J.J. Thompson

Sir Joseph John "J. J." Thomson, OM, FRS (18 December 1856 – 30 August 1940) was a British physicist.

In 1897 Thomson showed that cathode rays were composed of a previously unknown negatively charged particle, and thus he is credited with the discovery and identification of the electron; and, in a broader sense, with the discovery of the first subatomic particle. Thomson is also credited with finding the first evidence for isotopes of a stable (non-radioactive) element in 1913, as part of his exploration into the composition of canal rays (positive ions). He invented the mass spectrometer.

Thomson was awarded the 1906 Nobel Prize in Physics for the discovery of the electron and for his work on the conduction of electricity in gases.

In 1912, as part of his exploration into the composition of canal rays, Thomson and his research assistant Francis William Aston channeled a stream of neon ions through a magnetic and an electric field and measured its deflection by placing a photographic plate in its path. They observed two patches of light on the photographic plate (see image on right), which suggested two different parabolas of deflection, and concluded that neon is composed of atoms of two different atomic masses (neon-20 and neon-22), that is to say of two isotopes. This was the first evidence for isotopes of a stable element; Frederick Soddy had previously proposed the existence of isotopes to explain the decay of certain radioactive elements.

JJ Thomson's separation of neon isotopes by their mass was the first example of mass spectrometry, which was subsequently improved and developed into a general method by Francis William Aston and by Arthur Jeffrey Dempster.

In 1905 Thomson discovered the natural radioactivity of potassium.

In 1906 Thomson demonstrated that hydrogen had only a single electron per atom. Previous theories allowed various numbers of electrons.

Earlier, physicists debated whether cathode rays were immaterial like light ("some process in the aether") or had mass and were composed of particles. The aetherial hypothesis was vague[citation needed], but the particle hypothesis was definite enough for Thomson to test.

Thomson first investigated the magnetic deflection of cathode rays. Cathode rays were produced in the side tube on the left of the apparatus and passed through the anode into the main bell-jar, where they were deflected by a magnet. Thomson detected their path by the fluorescence on a squared screen in the jar. He found that whatever the material of the anode and the gas in the jar, the deflection of the rays was the same, suggesting that the rays were of the same form whatever their origin.

Awards:

Royal Medal (1894)

Hughes Medal (1902)

Nobel Prize for Physics (1906)

Elliott Cresson Medal (1910)

Copley Medal (1914)

Franklin Medal (1922)

In 1991 the thomson (symbol: Th) was proposed as a unit to measure mass-to-charge ratio in mass spectrometry in his honour.