Metals under Influence of Alpha Hays. 



413 



lated from the case by amber, guard tube, and ebonite, and the 

 joints made tight by sealing wax ; it was connected with a 

 Dolezalek electrometer, which, with 80 volts on the needle, had 

 a sensitiveness of about 650 mm divisions per volt. It was nec- 

 essary to reduce somewhat the current-sensitiveness of the 

 instrument ; for this purpose a small mica condenser of about 

 150 cm capacity was put in parallel with the electrometer. 



With a positive potential on the case, the saturation current 

 was reached with 40 volts ; and when the brass sector was 



Fig. 3. 



CENTIMETERS 



1 2 3 4 5 



interposed in the path of the rays the current fell to a negligi- 

 ble value. On the other hand, it was impossible to approach 

 saturation with a negative potential on the case, even at —320 

 volts ; the negative current when the rays were stopped by the 

 disc was always large (20-40 per cent of the total), and increased 

 with the negative potential on the case. This part of the cur- 



