On the Ionisation of Atmospheric Air. 



157 



It might be considered as possible that the conducting power of 

 the air was due to some effect of the walls of the apparatus, related 

 perhaps to the Russell* photographic effect and the nucleus-producing! 

 effects of metals. These effects, however, are in the case of brass 

 certainly very slight (I have not been able to detect any cloud-nuclei 

 arising from the presence of brass) ; they are enormously greater in 

 the case of amalgamated zinc. Yet the presence of a piece of amal- 

 gamated zinc in the apparatus was without effect on the rate of 

 leak. If then the walls of the vessel influence in any way the 

 ionisation of the air in the vessel, this influence is not proportional 

 to the photographic or nucleus-producing effects of the metals. 



To find the loss of electricity corresponding to the observed fall of 

 potential of the leaking system, the condenser was removed, and the 

 capacity of the Exner electroscope, with the connecting wires and the 

 rod supporting the leaking system, was first determined by finding the 

 fall of potential resulting from contact with a brass sphere of which 

 the radius was 2*13 cm. The sphere, suspended by a silk thread, was 

 in contact with a thin earth-connected wire, except when momentarily 

 drawn aside by a second silk thread and brought into contact with 

 the end of another thin wire leading to the electroscope. Except for 

 these two wires the sphere was at a distance great compared with its 

 radius from all other conductors. The rise of potential which occurred 

 in the leaking system after a momentary contact with the system con- 

 sisting of the supporting rod, electroscope, and connecting wires was 

 then compared with the simultaneous fall of potential of the latter 

 system. The loss of electricity corresponding to a given fall of 

 potential of the leaking system was thus obtained. It was found to 

 be sensibly the same for potentials in the neighbourhood of 100 volts as 

 for the higher voltages (about 200 volts) generally used, the variations 

 in capacity due to the change of position of the gold leaf being too 

 small to be detected. The system had a practically constant capacity 

 equal to 1*1 cm. 



It was possible now to compare the rates of leakage for different 

 strengths of the electric field. 



Brass apparatus used, air at atmospheric pressure. 



Initial difference of Fall of potential 



potential. per hour. 



210 volts. 4-1 volts. 



120 „ 4-0 „ 



The leakage of electricity through the air is thus the same for a poten- 

 tial difference between the leaking system and the walls of the vessel 

 of 210 volts as for one of 120 volts. On the view that the conduction 



* Kussell, ' Koy. Soc. Proc.,' vol. 61, p. 424, 1897 ; vol. 63, p. 102, 1898. 

 f Wilson, ' Phil. Trans./ A, vol. 192, p. 431. 



N 2 



