[m'lenxan] 



RADIOACTIVITY OF METALS 



39 



In making a measurement the connecting rod D was withdrawn 

 and the apparatus left intact. The inner cylinder B and the free 

 quadrants then slowly acquired a negative charge which finally reached 

 a limiting value. This value varied with the metal used in the con- 

 struction of the cylinders A and B, but did not vary with samples of 

 the same metal obtained from different sources. It was also the same 

 whether the needle of the electrometer was positively or negatively 



3 ^ 



charged. With lead, tin and copper the limiting value was reached in 

 about one hour, but with aluminium and zinc not before four or five 

 hours had elapsed. 



Readings for pairs of cylinders of these metals at 16 °C are set 

 forth in Fig. IL, where curves are drawn with the time expressed in 

 hours for abscissae and the negative potential attained by the inner 

 cylinder expressed in millivolts for ordinates. The limiting values for 

 the different metals are as follows: 



Millivolts. 



Aluminium 17S 



Zinc 160 



Lead 216 



Tin 95 



Copper 73 



In experimenting with lead cylinders the pressure of the air m 

 B was reduced from 750 mms. to 30 mms. of mercury and a slight 

 drop in the limiting potential of about three or four per cent was 

 observed. 



Measurements were also made with different gases at atmospheric 

 pressure between these cylinders, and it was found that the limiting 

 potential slightly increased with the density of the gas used. A set 



