Potentiometer
A potentiometer is used for precise measurements of voltage, and sometimes current. It also can be used as a three-terminal potential (voltage) divider, which is a type of rheostat. Nearly every type of electronic instrument uses a potentiometer. The most common use is for volume control for radios, televisions and other amplifiers.
Potentiometers make use of rheotats, which are variable resistors. A rheostat is a coil of wire that has been wrapped around an insulating tube and attached to two terminal points. Next to the coil are metal "brushes" that make contact with the coil. When a knob (or "slider") is moved, it causes the position of the brushes on the coil to change, resulting in the resistance between them, and so the terminals of the coil, to change. Heat is produced by resistance, but a rheostat can handle large amounts of current because the coil easily radiates the heat away.
The potentiometer works by balancing one voltage against another. If two voltages are connected together so that one opposes the other, the electromotive force (emf or "driving force") is the difference in the two voltages. A galvanometer internal to the potentiometer can be added to the circuit, and a reading of the current flow and the inbalance between the voltages can then be made. If the galvanometer shows no deflection, the voltages are equal and completely balanced. In practice one of the voltages is known, usually by a "standard cell" (a cell whose emf is known). A standard cell has an absolute value which remains stable for years.
The sliding-wire potentiometer makes use of a taut wire, which has uniform resistance, that is attached between two terminals. The unknown voltage located at any point on the wire is proportional to the distance from the terminal. Two leads are taken from the source of the voltage. One lead is attached to the end terminal of the wire; the other end is attached to a hand-held contact after passing through a galvanometer. When the contact touches the wire, the galvanometer registers the imbalance. The contact is moved along the wire until the galvanometer reads zero. This procedure is repeated substituting the standard cell for the unknown voltage and a new distance is measured. With the voltage of the standard cell known the two distance measurements are made, and it is possible to calculate the unknown voltage. This is essentially how the Wheatstone bridge measures resistance.
A "voltage divider" is used when large voltages are to be measured. The divider is comprised of a series of resistances which are tapped at various points along its length. After the voltage is reduced the potentiometer can be used to measure the remaining voltage.
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