[sHAw] CATHODE RAYS FROM GASES 15 



metres from each of their surfaces, but a patch of luminosity was left 

 isolated between the gauzes. This patch could be decreased by charg- 

 ing the gauzes negatively, a potential difference from the box of about 

 45 volts being found sufficient to cause the disappearance of the glow. 

 The exact point of disappearance was determined in the following 

 way. The two left-hand gauzes were charged to a potential less than 

 the amount required for the elimination of the glow, while those on 

 the right were charged considerably in excess of this, thereby insuring 

 minimum illumination on this side. The charge on the left-hand 

 side was then slowly increased until, on the reversal of the electrical 

 connections, no flicker could be observed. This process was rendered 

 necessary by the presence of diffused light from the beam above. 



This measurement was repeated over the whole range of pressure 

 in which the Wehnelt Cathode would work, but in every case the 

 value of the minimum potential required to eliminate the glow be- 

 tween the gauzes lay between 44 and 46 volts. The potential differ- 

 ence in the discharge tube was varied from 60 to 1,000 volts; but the 

 same constancy was maintained in the stopping potential for the glow. 

 The introduction of carbon dioxide also failed to produce a difference 

 in this figure. 



These experiments can be explained in the same manner as 

 those of Professor Thomson, and support the view "that the energy of 

 the secondary rays is determined by the atoms emitting them and not 

 by the energy of the primary rays." When the gauzes were charged 

 negatively to 45 volts or over, no electrons with the capacity for pro- 

 ducingluminosity entered the space between gauzes. If charged to 

 less than this value some passed through so slowly that no appreciable 

 glow was produced near the gauze but on reaching the other side the 

 electric field accelerated their motion and a perceptible haze was again 

 produced. In the same way it was possible to obtain another detached 

 glow below the lower gauze. 



By charging the lower gauze to a high positive potential the glow 

 between the gauzes was intensified, the electrons being accelerated as 

 they approached. It was thought that if the lower gauze was kept 

 charged to a high positive potential and if a gauze covering was placed 

 over the box C, that this stopping potential could be determined more 

 accurately. The idea was that any electrons passing through the 

 upper gauze would be accelerated, and if in sufficient numbers, cause 

 a glow and signalize their presence. In this way one could differ- 

 entiate between that potential required to stop those electrons which 

 were initially fast enough to penetrate and cause a glow and that 

 potential required to stop all electrons. This point was examined in 

 the same way as before and it was found that a negative potential 



