rheostat

A rheostat is a variable resistor used to control current by adjusting resistance in an electrical circuit. Historically, it refers to a two-terminal device in which a sliding or rotating contact selects a portion of a resistive element to change the effective resistance. In strict use, a rheostat is intended to limit current with two connections, while a potentiometer is a three-terminal device used as a voltage divider. The term rheostat is often used more loosely to describe any adjustable resistance.

The classic construction is a wire-wound resistive element on an insulating former. A sliding contact (slider) or a rotary contact moves along the element to select a specific resistance value. Common materials include nichrome and other high-temperature alloys. Because the device can dissipate substantial heat, rheostats are typically larger than small variable resistors and are rated for higher power. The resistance range can span from a fraction of an ohm to tens of kilohms, and the actual power dissipated follows P = I^2R or P = V^2/R, depending on circuit conditions.

Applications and usage have evolved over time. Rheostats were widely used for lamp brightness control and DC motor speed regulation in early equipment. In modern electronics, low-power controls more often employ potentiometers or electronic controllers, while rheostats remain in some laboratory power supplies and high-current motor controllers where rugged, continuous adjustment is required. They are valued for simple, direct control but are less common in compact modern devices due to size, efficiency, and wear considerations.