There is another quantity that is similar to resistance. It is called “reactance.” Like resistance, we measure reactance in ohms, and it follows the rule of Ohms Law.
Reactance only occurs in electrical or electronic circuits if the current flow is changing rapidly. It is usually important in “alternating current” (AC) circuits where the current periodically changes direction and amplitude at some rate called “frequency.” However, reactance does not exist in “direct current” (DC) circuits where current flows in one direction and its amplitude is not changing rapidly.
Reactance occurs because all circuit elements have “inductance” and “capacitance.” In AC circuits, the capacitance of wire wound resistors is seldom large enough to be considered, so we will ignore it in this discussion. However, the inductance of wire wound resistors can be critical!
All conductors have some inductance. When the conductor is coiled, as it usually is in wire wound resistors, this inductance becomes larger. In AC circuits, inductance causes “inductive reactance.” Inductive reactance and resistance add, increasing the resistor’s value.
Inductive reactance increases as the frequency of the alternating current increases. For example, a resistor might have enough inductance to create one ohm of reactance at a frequency of 60 Hz (cycles per second.) If we increase the frequency to 6,000 Hz (an audio frequency), the reactance would increase to 100 ohms. Increasing the frequency to 6,000,000 Hz (a radio frequency), increases the resistor’s reactance is 10,000 ohms.
Obviously, the inductance of wirewound resistors can be significant in AC circuits! When reactance is important in AC circuit applications, Riedon can wind the wire in a special way to eliminate or reduce the resistor’s inductance.