﻿370 



Dr. Norman. Campbell on the 



conveniently expressed, is (ii—i^jii- If there is only- 

 reflexion and no ionization or only ionization and no 

 reflexion, then R is a measure of the reflexion or the 

 ionization respectively. 



F ' Fi*. 1. 



S 



o 

 1 1fl 



« 



~-*o 



i 2 can easily be measured by making U negative ; it was 

 found that i 2 was saturated when U= —20 volts ; this value 

 was sufficient to prevent any of the rays leaving P from 

 striking it again. To measure t] accurately is not quite so 

 easy. The most obvious way to do so is to connect B to P 

 and measure the total current received by the whole of the 

 lower part of the apparatus. But it was found that, in order 

 that the rays striking P should be homogeneous in speed, 

 and all have the speed Y=Vi— V 2 it was necessary that 

 V 2 should be not less than 40 volts, and in a direction to 

 retard the rays entering B. (That is to say, besides the rays 

 of speed Vx entering B, there are some others of a speed 

 between and 40 volts, which probably represent the result 

 of the ionization of A by the rays falling on it.) But if 

 the tube projecting from A into B is at a potential con- 

 siderably higher than P or B, many of the rays leaving P 

 will be attracted to the tube, instead of to B. and the current 



