Thomson J(oseph) J(ohn)

Thomson  (tmsn), Sir J(oseph) J(ohn) 1856-1940.

British physicist who discovered the electron in 1897. While experimenting with cathode rays, he deduced that the particles he observed were smaller than an atom. Thomson also made noteworthy studies of the conduction of electricity through gases. He was awarded the Nobel Prize for physics in 1906.

Biography Nowadays we take for granted the existence of electrons, but this was not true just over 100 years ago, when the atom was thought to be a single unit that had no parts. The breakthroughs came in the late 1890s, when the British physicist J. J. Thomson was studying what we now call cathode-ray tubes. As an electric current passed from the cathode at one end of the tube to the anode at the other, raylike emanations were seen to proceed from the cathode to the anode. Thomson examined the nature of the rays' charge by bringing a positively charged and a negatively charged plate near the path of the rays, and observed that the rays were deflected toward the positive plate, suggesting they had negative charge. A series of experiments in which various objects were placed in the path of the rays showed that they also had momentum (they would cause a small paddle wheel to turn, for example). If they had momentum, that meant (in the physics of the time) that they had mass, suggesting that the rays were composed of tiny particles. Other experimental results, some by other scientists, suggested that the ratio of the charge to the mass of these particles had to be less than one-thousandth the ratio for charged hydrogen atoms. By examining both the energy of the rays and the amount by which an electric charge deflected them, Thomson was able to calculate that these particles had one two-thousandth the mass of a hydrogen atom. The particles, first named corpuscles, were later called electrons. (The term electron was not completely new; it had been invented in 1891 for the rays themselves.) Thomson was thus the first to discover that particles smaller than atoms existed, and for his pioneering work he was awarded the 1906 Nobel Prize for physics.