59G ]\Ir. G. C. Simpson on 



made of zinced iron wire nettino-. Inside this cao-e different 

 metals were hung on an insulating support and then con- 

 nected by a wire of the same material to an electrometer 

 outside. The metal to be investigated was first connected to 

 the earthed cage and a zero reading of the electrometer taken; 

 then, on insulating, it was found that the metal began to 

 become charged, until in a little over an hour a steady 

 deflexion of the electrometer-needle showed that the charging 

 had ended. The final voltao-es obtained varied in sisn and 

 amount from metal to metal, being as follows : — 



Copper... +-70 volt. Lead +'23 volt. 



Iron +'4:6 „ Magnalium... —'2% „ 



(magnesium aluminium) 

 Tin -f-25 „ Sodium —'TO „ 



It will be seen at once that these numbers are practically 

 those of the volta difference of potential between the different 

 metals and zinc. The zinced iron cage and the metal 

 within form the poles of a battery with the slightly ionized 

 air between actiog as the electrolyte. The only effect of 

 greatly increasing the ionization of the aiv by means of 

 Kontgen rays was to increase the rate at which the final charge 

 was obtained, the charo-e itself remainin g- unaltered. Thus we 

 see that Zeleny's results do not apply to conductors in the 

 ionized air of the atmosphere — at least with the air at rest, 

 air in motion will be considered later. A conductor in 

 ionized air must be looked upon as a conductor in an electrolyte, 

 the charge it obtains being determined by its position in the 

 Yolta series and not by the mobility of the ions. It is easy to 

 see that a conductor in ionized air at rest will not become 

 " charged negatively until it produces in the surrounding air 

 an electrical field strong enough to neutralize the difference 

 in the mobility of the ions.''' Consider a space filled with 

 ionized air, there being an equal number of positive and 

 negative ions in a cubic centimetre. Xow imagine a con- 

 ducting sphere suddenly placed in the middle of the space. 

 For the first instant more negative than positive ions will 

 reach it and be absorbed, but the corresponding positive ions 

 are all within molecular distances of the sphere so that no 

 field can be set up outside the immediate neighbourhood of 

 the surface. There may be something of the nature of a 

 double layer at the surface, but its presence cannot be 

 experimentally observed, for aU conductors have the same 

 layer. We must accept then, as the result of both theory and 

 experiment^ that in air at rest there is no tendency for an 

 insulated conductor to become charged negatively as a result 



