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Edward Purcell

Edward Mills Purcell was an American physicist who received half of the 1952 Nobel Prize for Physics for his development of a new method of ascertaining the magnetic properties of atomic nuclei.

Edward Purcell

Known as nuclear magnetic resonance (NMR) absorption, the method arose from the application of radar theory to the magnetic fields of atoms and was a significant advance over the magnetic resonance detection technique developed earlier by Isidor Rabi. Felix Bloch, with whom Purcell shared the Nobel Prize, independently made the same advance.

Purcell was born on August 30, 1912, in Taylorville, Illinois. From his parents, Edward A. and Mary (Mills) Purcell, he inherited an interest in electronics and teaching. His mother had been a teacher before she married and his father taught for many years before accepting a management position at a telephone company. Through his father’s work, Purcell had access to copies of the Bell System Technical Journal and surplus technical equipment with which he experimented at home.

Purcell went on to further develop these skills at Purdue University in West Lafayette, Indiana, where he majored in electrical engineering. He graduated in 1933, along the way developing a penchant for physics. Karl Lark-Horovitz, the head of the physics department, encouraged Purcell and helped him win an exchange fellowship at Karlsruhe Technical Institute in Germany, where Purcell studied for a year after graduation. While traveling to Germany, he met fellow exchange student Beth Busser. Four years later, she became his wife. The couple eventually had two sons.

When Purcell returned to the United States, he studied physics at Harvard University, where he received an M.A. in 1935 and a doctorate degree in 1938. He taught at Harvard for three years before interrupting his academic career to help with the war effort. At the Radiation Laboratory of the Massachusetts Institute of Technology, Purcell was head of a research group seeking to advance World War II military radar technology. Isidor Rabi, who had by that time already devised a method of observing atomic magnetic properties, oversaw the project to which Purcell’s group contributed. When the war ended, Purcell remained at the MIT laboratory for a brief period to put together summaries of the research. It was during this time that he decided to attempt to bring together his recent work with radio waves and his interest in Rabi’s molecular-beam magnetic resonance detection method, for which Rabi had won the 1944 Nobel Prize in Physics.

In order for the spin of an atomic nucleus to be determined via Rabi’s technique, a sample needed to be vaporized and then exposed to a magnetic field. The method developed by Purcell in the mid-1940s, however, eliminated the vaporization requirement and simplified the measurement process. Purcell’s process entailed the use of two magnetic fields. The first field was held stationary and was supplied by a strong electromagnet. Once a sample was placed in this field, a second field was applied , generated by a magnet driven by radio waves. As Purcell discovered, the atoms in a sample will only absorb energy and vibrate at a particular frequency (termed the signature frequency) of radio waves. By varying the radio waves powering the second magnet used in his method, Purcell showed that this frequency could be easily determined.

In 1946, Purcell resumed teaching at Harvard, where he remained, advancing to higher and higher ranks, throughout the rest of his career. His research in the early 1950s resulted in one of the earliest applications of nuclear magnetic resonance absorption in a field other than physics. Working with a graduate student, Harold Ewen, Purcell constructed a radio telescope that used nuclear magnetic resonance to measure radio emissions in space. The pair used the device to detect hydrogen’s signature frequency, enabling them to be the first to perceive interstellar hydrogen. Their technique became popular among astronomers and launched the field of radio astronomy.

Although much of his research following the invention of the radio telescope was in astrophysics, Purcell made a notable contribution to the field of biophysics during his later years. In 1977, working with Howard Berg, he investigated the motion of flagellated bacteria. Purcell proposed a successful explanation of bacterial locomotion, which, as Berg discovered, involves helical flagella that rotate like corkscrews. In 1984, the pair won the Biological Physics Prize of the American Physical Society for their work.

Purcell retired from teaching at Harvard in 1980, but continued research for several years. He died on March 7, 1997, in Cambridge of respiratory failure. In addition to the Nobel Prize and the Biological Physics Prize, Purcell’s other honors included the Ørsted Medal (1967) and the National Medal of Science (1979). He was also elected a member of the National Academy of Sciences and a foreign member of the Royal Society of London, and served as president of the American Physical Society and as an advisor to the President's Scientific Advisory Committee.