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Oral History Transcript — Dr. Edward Purcell

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Interview with Dr. Edward Purcell
By Paul Henriksen
June 29, 1982

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Edward Purcell; June 29, 1982

ABSTRACT: Work at Massachusetts Institute of Technology Radiation Laboratory. Significant contributions to solid state physics. The interview deals briefly with Purcell's early acquaintance with Karl Lark-Horovitz at Purdue, while Purcell was an undergraduate electrical engineering major there. The bulk of the interview concerns Purcell's work at the MIT Rad Lab during World War II: how he started working there in 1940, his work with the magnetron group, the problems with the transmit receive switch, theory vs. experiment at the Rad Lab, the Steering Committee, work with the propagation group, the 1.25 cm radar fiasco, design of permanent magnets, postwar applications of Rad Lab research, and relations with British engineers. Purcell then discusses the influence of the Rad Lab on his subsequent career, his work with R. V. Pound, and H. C. Torrey at Harvard on NMR and the 21 cm line of hydrogen, and postwar uses of microwaves for physics research.


Undergraduate in Electrical Engineering at Purdue. LH had just come to Purdue to a zero department and quickly made it into a research place. Wanted to take a course in independent physics research. Got hooked on physics at that time. Worked with Walerstein and later Hugh Yearian. Yearian was not a PhD yet but was extremely experienced. Worked with Yearian in his senior year. Did not have to report directly to Lark-Horovitz. "The Lark ran it as sort of a European style with him at the top of the pyramid and gave orders to everybody down the way." First published paper was with Lark-Horovitz on making thin films for electron diffraction. Carried out LH idea. Graduated in electrical engineering. Changed to physics, LH helped him get an exchange scholarship to Germany. Sure that LH letter helped him get into Harvard. Purdue was an interesting place at that time. LH was giving courses in topics that were in the very forefront of experimental physics at that time. Very quick with a lot of things, but he wouldn’t write things up and publish them. Doesn't know why. He was very imaginative, very quick and impatient with the graduate students. LH went to Berlin to visit in 1933 and talked to a man named Rupp. Story of LH and the making of thin films on rock salt. Illustrates LH desire to try new things even if they were possibly wrong. Some of the graduate students lived in the basement.

Purcell would go to Walerstein or Yearian with questions. LH had the intellectual tradition of a liberal Viennese Jew. Loved music. Made a little island of Central European Judaism in West Lafayette Indiana.

PhD in 1938. Then helped build the Harvard cyclotron. Did not do any nuclear physics experiments with it. Wanted to try an air core betatron.

Began at RL in Dec. of 1940. People at Harvard were already there. Bainbridge and Rabi were involved. Asked Purcell to come up to a meeting at Harvard. Purcell worked there part time until the term was over and then went full time in January. Lawrence and some people at MIT were involved in setting it up.

First work was in the magnetron group along with Ramsey. Does not recall if he had a choice in where to work. Were magnetron, receiver, pulser and antenna groups at that time. First assignment for RL was to make an airborn 10 cm radar for a night fighter. Had to make a pulser. Just tested magnetrons, did not have the facilities for making them. The magnetrons came from Bell. Then came the problem of the Transmit-Receive switch. [It was very desirable from the standpoint of limitations on size of radar sets for airplanes that the sets use the same antenna for sending and receiving. But the powerful outgoing signal would burn out the sensitive crystal rectifier if it was in the circuit when the pulse went out. They needed a switch that would allow the receiving circuit to be switched in and out in fractions of a second. They solved the problem by developing a gas discharge tube that would arc and therefore short out part of the circuit.] Zacharias was in the antenna group at the beginning. Lists other people who were there.

All in two big rooms, but in a well-defined group. Reported to Rabi. Spent two years in the roof building in Group 41 Fundamental Developments. Ramsey was first leader of 41, then Purcell took over. Also involved with Propagation, head of the group for a while. Purcell's group was doing 3 cm. and later K band. Did much more basic research, and being a free wheeling group, when they made models of 3 cm work they were free to tryout ideas. Had Stewart Foster in his group. Brilliant inventor, but couldn't fit in with the antenna group. Description of Group under Otto Halpern. Purcell's group was a very small and informal group. Had Dicke and Berringer in the group. Let them do what they wanted. Story of mortar shell tracking by radar and the man who founded Tektronix.

Oscilloscopes developed during the war. Needed faster phosphors. British started making the oscilloscope tubes with a flat face. Thinks the British did the first work on dark-trace tubes: the "dark face tube and the flat face tube." Remembers during his trip to England in 1942, that he went to Bristol to see a man named Sutton who was a professor at Bristol and had been doing defense work in his lab. He was making flat face tubes very simply. He had lot of high school kids trained as technicians to make these tubes. Purcell commented on his return to the Rad Lab, how simple it seemed for Sutton to make these tubes. Dark tubes were made with the halides and erased with infra-red. Project never did succeed or turn out to be important.

Fiasco in Purcell's group. 3 cm airborn interceptor conical scanner for aircraft was too complicated to work.

Needed a gyrator circulator. Needed an insulating ferromagnet. Stuck with reciprocity theorem during the war. Solid state thing that came too late. More important development was permanent magnetic material.

In 1940 knew the magnetron needed a permanent magnet for field units. Art of designing permanent magnets was entirely unknown. Bainbridge came across an article in the Philips Technical Journal in 1939 or 40 that told all you need to know about permanent magnets. Then at Bell Labs with Bozorth more was done. Gradually AlNiCo 5 became available. Needed stronger fields for shorter wavelength.

Screwdriver effect. Screwdrivers touching the magnet would demagnetize the alnico. Modern position in magnetic material came during the war. Also didn't have polyethyline insulation in coax cable.

British even started that. First plants for polyethyline at that time. At that time had to use beads to insulate the coax cable. Microwaves for physics research. Microwave spectroscopy, paramagnetic resonance. Nuclear magnetic resonance work was done at 30 Mc because that was the radar frequency and the equipment was available. Microwave ovens also, developed after the war, by Raytheon who was making magnetrons in large quantities. Problems with microwave ovens and mode stirrers. Came directly from their terrible mistake about K band, which they discovered when they tried to measure water vapor absorption. [Water vapor absorbs strongly at 1.25 cm, thus rendering the K band radar useless when water was present.] Dicke tried to make measurements of the absorption with his radiometer.

Applications of Rad Lab work. Precision time measurement came from the Loran and Shoran work. Gas discharge tubes for the TR problem. Understood microwave circuits very well after three years, also magic tees. Harp project under Otto Halpern.

Manufacture of magnetrons and tubes advanced. Advances were in manufacturing practices for putting hard vacuum devices together. Only U.S. made metal tubes and double tubes. British never made double triodes or anything. They always made the tubes out of glass. There used to be good natured fights between British and Americans. Over who had the fewest tubes in their receivers. British counted American double tubes as two tubes, or valves. Other differences in relations with British industry. Rad Lab people were used to dealing with places like Bell Labs, Raytheon etc. Rad Lab physicists on an even basis with the Bell Labs people. Purcell's opposite number in England was a university scientist who had to deal with GEC Wembley for example. University scientist treated the British manufacturers like tradesmen. "One felt this sort of social difference." Didn't have the mutual give and take due to the way British industry was set up. British engineers were not trained at a place like MIT; they had come up as apprentices in industry. Severe handicap for them.

Pulse forming line was simultaneous invention. And rectifiers. Skinner discovered tapping.

Working at the Rad Lab was the foundation of Purcell's career. Everything he did after came from the RL. Had Bob Dicke in his group inventing the radiometer, forced by the k band problem to investigate the absorption at resonance of water vapor. Thrown together with Ramsey and Zacharias who had worked with Rabi in his molecular beam lab before the war. Rabi was his superior. Learned about molecular beam resonance, and signal to noise, so when deciding after the war which experiments to do with the new equipment, they knew if they could do them or not.

So in the NMR, done at the Rad Lab yet, they could calculate how big the signal should be. And about saturation on the 21 cm experiment on hydrogen. First radio astronomy experiment used a plywood horn that he knew how to build. Had a very sensitive receiver in the room. Knew how everything should turn out, except if they would see the 21 cm line. "My whole life as a physicist, basically, traces to that." First nuclear resonance was done by Pound and Torrey. Pound had a B.A. from Buffalo, knew more about microwave amplifiers than anyone else. Had a parametric amplifier working on his bench. Torrey looked out for the Purdue contract. Torrey had worked with Rabi.

Work style at the Rad Lab. Memories of England in 1942.

Torrey was in group 53. Purcell had no subcontractors of its own. Reflex klystron tubes at Oxford and Raytheon. Informal contacts with industry was getting out of hand. Steering Committee tried to make the scientists stop by decreeing that all letters to Bell Labs had to be from DuBridge to Bown. So they wrote letters and signed DuBridge's name to them. Military relations were good on the whole.

New people were assigned to groups by Wheeler Loomis. Bob Dicke was assigned to Purcell's group. Spent his first afternoon there trying to tune a local oscillator. Lab Policy on fundamental research? Didn't need a policy. People concerned with the job they had to do, but they were mostly physicists, who knew something might be found by accident. Vic Neher made the first I cm local oscillator. Guy Stever, later science advisor to Nixon, was Neher's assistant. He used the 1 cm to look at absorption of ammonia in a waveguide. Rabi didn't think some of the propagation work was useful, but it turned that anomalous propagation was important to the Navy.