Text 3 inductance

When a current flows through a conductor it sets up a magnetic field in the neighborhood of the conductor. This is negligible in us effects in a number of cases, but there are also many cases where this magnetic field exerts a profound effect upon the circuit. The magnetic field created by the current is represented by lines of magnetic flux, these lines consisting of closed loops which are interlinked with the electric circuit, itself necessarily a closed circuit. If the current is steady, the magnetic flux is constant and produces no effect upon the circuit, but if the current changes then the strength of the magnetic field also changes. If the current increases, the total number of lines of magnetic flux is increased, so that the total number of flux-linkages is also increased. It is, however, a fundamental law that whenever the number of flux-linkages changes, an e.m.f. is induced in the circuit linked with the flux. This e.m.f, is proportional to the rate of change of linkages, and one volt is induced when the linkages change at the rate of 10^s linkages per second.

Unit of Inductance.—The unit of inductance is the henry, and a circuit is said to possess an inductance of 1 henry if 1 volt is induced when the current changes at the rate of 1 ampere per second. The symbol for inductance is L, so that a circuit possesses an inductance of L henries if L volts are induced due to a. rate of change of current of 1 ampere per second.

This induced e.m.f. always acts in such a direction as to oppose the change of current in the circuit, and also the magnetic flux linked with it. Thus, if the current is rising, inductance tends to oppose its growth, and if the current is falling inductance tends to oppose its decay. Examples of this effect are found in the field circuit of an ordinary generator or motor, the field circuit being highly inductive. When switched into circuit the current does not immediately rise to its full value, but grows relatively slowly, while when the switch is opened, the current tends to continue as evinced by the spark at the opening contacts.

Mutual Inductance,—When inductance is .due to flux linking with the same circuit that carries the current, it is called self-inductance, in order to distinguish it from inductance due to flux linking with the circuit due. to current in a neighboring circuit which is called mutual inductance. In the latter case, two circuits are said to possess mutual inductance if a change of current in one circuit causes an e.m.f. to be induced in the, other circuit.

Two circuits are said to possess a mutual inductance of L henries if L volts are induced in one circuit due to a rate of change of current of 1 ampere per second in the other circuit. The circuit in which the current is changing is called the primary circuit, while the circuit in which the e.m.f. is induced is called the secondary circuit. When two circuits possess mutual inductance, either circuit can be employed as the primary, the value of the mutual inductance being the same, no matter which circuit is the primary and which is the secondary.