﻿794 Ionization by Positive Hays. 



about 5 x 10 8 cm. sec, corresponding to an energy (for a 

 single electronic charge) o£ 5 X 10 5 volts ; such rays are about 

 3 mm. from the end of their range in air. Their energy 

 when they cease to penetrate must be considerably less, for 

 it falls rapidly towards the end of their range, but no definite 

 limit can be assigned. Moreover, it is clearly rash to assume 

 too great a similarity between the properties of doubly 

 charged helium atoms and singly charged hydrogen atoms. 

 But at least nothing is known inconsistent with the view 

 that the first stages of the conversion of canal-rays to a-rays 

 begins at about 38,000 volts and is not complete at 50,000 

 volts. It is hoped that more light may be thrown on the 

 matter by an improved apparatus in which higher values of 

 V can be investigated (it does not seem impossible that 10 5 

 volts might be attained), the reliability of the measurements 

 somewhat increased, and more information as to the nature of 

 the particles obtained by deflecting them in a magnetic field. 

 12. In conclusion a numerical coincidence may be noted 

 which is probably without theoretical significance. The 

 maximum value of the ionization per incident particle ob- 

 served in these experiments is within 10 per cent, the same 

 as the maximum value observed with cathode rays, namely 

 that at about 280 volts energy. The copper plate was 

 throughout in the " state A"" described in a recent paper*. 



Summary. 



The liberation of electrons from the surface of a copper 

 plate struck by the positive particles from heated phosphates 

 has been studied. 



Figs. 2 and 3 give the relation between P, the number of 

 electrons liberated by each positive particle, and V, the P.D. 

 through which the particles have fallen. 



Fig. 4- gives the relation between V and E, the reflexion- 

 coefficient of the positive rays. 



The significance of the results shown in fig. 2 is discussed. 

 It is suggested that the fall in P for the largest values of V 

 may be due to the beginning of the stage at which the 

 particles can penetrate the surface-layer of the metal, exhi- 

 biting one of the properties which distinguish a-rays from 

 canal-rays. At the highest potentials which could be studied 

 no indication was obtained of the acquirement by the positive 

 particles of the other distinctive properly of a-rays, the great 

 ionizing power. 



Leeds, March 1915. 



* N. Campbell, Phil. Mag. xxix. p. 369 (1915). 



