Self-induction

By means of a transformer, alternating currents can be changed in voltage with comparatively small energy losses, these losses seldom exceeding some 2.5 or 5 per cent. That is the reason why most of the world uses a. c. High voltages are necessary on transmission and distribution lines, while low voltages are required for safe use.

In order to understand alternating currents, we must consider in detail two properties of an electric circuit. One of them is resistance. The other is self-inductance, the property having been discovered by Faraday.

When an electric current flows in a circuit, a magnetic field is produced by it. The direction of this field is correlated with that of the current.

Thus, the current in a conductor always produces magnetic field surrounding or linking with the conductor. This current changing, the magnetic field will change as well; and whenever there is a change in the magnetic field surrounding a conductor, an e. m. f. is induced in the conductor. This e.m.f. is called a se1f-induced e.m.f. because it is induced in the current-carrying conductor. The relationship between the current and the induced e.m.f. is a fundamental characteristic of an electric circuit. It was examined by the outstanding Russian scientist Lenz. He discovered the following: when the current in a circuit increases, the flux linking with the circuit also increases; this flux induces an e.m.f. in the conductor in such a direction as to oppose the magnetic flux increase. The current decreasing, an e.m.f. is induced in the direction which coincides with that of the current, thus, opposing the decrease of current. Lenz summed up the inductive action of currents and magnets, as follows: "in all cases of electromagnetic induction the induced currents have such a direction that their reaction tends to impede the change that produces them." He stated that the self-induced e.m.f. impedes any current change and tends to support the former current value. The above is known as Lenz's Law.

An electric circuit in which an appreciable e.m.f. is induced, while the current is changing, is called an inductive circuit, and we say that the circuit has self-inductance. In other words, the inducing of an e.m.f. in a circuit by a varying current in that very circuit is called self-induction.

A solenoid will have appreciable self-inductance since there will be a considerable magnetic field linked with the solenoid. Two parallel conductors forming an electric circuit will have relatively small self-inductance because the flux linked with them is small.

Remember that the induced e.m.f. is proportional to the rate the lines of force are cut at. Therefore, if the coil has many turns and produces a strong magnetic field, and if the current is stopped very quickly, the rapid collapse of the field will greatly increase the rate at which the lines of force are cut. But the cutting of the lines of force occurs only during the very short time that the magnetic field is collapsing, so only then is the extra current induced.