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rheostat(rē`əstăt'), device whose resistance to electric current depends on the position of some mechanical element or control in the device. Typically a rheostat consists of a resistance element equipped with two contacts, or terminals, by which it is attached to a circuit: a fixed contact at one end and a sliding contact that can be moved along the resistance element. Electric current enters and leaves the resistance element through the contacts. By moving the sliding contact toward or away from the fixed contact, the length of the resistance element through which the current travels can be decreased or increased. In this way the current through the circuit can be increased or decreased. Rheostats are widely used for such purposes as controlling the speed of electric motors and dimming electric lights, but they have been replaced in many applications by systems based on semiconductor devices that waste far less power. See potentiometerpotentiometer.
1 Manually adjustable, variable, electrical resistor. It has a resistance element that is attached to the circuit by three contacts, or terminals. The ends of the resistance element are attached to two input voltage conductors of the circuit, and the third
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an electric device for controlling and limiting current or voltage in an electric circuit. The principal component of a rheostat is a conducting element with variable resistance. The magnitude of this resistance may be varied smoothly or in discrete steps. When it is necessary to vary the current or voltage within narrow limits, the rheostat is connected to the circuit in series; this is done, for example, when the starting current of electrical machines is to be limited. Potentiometer-connected rheostats are used to vary current or voltage over a wide range, from zero to a maximum value; in this case, the rheostat is a variable voltage divider.
According to their purpose, rheostats may be divided into such types as starting, starting-and-controlling, loading, and field rheostats. With respect to the method of heat dissipation, air-cooled, oil-cooled, and water-cooled types are distinguished. Depending on the material of the conductive element, rheostats are classified as metallic (most widely used), liquid, and carbon.
The simplest metallic rheostats are sliding-contact devices. In this design, the resistance is varied by moving a sliding contact finger along the turns of a wire made of a material of a high resistivity, such as manganin, constantan, nichrome, iron-chromium-aluminum alloy, or steel. The wire is wound on a cylindrical form made of an insulating material, such as porcelain or steatite.
A liquid rheostat consists of a vessel filled with an electrolyte in which electrodes are placed. The electrolyte is a 10–15 percent solution of Na2CO3 or K2CO3 in water. The resistance is varied by changing the distance between the electrodes or by changing the depth of immersion of the electrodes.
Carbon rheostats may be constructed in the form of piles of thin carbon disks. The resistance of such a rheostat is varied by changing the pressure applied to the piles.
REFERENCEChunikhin, A. A. Elektricheskie apparaty. Moscow, 1975.
T. N. DIL’DINA