The 'Plum-Pudding' Atom

J J Thomson at work in the Cavendish LabImage supplied by and copyright the Cavendish Laboratory, Cambridge

J J Thomson's discharge tube, c. 1897 (replica)

On loan from the Cavendish Laboratory, Cambridge

Thomson used this tube during his experiments with cathode rays that led to the discovery of the electron. The tube was evacuated, and the cathode rays entered from the left. Thomson found that the rays were deflected towards the upper metal plate when it was positively charged. This showed that the cathode rays consisted of negatively charged particles, now called electrons. The graph paper pasted to the end of the tube allowed Thomson to calculate the amount of deflection.

Letter from J J Thomson to A W Porter FRS (March 1896)

(Royal Society manuscript MM/9/22)
J J Thomson FRS, 'On the Structure of the Atom', Philosophical Magazine Vol 7, No 39 (March 1904)

In this letter Thomson describes his experiments with cathode rays in tubes similar to the one displayed here. His article 'On the Structure of the Atom' outlined his theories about how 'corpuscles' (that is, electrons) were arranged in and around atoms.