THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



DECEMBER 1915. 



LXXX. Note on the Velocity of Electrons expelled by X-rays. 

 By 0. G. Barkla, F.R.S., and G. Shearer, M.A., 

 University of Edinburgh *. 



I^HE relation between the maximum velocity of ejection 

 of electrons from substances exposed to ultra-violet 

 light, and the frequency of the radiation, has been shown by 

 Richardson & Compton f and by A, L. Hughes J to be given 

 by the equation ^mv 2 = hi — E , where n is the frequency of 

 the light and h and E are constants for a particular sub- 

 stance, but vary from substance to substance. The small 

 irregularities in the values of k observed by Richardson k 

 Compton were considered by them to be within the limits of 

 experimental error, but in the case of the experiments of 

 Huohes the variation seemed to be a real one ; indeed it was 

 apparently a regular variation connected with the atomic 

 volume and probably the valency of the particular substance. 

 The extreme values of k were those found for calcium and 

 zinc, that for zinc being nearly 1*2 times that for calcium. 

 When the frequency of radiation is very high, E is 

 negligible and the maximum velocity of emission from zinc 

 is nearly 10 per cent, greater than that from calcium. If 

 such a law held in the case of ejection by radiation of the 

 still higher frequency of X-rays, this difference in velocity 

 ou <>"ht to be easily observable, for the maximum distance 

 traversed in air by the electrons from zinc would be more 

 than 1*4 times that of the electrons from calcium. (This 



* Communicated by the Authors. 



f Phil. Mag. Oct. 1912. 



% Phil. Trans. Roy. Soc. A. ccxii. pp. 205-226 (1912). 



Phil. Mag. S. 6. Yol. 30. No. 180. Dec. 1915. 3 C 



