﻿796 Mr. G. Shearer on the 



The vessel was connected by wide-bore glass tubing to a 

 Gaede mercury pump. A charcoal-filled tube was attached 

 and cooled with liquid air. By these means the vessel was 

 kept at a very low pressure so as to make all ionization effects 

 negligible. As an additional precaution, hydrogen was used 

 as residual gas in some of the experiments. 



The substances to be examined were mounted on the faces 

 of the cube, and were all of sufficient thickness to give the 

 maximum electron emission. 



The source of X-rays was a Coolidge tube with a tungsten 

 anticatliode. This was actuated by a Butt induction coil 

 and mercury interrupter. Owing to the smallness of the 

 effect to be measured, it was not possible to use a mono- 

 chromatic source of X-rays. 



The rod carrying the cube was connected through earthed 

 shielding tubes to a string electrometer. The rate at which 

 this acquired a positive charge was taken as a measure of 

 the electron emission, and was determined for various sub- 

 stances. In order to correct for small variations in the 

 intensity of the rays during a set of observations, a standard- 

 izing ionization chamber was fitted. This was connected to 

 a Dolezalek electrometer of low sensitivity, a steady deflexion 

 method being used. 



(3) General Results, — The early results with this apparatus 

 showed that the electron emission was of a more complicated 

 nature than had been anticipated. 



If the cube was allowed to charge up to a considerable 

 potential, it was found that the rate of charging up fell off 

 rapidly at first and only became steady after a potential of 

 from ten to twenty volts had been reached.. Such an effect 

 might have been due to ionization effects, but a simple 

 calculation showed that the magnitude of the effect was very 

 much larger than that due to the ionization of a gas at a 

 pressure of 01 mm., and the pressure in the vessel was 

 certainly less than "001 mm. It appeared, therefore, that 

 this effect was due to the presence of a large number of slow 

 electrons. When the cube attained a voltage of from ten to 

 twenty volts the electric field was of sufficient strength to 

 prevent the escape of such electrons. 



The existence of these low-speed electrons was also 

 suggested by certain observations on the effect of the 

 magnetic field used to deflect the electrons from the window. 

 It was found that this field reduced the emission from the 

 cube although the stray field at the surface of the cube was 

 not more than a few Gauss. 



