72 Prof. J. A. Fleming on the 



Table No. 8. Lamp No. 1. Electrostatic Voltmeter. 



Working volts of the lamp. 



Static potential-difference in 



volts between platinum wire 



and positive electrode of 



the lamp. 



62 



79 



97 



118 



53 



75-5 

 85 

 107 



The figures in the above Table No. 8 show that when the 

 surface of the collecting-plate is very small and is placed 

 some distance from the base of the negative leg of the carbon 

 it is brought down only to the potential of some point (probably 

 the nearest point) on the carbon conductor, and that there- 

 fore the potential- difference between the plate and positive 

 electrode of the lamp is somewhat less than the potential- 

 difference between the working terminals of the lamp. At 

 the same time, however, the electrostatic voltmeter shows no 

 measurable potential- difference between the negative terminal 

 of the lamp and the platinum wire, and the most sensitive 

 galvanometer between these points gives no indication of any 

 current. 



By means of the electrostatic voltmeter it was, however, 

 ascertained that in those cases in which the metallic plate 

 presented considerable surface (several square centimetres) 

 and was placed so that some portion of it was not removed by 

 more than* a centimetre or two from the base of the negative 

 leg of the carbon, it was brought down almost immediately 

 to the potential of the negative terminal of the lamp. If the 

 middle plate is placed at a little distance from the carbon 

 loop then, on testing by the condenser method, it is found 

 that the plate is not instantly brought down to the potential 

 of the negative terminal, but that some few seconds have to 

 elapse before this is the case. 



§ 11. A series of experiments was then undertaken in order 

 to determine the effect of varying (1) the surface, and (2) the 

 position of the metal plate in the bulb, and in these experi- 

 ments the plate was sometimes of platinum and sometimes of 

 aluminium. In all cases the vacuum was a very perfect one, 

 any occluded gases in the plates being got rid of by special 

 means. 



