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A rheostat is a device which is used to vary the resistance in an electrical circuit without interrupting the circuit.  Rheostats are also used in a number of electrical applications and a variety of industries. Many companies manufacture these devices and people can also make their own, as is sometimes done in science classes to introduce students to the topic of electrical resistance.

This device relies upon the fact that the current flowing through a circuit will vary depending on the amount of resistance it encounters. Low resistance means high current, because there is nothing to impede the current, and high resistance means low current. This characteristic of electrical circuits can be harnessed to change the performance of a circuit to meet specific needs.

The simplest kind of rheostat uses a coil or rod of wire. A slider can be moved along the wire to create more or less resistance in the circuit. As the slider moves along the wire, it either increases the length of wire which the current must pass through to complete the circuit, or it decreases it. Increases create more resistance, resulting in less current flowing through the circuit, while decreases work in the opposite way.