Metals under Influence of Alpha Hays. 



407 



the two were mounted side by side under the same conditions, 

 the double electroscope had a sensitiveness about three time9 

 that of the tilted form, for equal stability. As used in the 

 present experiments the capacity of the leaf and its connections 

 was about 5 CI ", and the sensitiveness was so adjusted as to give 

 about 25 or 30 divisions on the scale in the microscope for 

 0*1 volt. The key, R, which was connected to a potentiometer 

 arrangement, allowed the leaf to be insulated, grounded, or 

 charged to any desired potential, and the volt sensitiveness 

 was taken immediately after each reading. 



The case containing the aluminium foilelectrode was exhausted 

 while the charcoal bulb was heated to a pressure of a few thou- 

 sandths of a millimeter, after which the bulb was cooled with 

 liquid air. The bulb was between the case and the pump and 

 thus formed a trap for the mercury vapor ; two liters of liquid 

 air were used, which lasted for about ten days. For that length 

 of time the air pressure could be kept continuously below 

 •0001""". The pressure of the mercury vapor in the case must 

 have been very small under these conditions. What vapor 

 there was from the rubber stop-cock grease used to cover the 

 joint between the base plate and cover, must have been con- 

 stantly distilling over from the case into the bulb ; that its 

 amount was small is shown by the fact that no visible traces 

 of the grease could be seen in the charcoal bulb after a week or 

 more of this distillation, although in the same time drops of 

 mercury of considerable size had distilled over from the pump. 



When the a-rays from the polonium were admitted into the 

 case through the aluminium-covered holes in the top, both the 

 insulated electrode and the case emitted electrons under their 

 action. In order to separate the two effects it was necessary to 

 charge the case ; when it was charged positively the electrons 

 which were set free from the case were prevented from reach- 

 ing the electrode, and the latter received a positive charge due 

 to the loss of electrons from its two surfaces ; when the case 

 was charged negatively, the electrode received electrons from 

 the case and emitted none itself. With + 25 volts on the case 

 the current of electrons from the aluminium foil was fully satu- 

 rated, and it was not increased by the application of +200 volts. 

 With +6 volts the lack of saturation was about 7 per cent. On 

 the other hand, with a negative potential on the case, it was 

 distinctly more difficult to reach the saturation value ; with 

 — 25 volts the current was 20 per cent less than with —200 volts. 

 The probable cause for this difference will be discussed in the 

 next paragraph. 



When the case was charged negatively, the current was 

 approximately twice as great as when the case was charged 

 positively. Thus, apparently, twice as many electrons were 



