66 ELEMENTS OF STATIC ELECTRICITY. 



But if it be objected that the case is not parallel, see- 

 ing that we live on the surface, it must be remembered 

 that we have an atmosphere above us which is a part of 

 the earth's matter; so that, although we live on the 

 solid surface, we do not live on the outer surface : and 

 the surface on which we live is practically equipoten- 

 tial over limited areas. 



Faraday, evidently, might have generated electricity 

 with insulated instruments, inside the cube, and con- 

 densed it on insulated conductors, without either dis- 

 turbing the electric conditions by which he was sur- 

 rounded, or being prevented by them : just as we do 

 without disturbing the earth's electricity, or being pre- 

 vented by it. But any connection by a conductor, 

 between his instruments and the cube, would have 

 caused the charge to disappear; just as a similar con- 

 nection with the earth produces the same result. 



THICKNESS OF ELECTRIFIED SURFACE. The idea 

 of surface condensation implies that an electrified sur- 

 face must be something more than a mere superficies. 

 It must have a certain degree of thickness, the elec- 

 tricity penetrating the conductor and surrounding air 

 to a certain depth, in proportion to the resistance of the 

 air, and the attraction or repulsion of the charge on 

 the conductor. Hence the amount of static charge 

 which may be condensed on a conductor, per unit of 

 surface, depends on the resistance of the air. 



CONVECTION. It has already been shown that dry 

 air is one of the best insulators ; but, since it is a fluid, 

 its resistance cannot be so great as that of a solid of the 

 same insulating power; for the air molecules, in contact 

 with an electrified surface, becoming charged, fly off 

 under the influence of repulsion and induction, while 



