reostate

Reostate, commonly spelled rheostat, is a two-terminal variable resistor used to control current in an electrical circuit by changing the resistance in series with the load. The term is sometimes spelled reostat or reostate in older texts.

Traditional rheostats use a resistive element such as a coiled wire or a carbon track, with a sliding or rotating contact (a wiper) that taps the element at different points to vary resistance. The device is wired in series with the load; as resistance increases, current decreases.

Rotary rheostats and linear slide rheostats are common forms. They are used for applications such as dimming lights, controlling DC motor speed, and laboratory experiments. They dissipate heat proportional to voltage and current; efficiency declines as more power is dropped across the resistor.

With the advent of solid-state controllers, rheostats are less common in new designs but may still be found in vintage equipment, educational kits, or where a simple, manually adjustable current control is desirable.