To control any electric or electronic device you vary voltage using a variable resistor. The two classes of variable resistors are rheostats and potentiometers and have distinct differences and applications.
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All variable resistors have a resistance track made of carbon, cermat (a mixture of metal and ceramic) or a coil of wire for lower resistances or precision. The wiper slides along the track to pick off a selected voltage.
A rheostat has two connections. One goes to one end of the track and the other goes directly to the wiper to vary the level of the source.
A potentiometer has three connections. The two ends of the track are wired across a voltage source and the third connection goes to the wiper to select a specific voltage from that source.
Rheostats connect in series with a voltage source to vary that source. Examples are light dimmer switches, heater controls and motor speed controls.
The track of a potentiometer wires across a fixed or variable voltage circuit and the wiper slides across it to pick off a controlling voltage. The best example of a potentiometer is a radio volume control knob.
A rheostat shuts off the current flow when adjusted to the off position. A potentiometer will always have current running through the track whenever the equipment has voltage applied.
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