "great minds run along similar lines"?

Another thing that came along for use in acoustics was holography. In 1969 I published a paper with students M. Wollman and R. Smith entitled "Acoustic Holographic System for Underwater Search" (Proceedings of the IEEE). Then a year later with two other students (Landry and Powers) I published "Computed Reconstructions from Phase-only and Amplitude-Only Holograms" and "Image Distortion in Reconstructions from Phase-Only Holograms" in Acoustical Holography and Acustica respectively. My students and I continued to do a lot of publishing on this subject and in 1971 I was chair of the Fourth International Symposium on Acoustical Holography held here in Santa Barbara. By then I had published well over seventy papers in referred journals and volumes, about twenty of which on various ultrasonic issues and instruments. There were more papers on Bragg-diffraction imaging, acoustical holography and tomography, optical heterodyne detection of acoustical fields, noise and threshold contrast in various acoustical systems, computer enhancement of acoustic images, acoustic lenses, opto-acoustic transducers, speckle and sensitivity, etc. The list is long.

### IRE and IEEE activities, publications

Morton:

Let's shift gears a bit. We haven't really talked about your IRE and IEEE involvement.

I have enjoyed a lot of involvement, almost all of it pleasant. I particularly liked the editorial activity. I was Editor of the Transactions on Electron Devices for ten years from 1961 to 1971, one of two founding Editors of the Journal of Quantum Electronics for two years from '63 to '65 and Editor of the Proceedings of the IEEE from '77 to '80. The other founding Editor of the Journal was Bob Kingston. Both of us had experience in editing and Karl Willenbrock, a member of the IEEE Board of Directors, needed two experienced people to get it going. There was no IEEE Group or Society at that time to start things off.

Later I became a member of the IEEE Executive Committee, and therefore automatically a member of the Board of Directors from 1969 to '70. That was fun too. The Board of Directors tends to hold meetings in glamorous places, such as Puerto Rico, Monterrey, CA and Hawaii. I enjoyed going those kinds of places. It was great also to work with dedicated people in the IEEE.

The leaders tended though - and maybe still do - to pick people just from the East Coast for the Executive Committee, other committees and boards. I was living on the West Coast at the time, but I was one of the few that did live that far away. I'm sure they made an exception in picking me because I had done a lot of things for the IEEE while in the East.

I had a lot of extremely pleasant travel. Generally the meetings were only one day long. I usually would arrive the night before, stay a full day, and then leave the third day and enjoy the trip. There were a lot of things in New York I enjoyed seeing. I guess New York is still a nice place, isn't it? Do you enjoy it?

Morton:

It's gotten nicer. I think the seventies and eighties were a low point.

I never went there on a trip that I didn't like. I guess there was a crime problem there, but I didn't see any signs of it. I recall one time visiting in New York at night, taking the A Train and getting off at some street near Columbia University. I was walking along a street and a taxicab driver drove up alongside and said, "You ought not to be walking down this street." When I asked why, he said, "Well, I wouldn't if I were you." He meant that I was in some danger, but I never saw anything I thought was dangerous. I jogged a lot in Central Park early in the morning and met all kinds of people there, but never saw anyone that looked like he might want to harm me.

Morton:

Do you remember if there was any talk in the sixties or seventies about whether the IEEE should move to the West Coast?

I never heard any talk like that in '69 or '70, which would be the two years I would have heard something about it. At that time they did moved part of the operation to New Jersey.

Morton:

Piscataway.

Right. That move was recommended by Ray Sears. He was very good at championing his point of view.

Morton:

You edited the Transactions for many years. What kind of perspective did that give you of the field?

It gave me a priceless perspective. I could easily keep abreast of what was going on in electron devices. I enjoyed and benefited from all the editing I did, and I'm still somewhat of an editor. Years ago I was a founding Editor of Wiley's International Journal of Imaging Systems and Technology and continue to serve as one of its Associate Editors. I've got a submitted paper here in my desk right now that I am going to assign myself to review as one of the referees. I like to do that sort of thing. It allows me to keep aware of new things coming along.

The Transactions on Electron Devices was one of the better Transactions in the IEEE. I no longer subscribe to it because I am no longer in that field. It started out quarterly and then moved to monthly. The papers in it were first-rate. We turned down a lot of papers - quite a bit more than we accepted. One out of every ten papers reviewed was accepted - something like that. That's still the case, isn't it?

Morton:

Yes, I think so.

The opportunity to read good papers was the main thing I enjoyed about it. I did not enjoy the Journal of Quantum Electronics as much. It was just starting up and there was much to do that did not concern the question of good papers. I think it's a first-rate journal now.

A few decades ago, one Transactions Editor could not be persuaded to appoint reviewers. I always had three reviewers.

Morton:

The editor did his own reviewing every time?

I'm not sure he did much reviewing. He simply published almost everything that was submitted. Some were good papers and some weren't. The IEEE has always had high standards for the papers it publishes. It had excellent editors. J.R. Pierce was once a Proceedings Editor, as was Mac van Valkenberg. In fact, J. R. Pierce turned down one of the papers Hugh Heffner and I wrote. It was the one I've talked about that we regarded as our best. Afterwards, when we sent it to the Journal of Applied Physics they accepted it immediately.

Morton:

Why do you think Pierce turned it down?

He thought we didn't have anything in it that hadn't already been published by the RCA people. However we really had a lot that was new. That particular paper was re-published in a reprint volume a few years later as I mentioned previously.

Morton:

Speaking of your experience as an editor, did you ever get the sense that the Electron Devices Society was going to disintegrate because of the proliferation of new types of electron devices?

I didn't ever get that feeling, but I think that issue arose later. I wasn't in that area after '72 or '73 and stopped subscribing to the ED transactions about that time. I always thought that Electron Devices was a marvelous society. It was one of the larger societies. Is it still?

Morton:

Very much so, though not the largest. Was any effort made to define the scope of the magazine so that it included certain things and not others?

Not that I know of. I believe that at the time I was associated with it there was not a lot of competition for its papers from other periodicals. The Journal of Quantum Electronics might have been a competitor, but in those days it really was not. It's possible to imagine one electron device journal dealing more or less with practical questions and another, with more or less theoretical questions. I don't know if such a division would make much sense.

Morton:

That's interesting. I guess it hadn't gotten to the stage where there were so many different communities working with devices that separate journals were needed.

I can't recall any problems of that kind actually developing in those days. The IEEE was trying to expand, and quantum electronics, for example, was an area into which Director Karl Willenbrock helped make it expand. It's still expanding, isn't it? How many Transactions are there now?

Morton:

There are thirty-six different societies.

Some of them have more than one magazine.

Morton:

Yes. I've lost track of the number.

The Computer Society probably has a lot, doesn't it?

Morton:

Yes, it does.

I used to be a member of the Editorial Board of the IEEE Press. I was Editor of the Special Issues Series. It was a well defined job - to encourage and take care of reprint volumes. I even served as an editor of a couple of reprint volumes in my own special issue series to move things along - one volume was on Modern Acoustic Imaging and the other, on Imaging Technology. Hua Lee was first listed Editor in both.

Morton:

Hua Lee?

Yes. Lee and Wade are listed as the Editors. Hua is currently the head of our ECE Department at UCSB. In my opinion these two and many other such volumes of the IEEE Press are very good.

I took the Lee and Wade volumes with me to the University of Guanajuato, Mexico, where I currently serve on occasion as a visiting professor and used them both. My students, although native Spanish speakers, read enough English to like them and get a lot out of them.

The nice thing about research is that it really moves. It's exciting to try to keep up with it. The Special Issue Series helps a lot in providing the background for current research..

Morton:

It's exciting but a little frustrating too.

Yes. But that is what engenders the new research. Problems can always be found that are interesting to look into carefully to see if one can make improvements. This is why the IEEE literature is important. I use it a lot. I'm still publishing papers with students. And my students are still writing PhD theses. Both in Mexico and here at UCSB.

I'll tell you how we're making one of my PhD students work hard. He's late in submitting his thesis, which is very standard. He has reached the point where we fined him one penny for a special kitty when the thesis wasn't completed a couple weeks ago. When it still wasn't done last week, we fined him two pennies. Then if and when it still isn't done this week, it will be four pennies. Etcetera.

Morton:

That will get expensive pretty quickly.

After one year of that sort of thing, Bush or Gore will be able to take the kitty and pay back the national debt.

I am presently writing an article on science and religion. My student makes me do a somewhat similar thing. I don't owe the kitty anything yet, but that article gets bigger and bigger as I write. A few weeks ago it was going to be four or five pages, and now it's up to fifteen pages and still growing. I'll have to put in a penny unless I can show my research group progress toward finishing each week. I'll start with a penny; then it will be two, and so forth.

### University of California at Santa Barbara

Morton:

I saw a teaching award you received from the University of California at Santa Barbara (UCSB).

The professors in the Academic Senate pick the Professor of the Year. They started giving this award in 1977 and I got the first one. Later the students also started giving a Professor of the Year award and I got one in 1988 or thereabouts. It was the year I taught a big sophomore class.

Morton:

You've been here quite a while. How has the engineering program at Santa Barbara developed over the years?

Marvelously. It's a lot better than it was in the old days when I first came here. The new guys are very good.

Morton:

What particular areas do they work in?

The best area in engineering, one that many of our electrical and computer engineering faculty members are in, is materials science. That's important work and our deans and department heads over the years have persuaded some of the most qualified people in the world to come here. Santa Barbara is a nice place to come to. The work is interdisciplinary. These engineers work closely with the physicists.

The Physics Department is absolutely outstanding. Walter Kohn was a Nobel winner last year. There will be others in the future. Our Jim Langer is currently President of the American Physical Society. I got a group e-mail from him this morning telling us how to get in touch with our representatives in Congress to ask them to vote for large appropriations of money for science.

Morton:

Did you ever collaborate with any of the materials people?

No, but I was instrumental in persuading one of our superstars, Herb Kroemer, to come here from Boulder, Colorado a few years ago. Also I did collaborate with the Physics people. I had a fine office and lab space in the Physics building as well as in the ECE building. I always had a lot of room before I became emeritus. The office I have at present is good, but nowhere near as roomy as before. Unfortunately I have a lot of books sitting on top of file cabinets, etc. My office isn't big enough for more book cases. I'm giving many old books away, but it takes a while to determine which books I can spare. An old book is like an old friend.

### Teaching strategies

Morton:

Getting back to your teaching, were you involved more on the teaching side of things or research? You mentioned you had a lot of graduate students. It sounds like you may have been their advisor and they may have been involved in research.

Around here it's supposed to be half-time teaching and half-time research. Both involve interaction with students. Most of my classes and my work at Stanford, Cornell and UCSB has been with graduate students. I used to teach a graduate class in a funny way. I don't think what I did ever caught on with other teachers but I liked the approach very much. I started it in 1977 and I think it was my approach that got me the first Professor of the Year award. I thought it was especially effective for graduate students and I taught that way for years. To me my method of teaching was innovative and good. It worked fine.

Morton:

What sort of course were you teaching?

It was a course on "imaging." I recorded my lectures on tapes that were passed out to the class each week. It was an evening class that met once a week for four hours. Instead of lecturing the students all that time, I gave copies of the tapes to each member of the class so that each could listen to the lectures over the week. There were usually 15 to 25 students in the class. I always told them if they wanted to speak with me about anything on the tape I was available at any time, day or night. They had my telephone numbers, office and home.

I got the "day or night" idea from Jim Mulligan, former President of the IEEE. He is now a Professor at UC Irvine. He was a great man and was President when I was on the Executive Committee and Board of Directors. He traveled all over the world but he frequently needed to call committee members. He called me at strange times sometimes after midnight, but I was always ready for his call. Remembering Jim, I told my students that they could call me anytime. Some of them took advantage of that and would do it at odd hours, but most of them didn't.

Morton:

id you get any midnight phone calls?

Yes. I got a few, but not too often from students. I think I got more at midnight from Jim Mulligan than from all my students combined.

In our four-hour class we worked every assigned problem and discussed any and all of the student's questions, with strong student participation. I also invited guest speakers on pertinent topics. There are a lot of good speakers located in the vicinity. I still taught that way the last time I taught that course in about 1993. I did not do that in Mexico, where I have taught virtually the same course three times in Spanish. But I did it that way here a number of times.

Morton:

You're not retired yet.

No. I don't use that word. I am Emeritus, but I'm not retired. The biggest part of my career is still to come.

### Career highlights

Morton:

The IEEE gave you its Fellow status based on particular work you have done. Do you consider that to be the biggest thing you have done thus far?

It was close. I think we wrote a good paper. In those days people didn't have photocopy machines so handy as nowadays to use for copying papers. I got 500 copies of that paper from the Journal of Applied Physics and gave almost all of them away as the result of requests. Now my supply is down to two or three.

Morton:

That's a lot of requests.

Nowadays people simply photocopy papers from the journals.

Morton:

What has been your biggest career-related frustration or disappointment as an engineer or as a professor?

I think it was the administrative activity. I wouldn't say that I was frustrated, but it was not as much fun. I have avoided that here at UCSB. I got my fill at Cornell and at Raytheon. I much prefer working with students - particularly now. My present students now are two or three generations younger than I am. I really enjoy them because they are so young. An older person likes to be around young people.

Morton:

You strike me as a fellow who would be a good administrator because you are a people person.

I didn't like it. I don't think I was good at Cornell. Cornell is a place where they seem to enjoy a lot of altercation. They did in those days, and I hear that it may still be the case. There was quite a bit more contention at Cornell than I ever saw at Stanford or here. You may know Sanjit Mitra. He was at Cornell when I was there. I liked him. I thought he was great. He did a lot of things for the IEEE. Back in the sixties he was teaching IEEE courses with no remuneration. He still does a lot of things for other people. He is a very good scientist and last year got the M. E. van Valkenberg Award from the Circuit Theory Society. It's their biggest award. Mac died a couple of years ago, and they made an award in his honor. People at Cornell gave Sanjit a lot of trouble. There was wrangling between them. Some of the professors didn't like him. I don't think they liked me very well either.

Morton:

Were these political fights, such as competing for the same things?

I don't think so, but I really don't know what the fighting was all about. Perhaps it's just that humans enjoy a certain amount of rancor. Bob Huttenback, our former Chancellor, used to say, "The reason there is so much fighting at universities is that the stakes are so low."

I don't know why they made life difficult for Sanjit. As far as I could see the stakes were zero. Why would they want to get rid of a star? They essentially kicked him out. He could have stayed, but they made it so miserable for him that he left. Why did they do this? I don't know. He went to the University of Maryland for awhile and then came west.

Carl Sagan was also at Cornell, and I think he also was given a lot of guff. Tremendous talent. I guess Carl was the best popularizer of science the world has ever known. Richard Feynman was a professor there. He was marvelous. He left for Cal Tech and got a Nobel Prize in Physics. Cornell had then, and maybe still has, a reputation for lots of bickering.

I've preferred being a professor over being an administrator. A professor can build his own little empire. My group is pretty small nowadays, but at one time I had thirty people including foreign scholars. We had a lot of fun, were congenial with each other and published a lot of papers. I have about 300 papers, mostly co-authored, as a result of having a big group of students and colleagues.

### Visiting teaching appointments

Morton:

You mention students in Mexico. Do you enjoy teaching elsewhere?