Resistor

A resistor is an electrical circuit component whose intended behavior is electrical resistance. An ideal ohmic resistor obeys Ohm's law, $V=IR$, and converts electrical energy into heat at power $P=I^{2}R$ through Joule heating. It provides the dissipative term in a series RLC circuit.

A carbon composition resistor uses a bulk mixture of carbon and insulating binder. The conducting network sets its electrical resistance. Its compact geometry has little winding inductance, but composition and contact fluctuations can cause excess electronic noise and changes of electrical resistance with temperature and aging. It differs from a carbon-film resistor.

A wirewound resistor uses resistive metal wire wound on an insulating former. Suitable alloys and heat-resistant construction provide stable electrical resistance and substantial electrical dissipation; parasitic inductance and interturn capacitance limit the frequency range over which it behaves as a pure resistor.

Resistor tolerance specifies the permissible manufacturing deviation of a resistor's electrical resistance from its nominal value under stated conditions. For nominal electrical resistance $R_{\mathrm{nom}}>0$ and fractional resistor tolerance $0\le \tau <1$, the specified range is $R_{\mathrm{nom}}(1-\tau )\le R\le R_{\mathrm{nom}}(1+\tau )$. This differs from measurement uncertainty: resistor tolerance describes a component specification, while measurement uncertainty describes knowledge of a measured value; the tolerance alone is not a probability distribution for that value.

Two nominally equal resistors can have different actual electrical resistances, changing their voltage divider ratio even without an external electrical load. In a loaded voltage divider, use the actual electrical resistances to calculate both the unloaded output and the Thévenin resistance governing voltage divider loading error. A mismatch from resistor tolerance and a change from voltmeter loading are distinct effects that can occur together.