emitted by Zinc bombarded by Alpha Rays. 

 Table VI. 



501 



Zinc surface deposited from zinc vapour in a high vacuum. 



Time since 



surface 

 was made. 



Electric field alone. 



Electric and magnetic field. 



Deflexion 



per 



minute. 



Saturation 

 current. 



Deflexion 



per 



minute. 



Saturation 

 current. 



ittin. 



10 



30 



90 



190 



mm. 

 53 



43 



25 



19 



lG6xlO- 5 e.s.u. 

 lo5 



79 



GO 



mm. 

 53 



43 



22 



15 



166XlO- 5 e.s.u. 

 135 

 69 



47 " 1 



One point which is brought out very prominently by these 

 readings is that for the first half hour the current between 

 P and X, under a potential difference of 80 volts was the 

 same whether a magnetic field, as high even as 1160 gauss, 

 was applied or not. From this it was manifest that during 

 this interval there was practically no emission of electrons 

 from the newly deposited zinc surface, under bombardment 

 by the alpha rays. It will be noted, too, that during the 

 interval the saturation current was about 165 x 10~ 5 e.s.u., 

 which was about twice as great as that saturation current 

 obtained in the previous experiment with the ordinary zinc 

 plate without the fresh deposit. This was very probably 

 due to the air-pressure in the apparatus being somewhat 

 higher immediately after the deposit had been than it was 

 when the observations were made with the zinc plate in 

 its original condition. Even with the Gaede pump in action 

 the effect of heating the furnace would be to drive off a con- 

 siderable quantity of air from the walls of the vessel into the 

 apparatus, and as the volume of the apparatus was consider- 

 able it would take time to remove this air again. That this 

 interpretation was the correct one is shown by the readings 

 taken in the second period extending from 55 minutes after 

 the deposit had been made up to 85 minutes after that time. 

 These, it will be seen, show that with increasing positive 

 potentials the current increased and finally reached a maxi- 

 mum of only 78 X 10~ 5 electrostatic unit. This would indicate 

 that during the first half hour the ionization constituent of 

 the current w r as very considerable, as it should have been 

 on account of the higher air-pressure, while at the end of 



