Von Baeyer & Hahn (1910) by use of a magnetic field, were the first to show that electrons emitted from a radioactive source consisted of several discrete energy groups superimposed on a continuous electron distribution. A diagram of their apparatus is shown in fig. 1. ß-rays emitted from a radio¬ active source A/can pass through a narrow slit B, and fall on a photographic plate 0. The deflecting magnetic field is perpendicular to the ß-rays and / in fig. 1, perpendicular to the plane of the paper. The intensity of the ß-ray groups recorded on the plate and their degree of resolution was low,because electrons leaving the source were not focused. After this initial discovery, the importance of the work was recognised and attempts were made to improve the apparatus in order to observe greater detail. Instruments designed for this purpose are known as ß-ray spectrometers.