As the sliding arm moves across the element, its point of contact on the element changes, effectively changing the length of the element. The rating of a variable resistor is its resistance at its highest setting. Variable resistors are also called rheostats or potentiometers. The resistance elements of rheostats are usually wire wound. They are most often used to control very high currents, such as in motors and lamps.
Potentiometers generally have composition elements.
They are used as control devices in radios, amplifiers, televisions, and electrical instruments. The actual resistance of a resistor may be greater or less than its indicated rating. The possible range of variance from the indicated rating is called its tolerance. Resistor Rating Color Code Composition resistors are color coded to indicate resistance values or ratings. The color code consists of various color bands that indicate the resistance values of resistors in ohms as well as the tolerance rating.