with the correlative generalization which follows the next thought
upon inductance.

Inductance of the Circuit. Inductance is the property of a circuit
by which change of current in it tends to produce in itself and other
conductors an electromotive force other than that which causes the
current. Its unit is the _henry_. The inductance of a circuit is one
henry when a change of one ampere per second produces an electromotive
force of one volt. Induction _between_ circuits occurs because the
circuits possess inductance; it is called _mutual induction_.
Induction _within_ a circuit occurs because the circuit possesses
inductance; it is called _self-induction_. Lenz' law says: _In all
cases of electromagnetic induction, the induced currents have such a
direction that their reaction tends to stop the motion which produced
them_.

[Illustration: Fig. 32. Spiral of Wire]

[Illustration: Fig. 33. Spiral of Wire Around Iron Core]

All conductors possess inductance, but straight wires used in lines
have negligible inductance in most actual cases. All wires which are
wound into coils, such as electromagnets, possess inductance in a
greatly increased degree. A wire wound into a spiral, as indicated in
Fig. 32, possesses much greater inductance than when drawn out
straight. If iron be inserted into the spiral, as shown in Fig. 33,
the inductance is still further increased. It is for the purpose of
eliminating inductance that resistance coils are wound with double
wires, so that current passing through such coils turns in one
direction half the way and in the other direction the other half.