A rheostat is a variable resistor that is used to control current. Rheostats are very similar to potentiometers, but they only use two connections instead of three. One connection is made at the end of a resistive element, and the other on the wiper. The rheostat does not divide voltage, and are typically designed to carry a substantial current.
Rheostats are primarily used as power control devices, light dimmers, and motor speed controllers. Because they are large and inefficient, rheostats have largely been replaced by potentiometers in today's industrial applications. However, because they are designed to handle a significant amount of power, rheostats still have some use in manually controlling pumps, fans and blowers where frequent adjustments are unnecessary.