On the ionisation of gases by collision and the ionising potential for positive ions and negative corpuscles

Abstract

The object of the present research was to investigate the ionising properties of positive ions using a direct method, and to try experimentally to determine the minimum energy which is necessary for a positive ion to produce fresh ions by collision with a gas molecule. At the same time it appeared to be of interest to measure the ionising potential for negative corpuscles under experimental conditions somewhat different from those previously adopted. 1. Description of the Apparatus . The apparatus in the present investigation was used in two forms, represented in figs. 1 and 2. The part common to both forms consists of two pieces of glass tubing, A and B - A, 3·5 cm. in diameter, B, a little less the latter being blown out to a slightly spherical form, as shown in the diagram. The joint at C was made air-tight with sealing wax. By this means it was possible to take out the lower part very easily a process which was very often necessary in order to renew the platinum strip P, or to cover it with a fresh salt. In the narrow upper part of the tube A a brass rod was sealed with a round brass plate E at its end. This rod was insulated and surrounded by a brass tube connected to earth, to prevent it acquiring a charge from the glass. The plate E was used as an electrode to measure the ionisation, and was connected to a Wilson's tilted electroscope adjusted to a sensitiveness of 80 divisions per volt. The ionisation chamber was a wire-gauze cylinder D, through the open top of which passed the electrode E. The sides of the cylinder were made of brass gauze and the bottom of platinum gauze, which was fixed in clamps at the end of a brass rod which passed through a side tube and which held the cylinder in its proper position. The platinum strip P was of thin foil, 2 cm. long and 1 mm. wide. It could be heated electrically by means of the thick copper leads G and H, the current being supplied from a battery of six accumulator cells connected in parallel and regulated by wire resistances. The heating circuit was insulated. The copper leads G and H were covered with glass in order to make them more rigid and to keep them cool. These glass tubes and the leads themselves were sealed into two tubes joined to the spherical part of the lower tube. The tube B could be adjusted at the joint C, so as to fix the strip P at any distance from the bottom of the cylinder D.