ON STANDARDS OF ELECTRICAL RESISTANCE. 155 
potential defined in the second passage quoted is identical with what we 
have called the electromotive force between the two points (16 and 27). 
When, instead of a difference of potentials, the potential simply of a 
point is spoken of, the difference of potential between the point and the 
earth is referred to, or, as we might say, the electromotive force between the 
point and the earth. 
The potential at all points close to the surface and in the interior of any 
simple metallic body is constant; that is to say, no electromotive force can be 
produced in a simple metallic body by mere electrical distribution; the 
potential at the body may therefore be called the potential of the body. The 
potential of a metallic 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 pro- 
portional 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 cireuit 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, 7. ¢. 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 produced in this case. The 
potential of a body moving in an electric field (7. e. in the neighbourhood of 
electrified bodies) is constantly changing, but at any given moment the 
potential of all the parts is equal. The use of the word “ potential” has the 
following advantages. It enables us to be more concise than if we were 
continually obliged to use the circumlocution, ‘‘ electromotive 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 con- 
ceived for statically electrified bodies; these surfaces and lines would be 
drawn on similar principles and possess analogous 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, 7. e. to the 
foree which a unit of electricity placed there would experience. The unit 
difference of potential is identical with the unit electromotive force ; and the 
electrometer spoken of as measuring electromotive force measures potentials 
or differences of potential. 
48. Density, Resultant Electric Force, Electric Pressure.—The three fol- 
lowing definitions are taken almost literally from a paper by Professor W. 
Thomson *. Our treatise would be incomplete without reference to these 
terms, and Professor Thomson’s definitions can hardly be improved. 
* Electric Density.—This term was introduced by Coulomb to designate the 
quantity of electricity per unit of area in any part of the surface of a con- 
* Paper read before the Royal Society, Feb. 1860. Vide Proc. R. 8. vol. x. p. 319 (1860), 
_ and Phil. Mag. vol. xx. Ser. 4 (1860) p. 322. 
