POTENTIAL AND EARTH CURRENTS. 181 



ducting surface; the earth's surface, the lower one; and 

 the dense lower stratum, the dielectric. And, as in 

 the Leydeii jar, any change of potential in either sur- 

 face produces the opposite electric condition in the 

 other surface. 



The upper surface, being insulated, corresponds to 

 the inner coating; and the lower uninsulated surface, 

 to the outer coating. But since those surfaces are of 

 vast extent, any limited area of upper surface would be 

 connected with a conducting surface at its outer edges ; 

 through which connection electricity would be repelled 

 from this area, or attracted to it, as the potential of the 

 surface below it had a greater or less intensity. But 

 the earth connection, of the lower surface, would be 

 exactly the same as that of the outer coating of the 

 Leyden jar. 



We live and move on the outer coating of this Ley- 

 den jar; on a surface practically equipotential within 

 limited areas ; and hence do not perceive electric action 

 taking place, no matter how highly charged the jar 

 may be, except when the tension becomes strong 

 enough to overcome the resistance of the dielectric, or 

 to render prominent or visible the action on either 

 side of it: 



This surface then, which we call neutral, is really a 

 charged surface ; but, like the outer coating of a 

 charged Leyden jar, quiescent, till brought into action 

 by connection with the inner coating, or by induction 

 between the two. 



ASCENDING AND DESCENDING CURRENTS. We 

 have seen how air currents are produced by the action 

 of an electric machine, and how light bodies vibrate 

 between electrodes connected with opposite surfaces of 



