312 Bumstead and McGouyan — Emission of Electron* by 



Table 1. 



Time 



1 hr. 



2 hrs. 



4 hrs. 



(i hrs. 

 146-5 



24 hrs. 



C a 



186 



174 



150 



129 



Foils 













2 



100 



100 



100 



100 



100 



3 



112-0 



111-2 



112-3 



113-0 



111-8 



4 



103-8 



103 



105 



103-3 



104-7 



5 



59-5 



60-3 



(54 



63-7 



66-2 



6 



23-1 



23 



25-3 



25-6 



26-6 



to increase with the time ; these values are on the decreasing 

 portion of the ionization curve, where a small change in the 

 range of the a-rays makes a large difference in the current. 

 The observed increase can he explained by supposing that the 

 progressive removal of occluded gas from the platinum foil 

 slightly diminishes its stopping power for a-rays and thus the 

 S-radiation from the emergence side is increased. 



After making the measurements which are recorded in the 

 last column of Table 1, a current of 12 amperes was passed 

 through the platinum foil. With this current a bright red 

 heat was obtained in the middle of the foil, fading away grad- 

 ually to the ends, which were cooled by conduction through 

 the clamps and copper rods.* The current was continued for 

 ten minutes, during which time the pressure rose from less than 

 •0001 mm to -004 mm . The charcoal bulb did not absorb the gas, 

 although the liquid air was left on over night ; this is doubt- 

 less due to the fact that the gas emitted by the platinum con- 

 tained considerable hydrogen which is not readily absorbed by 

 the charcoal. The liquid air was then removed, and the 

 Toepler pump was operated, while the charcoal was re-heated 

 to aid in sweeping out the hydrogen. When a pressure of 

 about -001 mm had been reached the liquid air was again applied, 

 and the pressure soon fell to less than •0001 mm . After two 

 hours the value of C 2 was 134 and the variation for the differ- 

 ent foils interposed did not differ appreciably from the last 

 column of Table 1. 



This test, however, was not very satisfactory on account of 

 the failure of the charcoal to remove promptly the gas emitted 

 by the heated platinum. Accordingly the charcoal bulb was 

 removed and a Gaede pump substituted for the Toepler pump. 

 When a vacuum of •0001 mm had been maintained for an hour, 



*A glass window in the brass case permitted observation of the foil. 



