How Rheostats Work
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Rheostats and Resistance
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A rheostat is a simple variable resistor with two contacts. A resistor is a device designed to resist the flow of electricity. The more resistance an electronic current encounters, the more slowly that current flows. By increasing or decreasing the resistance in a circuit, a rheostat changes the amount of current flowing through that circuit.
Inside a Rheostat
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Most rheostats contain a strip of some substance with fairly high resistance, such as carbon. One of the rheostat's two contacts attaches to the end of the carbon strip. The other moves along the strip as you adjust the rheostat slider or knob. When the sliding contact moves close to the fixed contact, the electric current only has to flow through a little bit of carbon to get from one contact to the other. This creates only a slight resistance. As the contacts get farther apart, however, the electricity has farther to travel along the carbon. This creates greater resistance, slowing down the current.
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Rheostat Design
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Rheostats come in two basic designs, sliding and rotating. With a siding rheostat, you pull a small lever up and down to slide the contact along a straight strip of some resistive material. A knob controls a rotating rheostat. As you turn the knob, the sliding contact moves along a spiral-shaped resistor. Many control knobs---particularly on older electronic equipment---are actually rheostats.
Uses
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Rheostats have many different applications. The control pots on most guitars, volume knobs on many stereos and dimmer switches on some lights are rheostats. Rheostats once were used in industrial control, but they're less common today. When a rheostat resists electricity, it wastes a portion of the energy, turning it into heat. Even so, rheostats remain useful in low-power applications like guitars because they're simple, and only bleed off a little power. With big, high-energy industrial motors, however, the power loss from a rheostat becomes more significant, making it less practical than a digital controller.
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