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Carbon resistor
The simple form of a carbon resistor consists of a tubular rod of carbon enclosed in an insulating ceramic tube. The value of resistance and the tolerance is usually marked on the surface of the component using coloured bands. Values from 10 ohm to 1 M ohm are typical.
Wirewound resistor
The simple wirewound resistor consists of a length of resistance wire spiral wound onto a tubular ceramic former and covered with a heatproof ceramic or vitreous enamel coating. The value of resistance and the power rating is usually marked on the surface of the component. Values from 0.1 ohm to 10k ohms are typical.
Variable resistor
The simple variable resistor consists of a circular carbon track fitted with a sliding metal contact driven by a rotary control shaft. With three terminals (two fixed connections at the extreme ends of the track and one connection to the slider) a component of this type is more correctly known as a potentiometer. Values from 1k ohm to 1 M ohm are typical.
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