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30 RICHARDSON'S BLUEBOOK OF PROJECTION
(11) In direct circuits the current strength is determined strictly by the resistance and the voltage applied, according to Ohm's Law, but with a.c. the resistance of the conductor is not the only factor tending to oppose and limit the flow of current. Hence, Ohm's Law does not have the same form for a.c. as for d.c. The form of Ohm's Law for a.c. is IR = Z, in which Z stands for impedance. The resistance of the conducting path, R, is one factor, but only one, in the complex of opposing forces that go to make up Z.
(12) One of the factors involved in Z, or impedance, is magnetism. Magnetism is also an effect manifested by direct currents, as has been seen. Ohm's Law is not strictly true during the instant when direct current is turned on, nor at the moment when it is cut off. There is a very brief interval of time, while a magnetic field is building up around the wires, when the current does not reach the full strength required by the voltage and resistance of the circuit. The appearance of normal current, as called for by Ohm's Law, lags momentarily behind the application of the voltage. But when the magnetic field is once established, the relations indicated by Ohm's Law obtain, and continue to exist (unless the magnetic field is disturbed) until the current is switched off. When this happens the field is said to "collapse about" the wire, and in doing so generates a sudden surge of current stronger than that called for by Ohm's Law. In d.c. circuits that contain magnetic elements (for example, circuits that supply the "field" windings of loud speakers) opening of the switch is accompanied by a surge of current which flashes across the partially-opened contacts, often melting the edges of the switch blades and studs. For this reason spring-operated switches, that open very rapidly when their handle is operated, are used in such circuits. During the instant the switch is opening the current is momentarily greater than Ohm's Law specifies.
(13) In an a.c. circuit the current is continually increasing and decreasing, rising to a maximum in one direction, dropping off to zero, rising to maximum in the opposite direction, and so on, scores or hundreds or thou