What Is a Rheostat?
A rheostat is a type of electrical component that provides the capacity to adjust resistance to electrical current. It is frequently equated with a potentiometer, which serves a similar function, though potentiometers tend to have three contacts while rheostats typically have only two. Rheostats come in two main types: slider style or rotary style.
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History
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George Ohm was a German mathematician who performed notable, experimental work with electricity in the first half of the 1800s. He discovered and formulated what is now referred to as Ohm's law. Ohm's law essentially states that there is an inverse relationship between current and resistance. The National High Magnetic Field Laboratory reports that this discovery allowed Charles Wheatstone to develop the first rheostat in the mid-1800s.
Design
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The majority of rheostats use a wire-wound coil. The wire coil is, of necessity, a conductive material that is tightly spiraled. In rotary-style rheostats, the coil is in a circular configuration. It would be reminiscent of a doughnut in shape. Slider-style rheostats have the coil laid out in a straight line. Think of a slinky toy, laid out in a straight line. To protect the coils, as well as the contacts which allow current flow, rheostats are frequently sealed into casings.
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Method of Action
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Rheostats are a direct application of Ohm's law. They function by increasing or decreasing resistance. As the resistance in the rheostat increases, it diminishes the available current. As the resistance in the rheostat decreases, it allows more current to pass through it. The position of the contact on the coil determines the amount of resistance. If the contact is touching the coil in place where there is little wire between it and the exit terminal, the wire exerts very little resistance and the current flow is high. As the amount of wire between the contact and exit terminal increases, the resistance increases.
Uses
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Rheostats have several applications. They are commonly found in small, residential appliances that require variable motor speeds, such as blenders or power tools. They are used to alter motor speed by adjusting the available current. They are also used as components of light dimmers; they increase or diminish bulb brightness through current adjustments. Rheostats are also found in some electronic circuits, arc lights, fans and medical equipment.
Current Status
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Rheostats are still common in many products today, but they are being phased out of some applications by better components. For example, they are being replaced in electronic circuits by silicon-controlled rectifiers, which perform the same function but exhibit a lower failure rate. They are also being phased out of some industrial applications in favor of three-contact potentiometers, for similar reasons.
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References
- Photo Credit Image by Flickr.com, courtesy of sookie
