The attainment of high potentials by the use of radium (1913)

Abstract The original aim of the work described in this note was to measure the energy and numerical importance of each of the many distinct kinds of β-particles emitted by a single radioactive substance. Calculation of the energy of a β-particle from observation of its deflection in a magnetic field involves assumptions which are as yet insufficiently supported by experiment. Theoretically both the energies and distribution of the particles could be directly measured by giving a gradually increasing positive charge to the source of radiation; for, when the potential of the source is +V, electrons possessing energy less than eV will be drawn back to the source of radiation. Unfortunately, more than a million volts would be necessary to stop the fastest β-particles, and no method is at present known of maintaining such a high potential in vacuo. It was thought that this difficulty might possibly be overcome by using the active material itself in order to produce the high potential according to the principle employed in Strutt's radium clock. If the source of radiation were perfectly insulated its potential would rise until the swiftest β-particles could no longer escape. The present note deals with experiments made to test whether this method were practicable. It was found that high potentials were readily obtained, but the attempt to attain to a million volts tailed through the difficulties of insulation encountered. But few experiments were completed, and many failed as the result of accident. This shows that, even if perseverance had been rewarded by greater success, technical difficulties, accentuated by every effort to improve the insulation, would probably have prevented the practical application of the method. It seemed, therefore, useless to pursue the matter further, until more is known of the reasons why the insulation of a vacuum breaks down. In these experiments the source of β-radiation was 20 millicuries or more of purified radium emanation contained in a thin bulb—marked B in fig. 1—of about 1 cm. diameter. The bulb, which was just thick enough to stop all α-radiation, was supported by a fine silica rod R inside an exhausted glass flask F of 1 litre capacity. The rod, of diameter about 0⋅8 mm., was freshly drawn from transparent fused silica. The surface of the bulb and the flask was coated with silver, which was found to retain a trace of conductivity when subsequently heated to 400° C., though it then became almost transparent. The potential gained by the bulb was measured by a simple form of attracted disc electrometer, a circular aluminium disc being hung from the arm of a horizontal silica spring, the other end of which was soldered with aluminium to a projection from one of the glass walls of the flask. By observing with a microscope the. displacement of the disc, the force of attraction exerted on it by the bulb was measured, and from this it was easy to calculate the charge and the potential acquired by the bulb. The force of a dyne displaced the spring by about 0⋅1 mm. The disc was hung just at the entrance to the mouth of the flask, so that the remainder of the flask wall served the purpose of a guard-ring.