§113. Measurement of the total energy emitted in the form of γ rays. It was early shown by Rutherford and Barnes that the γ rays could only account for a small fraction of the heating effect of radium and its products. Later Rutherford and Robinson, in the course of their work on the heating effect of the α rays (see § 32), were able to estimate the energy emitted in the form of γ rays by radium B and radium C as about 7 per cent, of the total disintegration energy of radon in equilibrium with its products. The method employed was to measure first the heating effect due to the α and β rays, the walls of the calorimeter being sufficiently thin to allow practically all the γ rays to escape, and then to determine the increased heating effect when a certain fraction of the γ rays were absorbed by lead screens placed inside the calorimeter. No great accuracy was possible as the small heating effect of the γ rays was measured as the difference between two large effects.

Ellis and Wooster devised a method of automatically compensating the large α ray and β ray effect, enabling that due to the γ rays to be measured directly. The calorimeter consisted of a hollow cylinder of four equal sectors, the two opposing ones A and B being respectively of lead and aluminium, while the intermediate ones were of insulating material. The difference of temperature between A and B was measured by a system of thermocouples.