4GG Prof. Bumstead and Mr. McGougan on Emission of 



foil slightly diminishes its stopping power for a-rays^ and 

 thus the ^-radiation from the 'emergence side is increased. 



After making the measurements which are recorded in the 

 last column of Table I., 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 

 gradually to the ends which were cooled by conduction 

 through the clamps and copper rods *. The current was 

 continued for 10 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 doubtless due to the fact that the gas 

 emitted by the platinum contained 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 G 2 was 134, and the variation for the different foils 

 interposed did not differ appreciably from the last column of 

 Table I. 



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, measurements were taken as before. 

 Then the strip was heated five times, for ten minutes at a 

 time, with intervals of fifteen minutes between, to avoid too 

 great heating of che clamps and copper rods ; the pump was 

 kept running continuously. Measurements of the S-ray 

 current were then made as before. The results are given in 

 Table II. 



Table IT. 



Foils 



Before 



After 



C 2 =150 



C 2 =134 



2 



100 



100 



3 



110 



108-2 



4 



102-7 



101-0 



5 



62-5 



61-0 



6 



24-7 



24-2 



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



