by Metals uncle)' tlie Influence of Alpha Rays. 911 



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 

 rending. 



The cas« containing the aluminium foil electrode was 

 exhausted, while the charcoal-bulb was heated, to a pressure 

 of a few thousandths of a millimetre, and the bulb was 

 cooled with liquid air. The bulb was between the case and 

 the pump and thus formed a trap for the mercury vapour. 

 Two litres of liquid air w r ere used which lasted for about ten 

 days. For that length of time the air-pressure could 

 be kept continuously below 'OOOl mm. The pressure of the 

 mercury vapour in the case must have been very small under 

 these conditions. What vapour there was from the rubber 

 stopcock grease used to cover the joint between the base- 

 plate and cover must have been constantly 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 dis- 

 tillation, 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 reaching 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 saturated, 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 

 emitted by the case under the influence of the a-rays as from 

 the two sides of the aluminium-foil electrode through which 



