﻿794 Mr. G. Shearer on the 



On the other hand, Robinson and Rawlinson * by the mag- 

 netic spectrum method showed that there were present 

 groups of electrons with special velocities. Kang Fuh Hu t 

 also obtained some evidence of the existence of these groups. 

 Simons X carried out experiments on the same lines as those 

 of Barkla and Shearer, and concluded that sub-groups of 

 electrons were present when the characteristic X-radiations 

 of the bombarded matter were excited. Recently the question 

 has been definitely settled by the experiments of De Broglie §. 

 By the use of the magnetic spectrum method be has shown 

 that the energies of these groups correspond to h (v — v K ), 

 h (v — v L ), &c, where v, v K , v L , &c, represent the frequencies 

 of the incident radiation and of the K, L, &c. radiations of 

 the matter bombarded. Similar results have recently been 

 obtained by Whiddington ||. 



While these experiments show that the electronic radiation 

 consists of a general emission corresponding to the " white " 

 radiation from the tube and groups of electrons whose 

 energies obey simple quantum relations, they tell us nothing 

 of the magnitude of the emission, nor of how this magnitude 

 depends on the type of matter from which the electrons are 

 ejected. 



Laub^[ showed that the efficiency of an element as a source 

 of electrons increased with its atomic weight, but made no 

 attempt to obtain a law governing the variation. Moore **, 

 as a result of some experiments on the relative ionizations 

 produced in various gases, deduced the law that the number 

 of electrons emitted per atom is proportional to the fourth 

 power of the atomic number. These experiments were of a 

 somewhat indirect nature, and the range of atomic number 

 investigated was small. 



The experiments described here were undertaken, 

 primarily, to throw light, if possible, on this question. 



(2) Experimental Arrangements. — In order to avoid the 

 difficulties of interpretation introduced by the use of ioniza- 

 tion methods, it was decided to measure directly the number 

 of electrons emitted by observing the rate at which an 

 insulated radiator acquired a positive charge under the 

 influence of X-ray bombardment. 



* Robinson and Rawlinson, Phil. Mag. xxviii. p. 277 (1914). 

 f Kang Fuh Hu, Phys. Rev. xi. p. 505 (1918). 

 t Simons, Phil. Mag. xli. p. 120 (1921). 

 § De Broglie, Journ. de Phys. (6) ii. p. 265 (1921). 

 j| Whiddington, Phil. Mag. June 1922. 

 «f[ Laub, Ann. der Phys. (4) xxviii. p. 782 (1908). 

 ** Moore, Proc. Roy. Soc. A. xix. p. 337 (1915). 



