FIG. 1,277.—Diagram showing alternating circuit containing inductance. Formula for calculating the ohmic value of inductance or "inductance reactance," is Xi = 2πfL in which Xi = inductance reactance; π = 3.1416; f = frequency; L = inductance in henrys (not milli-henrys). L = 15 milli-henrys = 15 ÷ 1000 = .015 henrys. Substituting, Xi = 2 × 3.1416 × 100 × .015 = 9.42 ohms.
The frequency of a current being the number of periods or waves per second, then, if T = the time of a period, the frequency of a current may be obtained by dividing 1 second by the time of a period; that is
| one second | 1 | ||||
| frequency | = | = | (4) | ||
| time of one period | T |
substituting 1 / T for f in equation (3)
| L | |||
| Xi | = | 2π | |
| T |
Fig. 1,278.—- Diagram illustrating effect of capacity in an alternating circuit. Considering its action during one cycle of the current, the alternator first "pumps," say from M to S; electricity will be heaped up, so to speak, on S, and a deficit left on M, that is, S will be + and M-. If the alternator be now suddenly stopped, there would be a momentary return flow of electricity from S to M through the alternator. If the alternator go on working, however, it is obvious that the electricity heaped up on S helps or increases the flow when the alternator begins to pump from S to M in the second half of the cycle, and when the alternator again reverses its pressure, the + charge on M flows round to S, and helps the ordinary current. The above circuit is not strictly analogous to the insulated plates of a condenser, but, as is verified in practice, that with a rapidly alternating pressure, the condenser action is not perceptibly affected if the cables be connected across by some non-inductive resistance as for instance incandescent lamps.
Capacity.—When an electric pressure is applied to a condenser, the current plays in and out, charging the condenser in alternate directions. As the current runs in at one side and out at the other, the dielectric becomes charged, and tries to discharge itself by setting up an opposing electric pressure. This opposing pressure rises just as the charge increases.
A mechanical analogue is afforded by the bending of a spring, as in fig. 1,279, which, as it is being bent, exerts an opposing force equal to that applied, provided the latter do not exceed the capacity of the spring.