In the ‘Proceedings of the Royal Society’ for 1919 (A, vol. 95, p. 408) we have given an account of an investigation of the effects of electron collisions with helium atoms. The apparatus and method there described have been somewhat modified, and applied to a similar investigation with argon. With this gas Franck and Hertz came to the conclusion that ionisation occurs when the velocity of the colliding electrons is raised to 12 volts, but the method of experimenting used by them was not able to distinguish between ionisation of the gas and the photo-electric effect of radiation produced by the collisions. The main features of the apparatus used in the present research have been described in the earlier paper; the modifications introduced will be seen by reference to the diagrammatic view in fig. 1. A short tungsten filament, F, which could be heated to incandescence by an electric current, was used as the source of electrons. This was supported horizontally and about 1·5 mm. below the top of the platinum thimble, E, which was plane and circular, and had at its centre a circular hole 1·5 mm. in diameter.
An experimental determination of the critical electron velocities for the production of radiation and ionisation on collision with argon atoms
