Prof. Thomson on the Measurement of Electromotive Force. 323 



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 



R=4tf^, 



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. 



Cor. 2. The electric potential throughout the interior of a hollow 

 metallic body, electrified in any way by external influence, or, if in- 

 sulated, electrified either by influence or by communication of elec- 

 tricity to it, is constant, since there is no electric force in the interior 

 in such circumstances. 



[It is easily shown by mathematical investigation, that the electric 

 force experienced by an electric point containing an infinitely small 

 quantity of electricity, when placed anywhere in the neighbourhood 

 of a hollow electrified metallic shell, gradually diminishes to nothing 

 if the electric point be moved gradually from the exterior through 

 a small aperture in the shell into the interior. Hence the one value 

 of the potential close to the surface outside, mentioned in Cor. 1, is 

 equal to the constant value throughout the interior mentioned in 

 Cor. 2.] 



8. Interpretation of measurement by electrometer. — Every kind 

 of electrometer consists of a cage or case containing a moveable and a 

 fixed conductor, of which one at least is insulated and put in metallic 

 communication, by what I shall call the principal electrode passing 



