Ionization by Charged Particles. 807 



reflected when the velocity o£ the primary rays is less than 

 11 volts. Reflexion of the rays, as distinct from scattering, 

 and ionization by them seem almost mutually exclusive 

 alternatives ; Lenard has already put forward this view, and 

 explains the fact on the ground that ionization or reflexion 

 occurs according as the primary ray does or does not enter 

 the atom which it strikes. 



6. It appeared that a further determination was desirable 

 of the distribution of the velocities of the electrons returned 

 from a plate struck by primary rays with velocities between 

 10 and 80 volts ; such rays were not investigated by Gehrts. 

 But after this determination had been made by his method, 

 doubts were thrown upon the results by the discovery that 

 the primary rays were not nearly homogeneous. Gehrts' 

 figures show that his rays were very nearly homogeneous 

 when their maximum speed was 10*4 volts, but he appears 

 not to have investigated the homogeneity of the faster rays 

 he used. The heterogeneity of the primary rays in my 

 arrangement arose doubtless from the fact that the rays in 

 their passage from the place where they were produced 

 (a zinc plate acted on by ultra-violet light or a Wehnelt 

 cathode), struck the metal diaphragm limiting the accele- 

 rating field and caused the emission from it of 8 rays which 

 became mingled with the primary rays. In order to avoid 

 this source of error a rather different form of apparatus was 

 employed (fig. 1). Ultra-violet light passes through the 



Fig. 1. 









\ 1 / 







m "™ ■* 



- 





O 1 



i I 



-1 L 1 ( j 



Cm** 



quartz window in the brass box A and falls on the zinc 

 plate Z, which is supported by a rod insulated from A. A 

 potential difference V x is established between A and Z. It 

 appeared that the current between A and Z was zero when Y x 

 was less than — 2 (Y 1 is counted positive when A is positive 

 to Z) , and saturated when Y ± was greater than + 2 ; con- 

 sequently all the electrons liberated from Z must fall on A 

 with a velocity which is very nearly V x . The rays liberated 



312 



