Actual Coils, Condensers, and Resistors

Author: J.B. Hoag

Actual coils of wire have resistance as well as inductance. In addition, they have a small amount of "distributed" capacity between turns. Hence a coil must be thought of as a series resistance and inductance, shunted by a small capacitance. At very high frequencies, the reactance of the condenser may be small enough so that practically all of the current passes through this bypass condenser. The coil then acts, electrically, as though it were a condenser.

The leads and plates of actual condensers have sufficient inductance so that, at very high frequencies, their inductive reactance must be included in the circuit calculations.

In a radio frequency circuit, the magnetic field set up inside a wire forces the current to travel only on the surface of the wire. This "skin effect" increases as the frequency is increased. Due to the reduced cross-section of wire through which the current flows, the resistance of the wire is considerably higher than the d.c. value. Conductors with large surfaces are needed at very high frequencies and, since no current flows in the inner part of the conductor, thin tubing serves just as well as solid wire of the same diameter.

The h.f. resistance of a coil may be many times its d.c. resistance, and also many times the h.f. resistance of the same wire when straightened out. This is because the currents are confined by the magnetic fields to only a small part of the total surface of the wires.

It should be noted that resistors do not offer pure resistance and that any and all types of units have capacity to any near-by solid object.