Sir Joseph John Thomson (1856-1940) was a British physicist who is credited with the discovery of the electron. First published in 1933, this forms the second of two books making up the third edition of a 1903 original by Thomson. The text was greatly enlarged for this edition, which resulted in its division into two parts, and incorporates numerous advances in research relating to the discharge of electricity through gases. Numerous illustrative figures are provided throughout. This book will be of value to anyone with an interest in Thomson and his contribution to the development of physics.