Inductance

Author: J.B. Hoag

When a battery is connected to a coil of wire, the current rises gradually instead of instantaneously to its final value. This is because of the need to establish the magnetic field of the coil. At first, as the field starts to form, the magnetic lines of force move out, cut the wires of the coil and generate a "back" e.m.f. which opposes that of the battery. This is called self-induction. When the battery is disconnected, the lines of force collapse, cutting the coils and generating an e.m.f. which is in the same direction as that of the battery, i.e., in such a direction as to keep the current flowing. In both cases, the back e.m.f. opposes the change which causes it. This is analogous to inertia or mass in mechanics, as, for example, in the case of a fly-wheel. When a force is first applied to the wheel, its inertia prevents its attainment of full speed of rotation. Again, when the force is removed, it requires some time for the wheel to stop rotating.

The inductance of a coil is measured in henries. It depends on the number of turns, the cross-sectional area, and the material (air or iron) inside the coil. A straight wire also has a small amount of inductance. The formulas for series and parallel combinations of inductances are the same as for resistances, provided the magnetic fields of the various coils do not interact with each other.