components like resistors are made for the express purpose of creating a precise quantity of resistance for insertion into a circuit. They are typically constructed of metal wire or carbon, and engineered to maintain a stable resistance value over a wide range of environmental conditions. Unlike lamps, they do not produce light, but they do produce heat as electric power is dissipated by them in a working circuit. Typically, though, the purpose of a resistor is not to produce usable heat, but simply to provide a precise quantity of electrical resistance.
The most common schematic symbol for a resistor is a zig-zag line:
Resistor values in ohms are usually shown as an adjacent number, and if several resistors are present in a circuit, they will be labeled with a unique identifier number such as R1, R2, R3, etc. As you can see, resistor symbols can be shown either horizontally or vertically:
Real resistors look nothing like the zig-zag symbol. Instead, they look like small tubes or cylinders with two wires protruding for connection to a circuit. Here is a sampling of different kinds and sizes of resistors:
Resistor Types
There are four main types of resistor: carbon, film, wirewound, and semiconductor. Carbon resistors are tiny insulated cylinders of carbon and ceramic dust with a terminal at each end. They have colored bands to show their resistance and tolerance values.
Film resistors are similar to carbon resistors, but use a wound metal film instead of dust for better tolerance. Wirewound resistors are also similar, but use a wound metal wire for low resistance and higher power rating.
Variable resistors, also called potentiometers, allow the resistance to be changed while in circuit. They are used in devices that control current, such as light dimmers, volume dials, and gamepads.
