Charged Bodies at Moderate Temperatures. 443 



volts at a temperature between 300° C. and 400° C. The 

 following contains the results of a number of additional ob- 

 servations at potentials of 1 or 2 volts and with both positive 

 and negative charges. 



§ 8. When wires of the same metal are connected with two 

 mutually insulated metals, and the atmosphere between these 

 two metals is rendered conductive by Rontgen rays, uranium 

 rays, or ultra-violet light, the wires acquire a difference of 

 potential * . In the case of ultra-violet light this difference 

 depends, amongst other things, as was first pointed out by 

 Righi, on the distance apart of the two mutually insulated 

 metals. 



To test whether or not a similar effect was produced when 

 the air between the plates was subjected to the effects of 

 thorium-radiation, an arrangement of the following nature 

 was used. 



Electrometer. 



A cylinder of zinc, C, had an opening, 0, in it. A rod of 

 zinc, A, in metallic connexion with the cylinder carried a zinc 

 plate, 1). A second rod, E, of brass carried a plate of zinc, D; 

 this rod E passed loosely through a solid paraffin collar P. 

 The distance between D and B could be varied. 



On B different metals were placed after having been 

 covered with a thin layer of a salt of thorium. It was found, 

 for instance, when D was connected to the insulated terminal 

 and B to the case of a quadrant-electrometer, that the electro- 

 meter-reading deviated from the metallic zero \ in a few 



* Phil. Ma^. ser. 5. vol. xliii. pp. 418-440 (June 1897). 



f By metallic zero is meant that electrometer-reading which is obtained 

 when the quadrants are all metallically connected to the case of the 

 instrument. 



2 G2 



