The condenser or other capacity acts as an effective barrier to the steady flow of direct currents.  Applying a fixed potential causes a mere rush of current to charge its surface to a definite degree, dependent upon the particular conditions.  The condenser does not act as such a barrier to alternating currents, for it is possible to talk through a condenser by means of the alternating voice currents of telephony, or to pass through it alternating currents of much lower frequency.  A condenser is used in series with a polarized ringer for the purpose of letting through alternating current for ringing the bell, and of preventing the flow of direct current.

The degree to which the condenser allows alternating currents to pass while stopping direct currents, depends on the capacity of the condenser and on the frequencies of alternating current.  The larger the condenser capacity or the higher the frequency of the alternations, the greater will be the current passing through the circuit.  The degree to which the current is opposed by the capacity is the reactance of that capacity for that frequency.  The formula is

Capacity reactance = 1 /_C_[omega]

wherein C is the capacity in farads and [omega] is 2[pi]_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.

[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.