This occurs in the effort of lines to shorten their paths, to lessen their density, to pass to better media.  Indeed, a close examination will show that wherever power is expended in developing current in a circuit, cutting lines of force, the energy expended is first employed in stretching the lines, which thus receive the energy required to permit them, in shortening, to cut the conductor and set up currents in the electric circuit in accordance with the potential difference developed in that circuit and its resistance.

I think we may also say, though I do not remember to have seen the statement so put, that whenever electric potential is set up inductively, as in self-induction, mutual induction, induction from one circuit to another, and induction from magnets or magnetic field, it is set up by the movement of lines of force laterally across the body, mass or conductor in which the potential is developed, and that whenever current is set up in a wire or an existing current prolonged, or an existing current checked by induction, self-induction, or induction from magnets, the action is a transfer of energy, represented by strained lines of force shortening or lessening their resistance, or lengthening and increasing the resistance in their paths.  The magnetic field is like an elastic spring—­it can in one condition represent stored energy—­it can be strained and will store energy—­it can be made to relieve its strain and impart energy.

[Illustration:  Fig. 1.]

Let us examine some known phenomena in this light.  Take the case of a simple wire, conveying current, say, in a line away from observer, Fig. 1.  There exists a free field of circular magnetism (so called), shading off away from the wire, and which is represented by concentric circles of increased diameter.  The superior intensity or strength of the lines near the wire may also be represented by their thickness.  This is often shown also by crowding the lines near the wire, though I am disposed to regard Fig. 1 as more nearly expressing the condition, unless we are to regard the lines as simply indicating a sort of atmosphere of magnetic effect whose density becomes less as we proceed outward from the wire, in which case either form of symbol suffices.  The direction of polarization of the lines may be indicated by an arrow head pointing in a direction of right-handed rotation in the path of the lines.  This is the typical figure or expression for all forms of simple magnetic circuit—­the form of the lines, their length, position, density, will depend on the shape of the conductor or conductors (when more than one) and the materials surrounding or in proximity to the wire or wires.