rheostat (rēˈəstătˌ) [key], device whose resistance to electric current depends on the position of some mechanical element or control in the device. Typically a rheostat consists of a resistance element equipped with two contacts, or terminals, by which it is attached to a circuit: a fixed contact at one end and a sliding contact that can be moved along the resistance element. Electric current enters and leaves the resistance element through the contacts. By moving the sliding contact toward or away from the fixed contact, the length of the resistance element through which the current travels can be decreased or increased. In this way the current through the circuit can be increased or decreased. Rheostats are widely used for such purposes as controlling the speed of electric motors and dimming electric lights, but they have been replaced in many applications by systems based on semiconductor devices that waste far less power. See potentiometer.