ABSTRACT
This chapter describes the experiments have been made with a view to determining the best conditions for measuring with accuracy the number of α particles expelled from radio-active substances. In 1910 we used a string electrometer, devised by Laby and constructed by the Cambridge Scientific Co. for the purpose of counting α particles. The instrument was found to have sufficient sensibility, and the movement of the fibre produced by the entrance of each α particle into the detecting vessel could be simply recorded on a moving photographic film. As is well known, however, the helium can easily be purified by passing it over charcoal cooled by liquid air. This method of purification of helium was used in all the experiments. It is far more certain to use a photographic method for recording the rapid movements of the fibre. For this purpose, the authors have found the registration apparatus constructed by Professor Edelmann very serviceable.
