Relations between Electrical Measurements. 517 



fore be called the potential of the body. The potential of a me- 

 tallic body varies according to the distribution, dimensions, 

 position, and electrification of all surrounding bodies. It also 

 depends on the substance forming the dielectric. 



In any given circumstances, the potential of the body will be 

 simply proportional to the quantity of electricity with which it 

 is charged ; but if the circumstances are altered, the potential 

 will vary although the total amount of the charge may remain 

 constant. 



In a closed circuit in which a current circulates, the potential 

 of all parts of the circuit is different ; the difference depends on 

 the resistance of each part and on the electromotive force of the 

 source of electricity, i. e. on the difference of potentials which it 

 is capable of causing when its two electrodes are separated by 

 an insulator or dielectric. The different parts of a conductor 

 moving in a magnetic field are maintained at different potentials, 

 inasmuch as we have shown that an electromotive force is pro- 

 duced in this case. The potential of a body moving in an elec- 

 tric field (i. e. in the neighbourhood of electrified bodies) is con- 

 stantly changing, but at any given moment the potential of all 

 the parts is equal. The use of the word u potential " has the 

 following advantages. It enables us to be more concise than if 

 we were continually obliged to use the circumlocution, " electro- 

 motive force between the point and the earth;" and it avoids 

 the conception of a force capable of generating a current, which 

 almost necessarily, although falsely, is attached to " electromotive 

 force." 



Equipotential surfaces and lines of force in an electric field 

 may be conceived for statically electrified bodies ; these surfaces 

 and lines would be drawn on similar principles and possess ana- 

 logous properties to those described in a magnetic field (10). It 

 is hardly necessary to observe that the magnetic and the electric 

 fields are totally distinct, and coexist without producing any 

 mutual influence or interference. 



The rate of variation of electric potential per unit of length 

 along a line of force is at any point equal to the electrostatic 

 force at that point, i. e. to the force which a unit of electricity 

 placed there would experience. The unit difference of potential 

 is identical with the unit electromotive force ; and the electro- 

 meter spoken of as measuring electromotive force measures po- 

 tentials or differences of potential. 



48. Density, Resultant Electric Force, Electric Pressure. — 

 The three following definitions are taken almost literally from a 

 paper by Professor W. Thomson*. Our treatise would be in- 



* Paper read before the Royal Society, February 1860. Vide Proceed- 

 ings of the Royal Society, vol. x. p. 333 (1860), and Phil. Mag. S. 4. vol. xx. 

 p. 322. 



