ELECTRON, the name suggested by Dr G. Johnstone Stoney in 1891 for the natural unit of electricity to which he had drawn attention in 1874, and subsequently applied to the ultra-atomic particles carrying negative charges of electricity, of which Professor Sir J. J. Thomson proved in 1897 that the cathode rays consisted. The electrons, which Thomson at first called corpuscles, are point charges of negative electricity, their inertia showing them to have a mass equal to about 12000 that of the hydrogen atom. They are apparently derivable from all kinds of matter, and are believed to be components at any rate of the chemical atom. The electronic theory of the chemical atom supposes, in fact, that atoms are congeries of electrons in rapid orbital motion. The size of the electron is to that of an atom roughly in the ratio of a pin’s head to the dome of St Paul’s cathedral. The electron is always associated with the unit charge of negative electricity, and it has been suggested that its inertia is wholly electrical. For further details see the articles on Electricity; Magnetism; Matter; Radioactivity; Conduction, Electric; The Electron Theory, E. Fournier d’Albe (London, 1907); and the original papers of Dr G. Johnstone Stoney, Proc. Brit. Ass. (Belfast, August 1874), “On the Physical Units of Nature,” and Trans. Royal Dublin Society (1891), 4, p. 583.