The carbon-composition resistor

Probably the cheapest method of making a resistor is to mix up finely powdered carbon (a fair electrical conductor) with some nonconductive substance, press the resulting clay-like stuff into a cylindrical shape, and insert wire leads in the ends (Fig. 6-5). The resistance of the final product will depend on the ratio of carbon to the nonconducting material, and also on the physical distance between the wire leads. The nonconductive material is usually phenolic, similar to plastic. This results in a carbon-composition resistor.

Carbon-composition resistors can be made to have quite low resistances, all the way up to extremely high resistances. This kind of resistor has the advantage of being pretty much nonreactive. That means that it introduces almost pure resistance into the circuit, and not much capacitance or inductance. This makes the carbon-composition resistor useful in radio receivers and transmitters.

Carbon-composition resistors dissipate power according to how big, physically, they are. Most of the carbon-composition resistors you see in electronics stores can handle 1⁄4 W or 1/2 W. There are 1/8-W units for miniaturized, low- power circuitry, and 1-W or 2-W components for circuits where some electrical ruggedness is needed. Occasionally you’ll see a much larger unit, but these are rare.