wherein C is the capacity in farads and ω is 2 π n, or twice 3.1416 times the frequency.

All the foregoing leads to the generalization that the higher the frequency, the less the opposition of a capacity to an alternating current. If the frequency be zero, the reactance is infinite, i.e., the circuit is open to direct current. If the frequency be infinite, the reactance is zero, i.e., the circuit is as if the condenser were replaced by a solid conductor of no resistance. Compare this statement 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.

Fig. 32. Spiral of Wire
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Fig. 33. Spiral of Wire Around Iron Core
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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.