162 Prof. E. Rutherford on Radioactivity 



a layer of thorium oxide was placed and covered with several 

 layers of foolscap-paper. The whole was enclosed in a lead 

 vessel A, with a door in the side to allow the plate C to be 



Fisr.l. 



readily moved. The crossed lines show the position of 

 insulators. The plate C was connected to the + pole of 

 a battery of 50 volts, the other terminal of which was to 

 earth. The plate B was connected to one pair of quadrants 

 of a delicate Thomson electrometer with a replenisher and 

 gauge, the other pair of quadrants of which was connected to 

 earth. 



With the arrangement in the figure, when B is insulated, 

 there can be no conduction-current from C along or through 

 the insulators, since the earth-connected vessel intervenes. 

 If the thorium-covered plate C was removed, and a brass 

 one of the same dimensions substituted, there was no appre- 

 ciable movement of the electrometer-needle. If, however, 

 the plate C, covered with thorium oxide, were left in the 

 vessel for several hours with the plate B charged — , on 

 removal of C and the substitution of a non-active metal 

 plate, the movement of the electrometer-needle showed that 

 B was receiving a -f charge. On reversing the battery, the 

 current was reversed in direction but equal in amount. The 

 current between the plates gradually decreased with the time, 

 and became inappreciable after a few days. By replacing 

 the thorium oxide, the experiment could be repeated. 



It was at first thought that possibly dust particles from the 

 thorium oxide might have escaped from under the paper and 

 in some way adhered to the upper plate. An examination of 

 the plate B, however, revealed no trace of thorium oxide on 

 its surface. The plate made the air a conductor in its 

 neighbourhood, as if it were covered with a thick layer of 

 radio-active substance. If the surface of the plate was care- 



