﻿Emission of Electrons by X-liays. 801 



to the fourth power of: the atomic number. This result was 

 obtained indirectly from observations on the relative ioniza- 

 tions of different gases. The heaviest element used was 

 chlorine, so that the range of Moore's experiments falls 

 almost entirely outside that of these experiments. Had it 

 not been for the fact that Moore's law fits in well with tho 

 absorption law of Bragg and Peirce, we might have con- 

 cluded that both his law and that deduced here are both 

 approximations to a more general law, the former being an 

 approximation holding for elements of low atomic weight, 

 while the latter is a better approximation for the heavier 

 elements. Bragg and Peirce * have shown that the X-ray 

 energy absorbed per atom is proportional to the fourth power 

 of the atomic number of the absorbing element. The com- 

 bination of this result with that of Moore suo-o-ests that the 

 number of electrons emitted is proportional to the X-ray 

 energy absorbed, a result pointed out by Moore. The range 

 of elements used in the experiments of Bragg and Peirce was 

 well within that of these experiments although outside that 

 of Moore's. On the other hand, if the above results are true, 

 no such simple law appears to hold. Even when allowance 

 is made for the approximate nature of the various assumptions 

 made in deducing the final result, it is extremely difficult to 

 see any way in which these experiments could possibly be 

 reconciled with a fourth-power law. 



In connexion with this result, it is of interest to refer to a 

 result obtained by Kaye f on the relative efficiency of 

 various metals as anticathodes in an X-ray tube. Kaye found 

 that the X-ray output of a tube increased linearly with the 

 atomic weight of the metal used as anticathode in the tube. 

 Later experiments by Duane and ShimizAi % showed that 

 the proportionality was to the atomic number rather than 

 the atomic weight. In these experiments we are dealing 

 with the transformation of electron energy into X-ray energy, 

 while the problem that is the subject of this paper is the 

 inverse one — that of the transformation of X-ray. energy 

 into electronic energy. It has been shown that there is a 

 linear relation connecting the number of electrons escaping 

 from a metal bombarded by X-rays and the atomic number 

 of the metal. These results imply that, when the number 

 of electrons entering a metal is kept constant — as in Kaye's 

 experiments, — the X-ray energy emitted is a linear function 



* Bragg and Peirce, Phil. Mag. xxviii. p. 626 (1914). 



t Kaye, Phil. Trans. A. 209, p. 123 (1908). 



t Duane & Shimizu, Phys. Rev. xiv. p. 525 (1919). 



