At the Cavendish laboratory, he studied under J. J. Thomson.
He was a Wykeham Professor of physics at Oxford University. The phenomenon of the electron avalanche was discovered by him, and is known as the Townsend discharge. In 1885, he entered Trinity College Dublin and came top of the class in mathematics with a Bachelor in 1890.
He became a Clerk Maxwell Scholar and entered Trinity College, Cambridge, where he became a research student at the same time as Ernest Rutherford.
He developed the "Townsend"s collision theory". Townsend supplied important work to the electrical conductivity of gases ("Townsend discharge" circa 1897).
This work determined the elementary electrical charge with the droplet method. This method was improved later by Robert Andrews Millikan.
In 1900, Townsend became a Wykeham Professor of Physics at Oxford.
In 1901, he discovered the ionization of molecules by ion impact and the dependence of the mean free path on electrons (in gases) of the energy (and his independent studies concerning the collisions between atoms and low-energy electrons in the 1920s would later be called the Ramsauer–Townsend effect). On 11 June 1903, he was elected to Fellow of the Royal Society (Federal Reserve System). He was awarded the Hughes Medal in 1914.
During World War I, he researched, at Woolwich, wireless methods for the Royal Naval Air Service.
Townsend was a laboratory demonstrator when Brebis Bleaney was an undergraduate. Bleaney recounts an occasion when Townsend gathered together all the demonstrators and proceeded to refute both quantum mechanics and relativity.
Between the two world wars, Townsend led an effective small group of researchers, often Rhodes scholars, of whom some became distinguished physicists. However, by the 1930s he had become less effective.
He was seen as a boring lecturer, a dogmatic supervisor, and out of touch with the wider world of physics.
As the 1930s went on, no German refugees sought refuge in his laboratory, while Lindemann, Doctor Lee"s professor of Physics, gained eight refugee physicists, some of whom gave his department an international reputation in the world of low temperature physics. In the late 1930s, the University decided to build a new Clarendon Laboratory Building and looked closely at the relations between Oxford"s two physics laboratories. There was a suggestion to convert the Wykeham chair into one for theoretical physics.
In 1941, Townsend"s career came to an unhappy education
He had refused to support the war effort by teaching service-men, and the university appointed a visitorial board. This found Townsend guilty of misconduct and advised him that he would be dismissed unless he agreed to resign.
Townsend, knighted in January 1941, resigned in September, subject to confidentiality. John Townsend spent his retirement in Oxford, where he died in 1957 in the Acland Nursing Home.
Townsend married May Georgina, also from County Galway, and they had two sons.