The action of capacity referred to the current wave is as follows: As the wave starts from zero value and rises to its maximum value, the current is due to the discharge of the capacity, which would be represented by a condenser. In the case of a sine current, the period required for the current to pass from zero value to maximum is one-quarter of a cycle.
Figs. 1,285 and 1,286.—Diagrams showing effect of condenser in direct and alternating current circuits. Each circuit contains an incandescent lamp and a condenser, one circuit connected to a dynamo and the other to an alternator. Since the condenser interposes a gap in the circuit, evidently in fig. 1,285 no current will flow. In the case of alternating current, fig. 1,286, the condenser gap does not hinder the flow of current in the metallic portion of the circuit. In fact the alternator produces a continual surging of electricity backwards and forwards from the plates of the condenser around the metallic portion of the circuit, similar to the surging of waves against a bulkhead which projects into the ocean. It should be understood that the electric current ceases at the condenser, there being no flow between the plates.
At the beginning of the cycle, the condenser is charged to the maximum amount it receives in the operation of the circuit.
At the end of the quarter cycle when the current is of maximum value, the condenser is completely discharged.
The condenser now begins to receive a charge, and continues to receive it during the next quarter of a cycle, the charge attaining its maximum value when the current is of zero intensity. Hence, the maximum charge of a condenser in an alternating circuit is equal to the average value of the current multiplied by the time of charge, which is one-quarter of a period, that is
maximum charge = average current × ¼ period (1)
Fig. 1,287.—Diagram showing alternating circuit containing capacity. Formula for calculating the ohmic value of capacity or "capacity reactance" is Xc = 1 ÷ 2πfC, in which Xc = capacity reactance; π = 3.1416; f = frequency; C = capacity in farads (not microfarads). 22 microfarads = 22 ÷ 1,000,000 = .000022 farad. Substituting, Xc = 1 ÷ (2 × 3.1416 × 100 × .000022) = 72.4 ohms.