Resistor, a basic component of electric circuits. Resistors are used to produce heat, as in an electric toaster or furnace; to produce light, as in an incandescent light bulb; to provide an electrical bypass, as in the shunt in an ammeter; to regulate the electric power entering a device, as in a light dimmer or radio volume control; and to set voltages within an electric circuit.

Carbon Resistor,Carbon Resistor, shown slightly larger than actual size. The stripes identify its value in ohms.

The resistors used in heating and lighting applications are almost exclusively metallic. Such materials as platinum, tungsten, and Nichrome (an alloy of nickel, iron, and chromium) are commonly used. A wire-wound resistor consists of a coil of wire made of Nichrome or a similar material wound on a ceramic core and covered with a protective ceramic material.

There are several types of resistors used in electronic circuits. A carbon resistor is made of carbon mixed with a binding material such as clay and molded into a cylinder. A film-type resistor is made of a thin film of carbon, metal, or metal oxide on a ceramic base. Resistors in integrated circuits typically consist of a thin layer of semiconductor material or a thin metallic film.

Resistors may be either fixed or variable. Variable resistors having two terminals are called rheostats; those having three terminals are called potentiometers.