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A variable resistor used to control current drawn by an electric motor, to dim lighting, etc. It may consist of a resistive wire, wound in a helix, with a sliding contact varying the effective length, or a series of fixed resistors connected between a row of button contacts. Or, for heavy loads, electrodes dipped in solutions can be used, the resistance being controlled by the immersion depth and separation of the electrodes. Rheostats are used to adjust generator characteristics, dim lights, and control the speed of electric motors.

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