A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal level, divide voltages, and bias active elements. A wide variety of types of resistors are available, with standard values ranging from a fraction of an ohm to several megohms.

The modern understanding of electric resistance began in 1827 when Georg Ohm published his work “The galvanic circuit investigated mathematically”. He presented his results in the paper “On the Influence of the Internal Resistance of Batteries on their Action” (1834). This became known as Ohm’s Law. The SI Unit for measuring electric resistance is named after him: the ohm (Ω).

Resistors can be made out of many different materials and compounds including carbon composition and metal film. They can also be combined together in what is called parallel or series connection schemes to create more complex impedance networks for applications such as filters or voltage dividers.