Resistors are devices used in electrical circuits to put a definite resistance in a circuit. Normally they are made of graphite or a thin carbon film coated on glass. Larger resistors are cylindrical and have four color bands that encode the value of the resistance. On computer boards they are tiny, rectangular devices barely a millimeter on a side with conductive ends that are soldered to the board. If they are designed to dissipate a large amount of power, they are made of high-resistance wire.