Background image: The Crookes tube. The shadow of the Maltese cross is thrown onto the tube face at right by a beam of electrons.
The electron is the lightest charged lepton, a spin-half particle with mass 9.1 x 10-31 kg and charge 1.6 x 10-19 C. It is subject to the electroweak interaction, but couples with all electrically charged particles including charged mesons, baryons and quarks. Heavier versions (flavours) of the electron are the muon and tao particles; since the electron is the lightest, it is the only one that is stable. Like all leptons it appears to be pointlike with no internal structure, except possibly at the superstring scale, and like all leptons it has an anti-matter companion. The antiparticle of the electron is the positron.
Electrons are constituents of all ordinary matter, and the structure of bound states of electrons about the atomic nucleus dictate the chemical properties of atoms and, in electromagnetic interactions with other atoms, the geometry and biochemical properties of molecules and ultimately of all living matter as well.
The existence of the electron was first conjectured by the British philosopher Richard Laming in 1838. In 1846, the German physicist William Weber conjectured that the force between electrons obeyed the inverse square law. But it was the Irish physicist George Johnstone Stoney who was the first to suggest that there was a ‘single definite quantity of electricity’, the charge of a monovalent ion. It was he who coined the term ‘electron’. Subsequent investigations by William Crookes in the 1870s and later J. J. Thompson 1896 on cathode rays yielded the first accurate measurements of the electron’s charge to mass ratio. In this period, H. A. Lorentz developed the electrodynamical theory of electrical motions, of importance to Einstein’s discoveries in special relativity and discoveries by Planck and Bohr leading to the ‘old’ quantum theory. Lorentz's electrodynamic theory of the electron was the first theory to describe the interaction of particles by means of fields.
The absolute value of the electron charge was first accurately measured by Robert Millikan in his ‘oil-drop’ experiments in 1909, for which he was awarded the Nobel Prize in 1923.