toral program and supervised his thesis, “The Fall of Potential in Electrical Discharges," which he completed in 1931.

Street then became a research fellow at the Bartol Research Laboratory in Swarthmore, Pennsylvania. Working with W. F. G. Swann and using an ion chamber for the measurement of cosmic radiation, he joined a pioneering investigation of the nature of cosmic radiation originating in outer space and its secondary radiation products. He also worked with Thomas H. Johnson, using Geiger counters and vacuum tube circuits as "telescopes" to determine that the incoming particles were deflected by the earth's magnetic field and therefore were electrically charged. Later Dr. Johnson moved their apparatus to the higher and lower latitudes of Mexico City to show there was an east-west effect, with the west intensity being greater, corresponding to positively charged particles being a dominant component of the primary cosmic radiation. Street's and Johnson's experiments were of great importance since, at the time, there was heated debate as to whether the primary cosmic rays were charged particles or uncharged photons.

Street continued his experimental research at Harvard, where he arrived as an instructor in 1932. It had become clear that the cloud chamber was key to resolving the debate on the nature of cosmic radiation; a charged particle creates a visible track in a cloud chamber, and the addition of a magnetic field allows measurement of its charge and momentum. With a series of grants from the Milton and Whiting funds and the help of his colleague Harry R. Mimno, as well as research associates and graduate students, Street designed and built a large electromagnet and cloud chamber for his research. This was a straightforward task for Street, who, like a number of physicists of his generation, benefited from his earlier training in electrical engineering. In 1934-35 Street assembled the electromagnet and cloud



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