An inverstigation of the ionising power of the positive ions from a glowing tantalum filament in helium

The ionising power of the positive ions emitted by a glowing solid has been investigated by Pawlow and by Bahr and Franek. The method used by these observers was that devised by Lenard for determining the ionising potential for electrons, and subsequently used by several experimenters for the same purpose. The positive ions from a glowing filament were accelerated towards a piece of platinum gauze by an electric field which could be varied. Those ions which penetrated through the gauze encountered a retarding electric field, greater than the accelerating field by a constant amount, which prevented any of them from reaching a collecting electrode placed a few centimetres beyond the gauze. If, however, any of the positive ions attained sufficient velocity to ionise the gas molecules on the further side of the gauze, the electrons produced would be attracted towards the collecting electrode, and their presence would be indicated by a deflection of the electrometer connected to it. The experiment consisted in gradually increasing the accelerating field until the presence of electrons beyond the gauze was indicated by the electrometer. Pawlow came to the conclusion that the ionising potential for positive ions in hydrogen was about 10 volts, but the results with the other gases tested were inconclusive and were taken to indicate that the ionising potentials were lower the hotter the source of the positive stream. Bahr and Franck experimented with air and with hydrogen, and could not obtain any definite evidence of the existence of a minimum ionising voltage for positive ions in either of these gases. They found that the point at which electrons could first be detected in the ionisation chamber varied with the density of the positive stream, and they concluded from their results that the gases were being ionised by the positive ions, and that, if there is a definite minimum ionising potential, it is lower than the ionising potential in the case of electrons.