PURCELL, EDWARD MILLS

(b. Taylorville, Illinois, 30 August 1912; d. Cambridge, Massachusetts, 7 March 1997), physics, nuclear magnetism, radio astronomy, astrophysics, biological physics, physics education.

Purcell was a complete physics professor. Through his research he helped originate nuclear magnetic resonance and radio astronomy. He won the Nobel Prize for Physics in 1952 for his magnetic resonance work. He was a gifted teacher, and throughout his professional career he devoted large blocks of time to his teaching responsibilities. He was active on his campus, in the physics community, and he served his nation in a variety of high-level advisory positions. He was a rare kind of man: he had no enemies, but an Abūndance of friends and admirers.

Early Influences Purcell was born in the small farming town of Taylorville located in the middle of Illinois. His father, Edward A. Purcell, was raised on a farm and his education was limited. He was the manager of the independent local telephone company, and his son had access to the backroom of the telephone office where technical equipment was kept, as well as to the basement where discarded equipment was available. This treasure trove provided Purcell a ready source of wire, magnets, and other electrical components. His mother, Elizabeth Mills Purcell, a Vassar graduate with a Master of Arts degree in classics, taught high school Latin. Purcell’s mother brought many books into the home and Edward took advantage of their presence. Literature and books were a familiar part of his childhood.

When Purcell was fifteen, his family moved sixty miles east to the small town of Mattoon where his father became the general manager of the Illinois Southeastern Telephone Company. The Illinois Southeastern, although not part of the Bell system, received the Bell System Technical Journal—which, because no one read it, Purcell’s father brought home. This provided young Purcell, as he said in an interview with Katherine Sopka of the American Institute of Physics, “a glimpse into some kind of wonderful world where electricity and mathematics and engineering and nice diagrams all came together.”

Purcell’s future as an experimental physicist was portended in his high school physics class. He was given a problem: a rope ran from a seat up over a pulley at the top of a flagpole and back down to the seat. How hard would a man have to pull in order to move himself up then flagpole? Purcell and a classmate solved the problem and got the answer that he would have to pull with half his weight. The physics teacher, on the authority of the textbook, said since the pulley was fixed, the person would have to exert a pull equal to his weight. Purcell and his friend went home, hooked up a system of rope, pulley, and spring scales. Purcell, who weighed 120 pounds, started the experiment by pulling on the rope. When the scales reached 60 pounds, he started moving up. They ran back to the school to tell their teacher of their triumph.

Education Upon completion of high school in 1929, Purcell went to Purdue University where he majored in electrical engineering. During his student days at Purdue, Karl Lark-Horovitz, a Viennese physicist with an international reputation, became head of the Purdue Physics Department and ultimately transformed a nondescript physics program into a respected department. In his junior year, Purcell signed up for an independent laboratory course offered in physics. Isador Walerstein assigned Purcell the task of examining some atomic spectra. To do this, Purcell had to assemble a spectrometer based on a long-unused Rowland grating. Following this project, Purcell built an electrometer to measure the half-life of a radioactive sample.

As a senior, Purcell continued independent research with a graduate student, H. J. Yearian. Purcell did an electron diffraction experiment and in the first photographic emulsion he developed, he saw Debye-Scherrer rings formed by the diffracting electrons. These independent projects gave Purcell a decisive push towards physics.

Purcell graduated from Purdue in 1933 with a degree in electrical engineering. He remained at Purdue the summer following graduation, continued his electron-diffraction research and wrote his first two papers based on the this work. Then, with the advice and the help of Lark-Horovitz, Purcell received an exchange-student fellowship to the Technische Hochschule in Karlesruhe, Germany. During his year in Germany, Purcell took physics courses and a range of other courses simply to improve his German.

On board ship en route for Germany, Purcell met Beth C. Busser, who was also an exchange student. She was headed to Munich to study German literature. Four years later, Beth and Edward were married in Cambridge, Massachusetts.

Purcell entered Harvard in the fall of 1934 as a graduate student in physics. In addition to courses in physics, Purcell took a complex variable course in the Mathematics Department and a course in cosmology taught by Alfred North Whitehead. It was a physics course in electric and magnetic susceptibilities taught by J. H. Van Vleck, however, that represented a turning point for Purcell. As a term problem, Van Vleck asked Purcell and the only other student in the course, Malcolm Hebb, to do a theoretical analysis of cooling by adiabatic demagnetization. Purcell and Hebb co-authored a paper that was the first significant paper on magnetic cooling, and it became a highly cited paper in the physics literature. Purcell has acknowledged that he came back again and again to the physics he learned in this term problem.

After Purcell “talked himself out of” his first thesis project, and demonstrated that his second thesis project was “hopeless,” he then successfully completed a thesis under the direction of Kenneth T. Bainbridge. Bainbridge, a mass spectroscopist, was interested in focusing charged particles by means of electric and magnetic fields, and he suggested that Purcell consider the focusing properties of a spherical condenser (now called a capacitor). For his dissertation, Purcell studied this system experimentally and also analyzed it theoretically; he published his results in 1938.

When Purcell completed his dissertation, he moved seamlessly into the lowest-ranked faculty position at Harvard: instructor. In 1938, the cyclotron, first developed by Ernest O. Lawrence at the University of California seven years earlier, had become an important tool in the most prominent frontier of physics in the 1930s—nuclear physics. Bainbridge was building a cyclotron at Harvard and Purcell joined this effort; he developed methods to improve the homogeneity of the cyclotron magnets.

Wartime Work The Harvard cyclotron became operational in 1939, but its future was not destined to be an academic research tool. Two dire situations changed everything. First, war had started in Europe and the Germans were conducting bombing raids over England. This put a stop to Purcell’s Harvard research. Second, nuclear fission was discovered in 1939 in Germany, and it was quickly recognized that the nucleus held the potential for weapons of unprecedented destructive power. This ended the cyclotron’s stay at Harvard, as it was needed for wartime research. Purcell went to the Massachusetts Institute of Technology (MIT) Radiation Laboratory; the Harvard cyclotron was disassembled, packed up, and shipped to a new location: Los Alamos, New Mexico.

The Radiation Laboratory at MIT began in October 1940 with the initial mission to develop microwave radar systems based on the magnetron, which had been developed in England. At the invitation of Isidor Isaac Rabi and Bainbridge, Purcell joined the Rad Lab, as it came to be called, in early 1941. Purcell quickly proved himself and became the leader of Group 41, the Advanced Development Group.

The magnetron, once called the most valuable cargo ever brought to the shores of the United States, emitted relatively high power microwaves with a wavelength of 10 centimeters. This wavelength made possible radar systems with sizes appropriate for mounting in aircraft. However, the immediate goal of Rad Lab leaders was to push to shorter wavelengths so that radar systems could become even more compact. Purcell’s group was responsible for developing sources that emitted 3-centimeter microwaves and, later, 1.25-centimeter radiation.

In the testing phase of the newly developed 1.25-centimeter microwave sources in the spring of 1943, a curious and troubling thing happened. In a somewhat unpredictable fashion, the effective range of the 1.25-centimeter radar systems decreased considerably. It was soon recognized that the range of the radar beams decreased when the moisture content of the air was high. The explanation followed that water had two rotational quantum states separated perfectly to absorb the 1.25-centimeter radiation. Purcell did not know it at the time, but this experience would have a lasting influence on his future research.

The MIT Radiation Laboratory was closed when World War II ended in 1945; however, the work did not end for Purcell and a few others. Many electronic devices were invented and refined by the Rad Lab scientists during the war years. There were a few select industries, such as Bell Labs, that participated in the development. These microwave circuits were new and had the potential for many practical applications. Rabi, the associate director of the laboratory, ordered a book-writing program to put all the details of the government-sponsored radar work in the public domain so that everyone could have access to the information. Purcell, along with C. G. Montgomery and R. H. Dicke wrote Volume 8, Principles of Microwave Circuits(1948), of this twenty-seven-volume series. These books, “the Rad Lab series,” became valuable references after the war, published by several different publishers over the years. Reprints of the Rad Lab books continued to sell into the 1990s—an amazing life of at least fifty years.

Academic Research Purcell officially returned to Harvard in the fall of 1946. He returned as an associate professor of physics and was promoted to full professor in 1949. Purcell spent his entire professional academic career at Harvard University.

In his writing for the Rad Lab series, Purcell described the water absorption problem that he had encountered in 1943. As already stated, the radar beam was attenuated in moist air because two quantum states of water absorbed energy from the radar beam. Purcell liked this two-state quantum system, which he first encountered in his graduate work and again with the water-absorption problem. Purcell had also learned about Rabi’s prewar molecular beam work and his discovery of magnetic resonance.

As the postwar writing period progressed, Purcell and two other physicists writing Rad Lab books, Henry C. Torrey and Robert V. Pound, went to lunch together. On the way to lunch, Purcell asked Torrey, a former Rabi student, about doing a Rabi-type resonance experiment— only in a solid instead of a gas. Torrey was skeptical, but upon reflection thought it might be possible.

During the evenings and on weekends in the fall of 1945, preparations were made at Harvard for doing an experiment designed to detect nuclear magnetic resonance in bulk matter. Pound, an electronic genius, designed the basic electronic equipment. A magnet was borrowed from Harvard physicist J. C. Street—the same magnet Street had used to obtain a photograph of a cloud-chamber track of a muon, which provided powerful early evidence for this new elementary particle. With this illustrious magnet and relatively simple electronic equipment, the experiment was ready to go in December.

Purcell bought paraffin wax at a local grocery store on his way to the laboratory where the experiment was set up. The paraffin, rich in hydrogen, was the sample they decided to use. Calculations were done by Torrey to determine the combination of radio-frequency radiation and magnetic field strength at which a nuclear resonance might occur. The resonant cavity, filled with paraffin, was designed to resonate at 30 megahertz. With the radio frequency fixed at 30 megahertz, a member of the Purcell team slowly varied the magnet current over the range from 65 to 80 amperes, which bracketed Torrey’s calculated value. All the time they watched the output meter for evidence of a nuclear resonance absorption.

The experiment almost failed. On Thursday evening, December 13, Purcell, Pound, and Torrey swept back and forth through the magnetic field strength they had predicted would yield a resonance. They saw nothing. Early Friday morning, they stopped and planned to try again on Saturday afternoon.

When the Purcell team did their experiment, it was generally believed that the nuclear spin relaxation times would be very long. As Purcell said in his Nobel address, “this question [relaxation time] gave us much concern.”

The nuclear relaxation time is the time required for the nuclear spins to come to thermal equilibrium with their environment. If this time is very long, it could make the nuclear magnetic resonance absorption impossible to observe, because the two nuclear spin states would have equal populations and there would be no net absorption. This long relaxation time was how they explained the failure of their first experiment. So Purcell arrived at the laboratory very early Saturday morning in order to turn on the magnet and allow many hours for the sample to come to equilibrium in the magnetic field.

By late Saturday afternoon, after many attempts to see the evidence of absorption, they were about to admit defeat and go home. Before shutting down, however, Pound suggested that they crank the magnet up to full strength and slowly decrease the current through the magnet and thereby decrease the magnetic field strength. In Pound’s laboratory notebook, in Torrey’s writing, are the words, “Dec. 15, 1945, proton resonance found at 83 amperes.…” This magnet current was about 10 amperes larger than the expected resonance value of 73 amperes. In a letter dated 24 December 1946, they sent their results to the editor of Physical Review.

One month later, Felix Bloch and his group at Stanford, by a completely different experimental method, observed the proton resonance in a sample of water. Their methods were so conceptually different that when William Hansen, a member of Bloch’s group, visited Purcell at Harvard, Purcell recalled that they talked for some time before they realized they were essentially doing the same thing.

After Purcell’s discovery, a scientist from the DuPont chemical company visited Purcell and asked him if he saw any practical applications of his discovery. Purcell responded that he could see nothing practical coming from nuclear magnetic resonance. Later, he laughed about his response as his work was quickly followed by nuclear magnetic resonance (NMR), a powerful tool for chemists, and later, magnetic imaging became a powerful diagnostic tool for physicians.

For their discovery of nuclear magnetic resonance in bulk matter, Purcell and Bloch shared the Nobel Prize in Physics in 1952.

As already noted, the uncertainty that faced the Purcell team when they started the magnetic resonance experiment was the nuclear spin relaxation time. With their discovery, however, it was clear that the relaxation time was much shorter than had been predicted. (Purcell recalled later that when he had sat all Saturday morning hoping the spin system would equilibrate, he had the magnet on 10⁸ seconds longer than necessary.) Nicolaas Bloembergen, a graduate student of Purcell, joined Pound and Purcell in experimental and theoretical work that resulted in the famous paper, “Relaxation Effects in Nuclear Magnetic Resonance Absorption” (1948), which explained the relaxation of nuclear spins. This paper, known as the BPP paper, established a record for the number of its citations.

The two-state system was the basis for Purcell’s nuclear magnetic resonance experiment and still another two-state system awaited Purcell’s attention. The transition between two hyperfine states of the ground state of the hydrogen atom has a wavelength of 21 centimeters. Purcell suggested to his graduate student, Harold I. Ewen,

that he look for radiation from this transition emitted by hydrogen atoms in interstellar space. With a horn antenna (mounted outside the window of Ewen’s laboratory) as a detector, this transition was detected on 25 March 1951, and became the basis for mapping the Milky Way and other galaxies. After this discovery, radio frequency astronomy became an active area of research. The horn antenna that Purcell and Ewen used to detect the hydrogen 21-centimeter signal was later mounted on a pedestal as a monument in Green Bank, West Virginia, where the National Radio Astronomy Observatory is located.

It happened that two other groups, one from the Netherlands and one from Australia, had previously tried to detect this transition and failed. Purcell submitted a manuscript to the journal Nature but told the editors not to publish it. Purcell communicated his results to the two other groups and gave them the details of his and Ewen’s experiment. The two groups were able to confirm Purcell’s results and he asked them to write it up and send it to Nature. He then asked the editors of Nature to publish the three articles sequentially in the same issue. In an act of rare generosity, Purcell shared this important discovery with the two other research groups.

With another graduate student, James H. Smith, Purcell discovered what became known as the Smith-Purcell Effect. In 1953, they passed a beam of energetic electrons close to and parallel with the surface of an optical diffraction grating. They observed that this produced visible light. The Smith-Purcell Effect has been applied to generate radiation in various frequency domains and free electron lasers have been proposed based on this effect.

Purcell had a total of eighteen PhD students—sixteen of them before 1960. After the NMR period, Purcell mostly worked alone. As Purcell acknowledged in the Sopka interview, he did not “develop a kind of coherent ongoing self-perpetuating program of research. So I was dabbling really in different things in kind of an opportunistic way, which is not particularly good for graduate students, and I just sort of slipped into the role of not taking graduate students.” In addition, membership on government committees consumed much of his time.

During the 1960s, Purcell turned his attention to astrophysics, which appealed to his interest in theoretical modeling. He examined the problem of light propagating through interstellar dust. Later, he became active in biophysics. Howard Berg, a Harvard Junior Fellow, showed that E. coli bacteria propel themselves by continuously rotating their corkscrew-like flagella. Purcell became interested in this and was able to describe E. coli hydrodynamically. For this work, Purcell and Berg were awarded the Biological Physics Prize in 1984 by the American Physical Society.

Academic Instruction Purcell was a devoted teacher, devoted in a broad sense. His interest was mostly at the undergraduate level, where he created several new courses for the Physics Department. He was the author or coauthor of two introductory-level textbooks: Physics for Science and Engineering Students, (1960, with W. H. Furry and J. C. Street) and Electricity and Magnetism: Berkeley Physics Course, Volume 2 (1963). The latter textbook was part of a curriculum reform effort in the early 1960s organized by the Commission on College Physics, of which Purcell was a member. Of the original five-volume textbook, Purcell’s book was the only one still in print in 2007.

Purcell was a senior fellow of the prestigious Harvard Society of Fellows. In this capacity, he helped select new junior fellows and for twenty years he had dinner with the junior fellows every Monday evening. Over these years, Purcell was an informal teacher to the junior fellows.

He was also a teacher of his colleagues. For twenty months, Purcell had a column in the American Journal of Physics called “Back of the Envelope,” in which he presented problems that could be solved in a few steps on the back of an envelope.

Government Advisor With the dawning of the nuclear age in 1945, leading physicists became public figures and Purcell was in demand as an advisor and a consultant. Purcell was a charter member of the President’s Science Advisory Committee (PSAC), which was created by President Dwight D. Eisenhower in 1957 following the orbiting of Sputnik I by the Soviet Union. This advisory committee, with leading scientists as members and MIT president James R. Killian, Jr. as chairman, had direct access to the U.S. president. Purcell had great influence on the members of PSAC. Robert Kreidler of the Sloan Foundation, who worked with Killian, said of Purcell: “Ed Purcell did not speak often, but when he did, there would be enormous silence in the room because everybody knew that whatever he said was going to be worth listening to with careful attention” (Killian, 1977, p. 123).

In 1958, Killian named Purcell to head a subcommittee of PSAC whose assignment was to define a space program for the United States. The drafting of their report was done by Purcell, Edwin Land (president, Polaroid Corporation), Herbert York (director, Livermore Laboratory), and Francis Bello (future editor of Scientific American). This report was disseminated throughout the United States and the world. Purcell was proud of this report, as its projections proved accurate as the space program developed and the Moon landings occurred. This report exerted an important influence on both the forming of the National Aeronautics and Space Administration (NASA) and on the Apollo Mission.

Purcell received many honors. In addition to the 1952 Nobel Prize, he was named the Gerhard Gale University Professor at Harvard. He was a also a member of the National Academy of Sciences (1951), a member of the American Academy of Arts and Sciences, a fellow of the American Physical Society and its president in 1970, and a foreign member of the Royal Society. He was awarded the National Medal of Science in 1979 for his service to the nation, and won the Oersted Medal in 1968, given by the American Association of Physics Teachers for his contribution to physics education.

BIBLIOGRAPHY

WORKS BY PURCELL

With H. C. Torrey and R. V. Pound. “Resonance Absorption by Nuclear Magnetic Moments in a Solid.” Physical Review 69 (1946): 37–38.

With Nicholaas Bloembergen and Robert V. Pound. “Relaxation Effects in Nuclear Magnetic Resonance Absorption.” Physical Review 73, no. 7 (1948): 679–712.

With H. I. Ewen. “Observations of a Line in the Galactic Radio Spectrum.” Nature 168 (1951): 356.

With W. H. Furry and J. C. Street. Physics for Science and Engineering Students. New York: Blakiston, 1952. “Research in Nuclear Magnetism.” Science 118 (1953): 431–36.

With S. J. Smith. “Visible Light from Localized Surface Charges Moving Across a Grating.” Physical Review 92 (1953): 1069.

Electricity and Magnetism. New York: McGraw-Hill, 1963.

“On Alignment of Interstellar Dust.” Physica 41 (1969): 100–127.

“Life at Low Reynolds Numbers.” American Journal of Physics 45 (1977): 3–11.

OTHER SOURCES

Killian, James R., Jr. Sputnik, Scientists, and Eisenhower: A Memoir of the First Special Assistant to the President for Science and Technology. Cambridge, MA: MIT Press, 1977.

Pound, Robert V. “Edward Mills Purcell.” Biographical Memoirs, Volume 78. Washington, DC: National Academy Press, 2000.

Rigden, John S. “Quantum States and Precession: The Two Discoveries of NMR.” Reviews of Modern Physics 58 (1986): 433–448.

———. Hydrogen: The Essential Element. Cambridge: Harvard University Press, 2002.

Sopka, Katherine R. Interview. 8 June 1977. Oral Histories, Center for History of Physics, American Institute of Physics, College Park, MD.

John S. Rigden