76.] ELECTRIC INDUCTION. 77
this condition is shewn to be fulfilled by the electric forces with the most perfect accuracy. Hence the law of electric force is verified to a corresponding degree of accuracy.
Surface-Integral of Electric Induction, and Electric Displacement through a Surface.
75.] Let be the resultant force at any point of the surface, and the angle which R makes with the normal drawn towards the positive side of the surface, then is the component of the force normal to the surface, and if is the element of the surface, the electric displacement through will be, by Art. 68,
Since we do not at present consider any dielectric except air, .
We may, however, avoid introducing at this stage the theory of electric displacement, by calling the Induction through the element . This quantity is well known in mathematical physics, but the name of induction is borrowed from Faraday. The surface-integral of induction is
and it appears by Art. 21, that if are the components of , and if these quantities are continuous within a region bounded by a closed surface , the induction reckoned from within outwards is
the integration being extended through the whole space within the surface.
Induction through a Finite Closed Surface due to a Single Centre of Force.
76.] Let a quantity e of electricity be supposed to be placed at a point , and let be the distance of any point from , the force at that point is in the direction .
Let a line be drawn from in any direction to an infinite distance. If is without the closed surface this line will either not cut the surface at all, or it will issue from the surface as many times as it enters. If is within the surface the line must first