Basic Electric Charges

The discovery of the electron as the first subatomic particle is credited to J.J. Thompson (England, 1897). He used an evacuated tube connected to a spark coil. As the voltage across the tube was increased, a beam of light became visible. This was referred to as a cathode ray. Thompson found that the beam was deflected by both electrical and magnetic fields. From this, he concluded that the cathode rays were made up of very small negatively charged particles; which he named electrons.

In 1909 Robert Millikan (United States) did his famous oil drop experiment to determine the mass/charge ratio for electrons. This led to the determination of that the mass was 9.11 x 10^-28 g.

Ernest Rutherford (England, 1911) performed a gold foil experiment which had tremendous implications for atomic structure.

Alpha particles ( helium nuclei) passed through the foil with few deflections. However, some deflections (1 per 8000) were almost deflected right back toward the source. This was unexpected and suggested an atomic model with mostly empty space between a nucleus, in which most of its mass was located and which was positively charged, and the electrons that defined the volume of the atom.

Further experiments shoed that the nucleus was made up of still smaller particles called protons and neutrons. Today the number of subatomic particles identified as discrete units has risen to well over 30 named units.