Originally published in 1942, this book was written by the renowned physicist and nuclear scientist Wilfrid Bennett Lewis (1908-87). The text presents an account regarding the technique of electrical counting and its role as an essential aid for research in nuclear physics, reflecting the discoveries of Lewis and his contemporaries at the Cavendish Laboratory. References are also included. This book will be of value to anyone with an interest in the writings of Lewis, nuclear physics and the history of science.