John Sealy Townsend

Sir John Sealy Edward Townsend (born June 7, 1868 in Galway , County Galway , Ireland , † February 16, 1957 in Oxford ) was a British physicist .

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Townsend studied at Trinity College , Cambridge . He was a research student at the Cavendish Laboratory under Joseph John Thomson . From 1900 he was a professor at Oxford .

Townsend did significant work on the electrical conductivity of gases ( gas discharge or Townsend electricity ). In 1897 he was the first to determine the electrical elementary charge using the droplet method he developed . This method was later improved by Robert Andrews Millikan ( Millikan experiment ). In 1901 he discovered the ionizability of molecules by ion impact and the dependence of the mean free path of electrons (in gases) on the energy ( Ramsauer-Townsend effect ). The fact that his otherwise recognized theory of gas discharges did not apply to noble gases and mercury, because ignition occurs despite high pressure and low voltage, was the trigger for the Franck-Hertz experiment , which experimentally confirmed Bohr's atomic model directly.

In 1903 Townsend was accepted as a member of the Royal Society . Since 1929 he was a corresponding member of the Académie des sciences . In 1941 he was knighted as a Knight Bachelor ("Sir").

The CGS unit Townsend ( unit symbol : Td ) is named after him, in which the reduced electrical field strength (quotient of electrical field strength and particle number density ) is measured:

{\ displaystyle {\ rm {1Td = 10 ^ {- 21} V \ cdot m ^ {2}}}}

^ List of former members since 1666: Letter T. Académie des sciences, accessed on March 8, 2020 (French).

personal data
SURNAME	Townsend, John Sealy
ALTERNATIVE NAMES	Townsend, Sir John Sealy Edward
BRIEF DESCRIPTION	British physicist
DATE OF BIRTH	June 7, 1868
PLACE OF BIRTH	Galway
DATE OF DEATH	February 16, 1957
Place of death	Oxford