History In the second half of the nineteenth century the phenomena accompanying the discharge of electricity through highly rarified gases received increasing attention from physicists. Thomson - the Centenary of His Discovery of the Electron and his invention of Mass Spectrometry, Rapid Communications in Mass Spectrometry, Vol.11, 2-16 (1997). Thomson, the Cavendish Laboratory, and Thompson's cathode ray tube and positive ray apparatus, see J. Used to determine the ratio of charge to mass (q/m) of the electron. External magnetic field can be applied to balance electric deflection. Metal plates can have electric field applied across them. Basic principle Rays from cathode in cylinder pass through 2 metal slots and between 2 metal plates onto glass ball. Four shorter glass tubes with internal wires extend perpendicularly from the cylinderical body (see 1st object on left in accompanying image). This object consists of a glass cylindrical body with a larger diameter glass ball at one end. Object ID EM.N-08013-A overall length 30.5 cm, glass ball diam. Thomson's cathode ray tube #2, replica of Cavendish Lab apparatus. Eliot Elisofon Photographic Archives, African Art.