CONCEPTIONS USED IN THE SURVEY OF A FIELD OF FORCE

The Intensity at a point.

The space in the neighbourhood of charges of electricity, considered with reference to the electric phenomena occurring in this space, is spoken of as the electric field.

A new charge of electricity, placed at any point 0 in an electric field, will experience attractions or repulsions from all the charges in the field. The introduction of a new charge will in general disturb the arrangement of the charges on all the conductors in the field by a process of induction. If, however, the new charge is supposed to be infinitesimal, the effects of induction will be negligible, so that the forces acting on the new charge may be supposed to arise from the charges of the original field.

Let us suppose that we introduce an infinitesimal charge ∈ on an infinitely small conductor. Any charge e1, in the field at a distance r1 from the point 0 will repel the charge with a force ∈e/r12. The charge ∈ will experience a similar repulsion from every charge in the field, so that each repulsion will be proportional to ∈.

The resultant of these forces, obtained by the usual rules for the composition of forces, will be a force proportional to ∈–say a force R∈ in some direction OP. We define the electric intensity at 0 to be a force of which the magnitude is R, and the direction is OP.