Experiments on the diffraction of cathode rays.—II

Abstract

§ 1. The following paper is an account of further experiments which have been made on the lines of those described in these ' Proceedings ’ (vol. 117, p. 600), and largely with the same apparatus. A beam of cathode rays passing normally through a very thin film of metal was found to produce a pattern of concentric rings on a photographic plate about 30 cm. away. These could be explained as a diffraction pattern due to the de Broglie waves of the electron, the atoms of the metal crystals being the diffracting system. In the present paper several points of uncertainty are cleared up, and the work extended to other cases. § 2. As mentioned in a note at the end of the previous paper, the discrepancy of 6 per cent, between the values of the crystal constants of aluminium and gold determined by X-rays, and those found by applying the de Broglie theory to the diffraction rings formed by the cathode rays, has now been explained. It was due to an error in the measurement of the energy of the cathode rays, and hence their wave-length. The energy was measured by a spark-gap connected in parallel with the discharge tube. In the earlier measurements a considerable length of leads and a rectifying valve were included with the discharge tube, and it now appears that an appreciable fraction of the potential fall occurred in these. When two spark-gaps were used, one connected as before, and the other directly across the discharge, there was 1-2 mm. difference in the readings. The following table shows the values of P, the voltage; D, the diameter of the ring corresponding to reflection from the (2, 0, 0) plane; and D√P (1 + P e /1200 m 0 c 2 ), which latter quantity should be constant for any one metal on the de Broglie theory (see previous paper, p. 603). The factor in brackets is the relativity correction, and in the experiments in question never differs from unity by more than 3 per cent.