First picture of a positron track, taken by Carl Anderson, 1932.
© Science Museum / Science & Society Picture Library
Carl D Anderson (1905-1991) discovered the positron by accident in 1932. He was photographing the tracks of cosmic rays in a cloud chamber to find the energy spectrum of secondary electrons produced by the rays. A lead plate, to slow or stop the rays divided the chamber, and a magnetic field applied to deflect the particles in different directions according to their mass. Many positive particles were observed which were too light and produced too little ionisation to be protons. Anderson concluded that they were positive electrons, or positrons.