354 Prof. J. J. Thomson : Experiments on 



rod. This rod before the measurement of the current was con- 

 nected with the earth, and the cylinder with one terminal of 

 a battery of small storage-cells, the other terminal of which 

 was connected with the earth. The battery contained 500 cells; 

 these were found sufficient in all cases to produce the satura- 

 tion-current, when the air was in the normal state a very 

 much smaller number of cells was sufficient to do this. As 

 the rod had to be strongly electrified in order to investigate 

 the induced radio-activity, there was some danger that the 

 ebonite disk with which it was connected might get charged 

 with electricity, and this electrification by leaking back to 

 the rod produce effects which were not due to the conductivity 

 of the air. To avoid this two ebonite disks were used, the one 

 nsed to support the rod whilst it was electrified was removed 

 before the current through the vessel was measured and 

 replaced by the second, which was carefully kept free from 

 electrification. 



In the earlier experiments four different rods were used as 

 electrodes, these were pieces of brass tubing of the same 

 length and diameter. The procedure was as follows : in the 

 morning the current through the vessel was measured, using 

 each of the rods as electrode in turn : the deflexion of the 

 electrometer in one minute, which is proportional to the cur- 

 rent through the vessel, rarely differed by more than about 

 one part in 75 for the four rods. During the day these rods 

 were subjected to different treatment; one was put aside to 

 serve as a standard, a second was connected with the negative 

 terminal of a Wimshurst machine, and exposed to the air of 

 the room, the Wimshurst machine was giving sparks about 

 3 cm. long ; the third rod was often connected with the positive 

 terminal of the machine and exposed to the air, while the fourth 

 rod was kept in the tank and connected in some experiments 

 with the negative terminal of the machine, in others with 

 the positive. The electrical machine was kept going all day 

 long, and tested from time to time to see that its electrification 

 did not reverse, and at the end of the day the current 

 through the tank was measured, using each of the rods as 

 electrode. If a rod had become radio-active, the current 

 through the tank with this rod as electrode would be greater 

 than it was when the rod was in its normal state, owing to 

 the additional ionization due to the rod. With the air in the 

 tank in its normal state I was never able to detect any change 

 in the rod due to its long negative electrification. The 

 volume of air in the tank, about 440 litres, was too small to 

 produce the radio-active effects observed by Elster and Geitel. 

 As the current coming up to the rod' in the tank was much 



