A resistor is a piece of material that obeys Ohm's Law. The name comes from its main property, it resists the flow of charge through itself, hence allowing us to control the current. Resistors can be made of various kinds of material, but whatever the choice it must conduct some electricity otherwise it wouldn't be of any use..

Two wires are connected to opposite ends of the resistor. When we apply a potential difference between the wires we set up a current from one wire to the other, through the resistor. The size of the current is proportional to the difference in voltage between the wires. The resistance (in units of Ohms) is defined as the ratio of the applied voltage, V (in Volts), divided by the current, I (in Amps), produced by the applied voltage. Resistors come in a wide variety of shapes and sizes, but the most common type is a cylinder with wires at the ends.

The value is usually displayed using standard colour code. Most resistors have a value in the standard E12 series.

Most of the resistors used in electronics have 'fixed' values, but resistors can also be made which have a controlled, variable resistance. These are sometimes called pots, and they're used for tasks like the volume control on a Hi-Fi amplifier.
*

Electronics homepage
Course contents


Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit2 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.