resistor, electrical component that opposes the flow of either direct or alternating current, employed to protect, operate, or control the circuit. Voltages can be divided with the use of resistors, and in combination with other components resistors can be used to make electrical waves into shapes most suited for the electrical designer’s requirements. Resistors can have a fixed value of resistance, or they can be made variable or adjustable within a certain range, in which case they may be called rheostats, or potentiometers.