334 



if electrified, experiences a real mechanical force in a direction per- 

 pendicular to the surface outwards, equal in amount per unit of area 

 to 2rcy> 2 , p denoting, as before, the electric density at the part of the 

 surface considered. This force may be called either a repulsion (as 

 according to the views of the eighteenth century school) or an attrac- 

 tion effected by tension of air between the surface of the conductor 

 and the conducting boundary of the air in which it is insulated, as it 

 would probably be considered to be by Faraday ; but whatever may 

 be the explanation of the modus operandi by which it is produced, it 

 is a real mechanical force, and may be reckoned as in Col. 5 of the 

 preceding Table, in grains weight per square inch or per square foot. 

 In the case of the soap-bubble, for instance, its effect will be to cause 

 a slight enlargement of the bubble on electrification with either 

 vitreous or resinous electricity, and a corresponding collapse on being 

 perfectly discharged. In every case we may regard it as constituting a 

 deduction from the amount of air-pressure which the body experiences 

 when unelectrified. The amount of this deduction being different 

 in different parts according to the square of the electric density, its 

 resultant action on the whole body disturbs its equilibrium, and 

 constitutes in fact the resultant electric force experienced by the 

 body. 



6. Collected formula of relation between electric density on the 

 surface of a conductor, electric diminution of air-pressure upon it, 

 and resultant force in the air close to the surface. Let, as before, 

 p denote the first of these three elements, let p denote the second 

 reckoned in units of force per unit of area, and let R denote the 

 third. Then we have 



7. Electric potential. The amount of work required to move a 

 unit of electricity against electric repulsion from any one position to 

 any other position, is equal to the excess of the electric potential of 

 the first position above the electric potential of the second position. 



Cor. 1 . The electric potential at all points close to the surface of 

 an electrified metallic body has one value, since an electrified point, 

 possessing so small a quantity of electricity as not sensibly to in- 

 fluence the electrification of the metallic surface, would, if held near 

 the surface in any locality, experience a force perpendicular to the 

 surface in its neighbourhood. 



