Townsend, John Sealy Edward (1868–1957)

Irish mathematical physicist who studied the kinetics of electrons and ions in gases. He was the first to obtain a value for the charge on the electron, in 1898, and to explain how electric discharges pass through gases. He was knighted in 1941.

Townsend was born in Galway and studied at Trinity College, Dublin. In 1895 he went to England, initially as a research student at Cambridge together with New Zealander Ernest Rutherford. From 1900 Townsend was professor at Oxford.

Townsend studied the conductivity of gases ionized by the newly discovered X-rays and in 1897 developed a method for producing ionized gases using electrolysis. In 1898, he began the first study of diffusion in gases that had been ionized (or electrified) by means of the so-called Townsend discharge of a weak current through low-pressure gases. In Townsend's collision theory of ionization, collisions by negative ions (electrons) could induce the formation of secondary ions, thus carrying an electric charge through a gas.

Townsend also studied the electrical conditions that lead to the production of a spark in a gas, and the confusing role played in this by the positive ions that are produced simultaneously with the electrons.

During the 1920s, Townsend was involved with the measurement of the average fraction of energy lost by an electron in a single collision.