A resistor is an electrical component designed to have an electrical resistance that is independent of the current flowing through it. The common type of resistor is also designed to be independent of temperature and other factors. Resistors may be fixed or variable. Variable resistors are also called potentiometers or rheostats (see below).
Resistors are used as part of electrical networks and incorporated into microelectronic semiconductor devices. The critical measurement of a resistor is its resistance, which serves as a ratio of voltage to current and is measured by the SI unit ohm. A component has resistance 1 ohm if a voltage of 1 volt across the component results in a current of 1 ampere, or amp, which is equivalent to a flow of 6.25 × 1018 electrons per second in the opposite direction.
Any physical object is a kind of resistor. Most metals are conductors, and have low resistance to the flow of electricity. The human body, a piece of plastic, or even air has a resistance that can be measured. Materials that have very high resistance are called insulators.
The relationship between voltage, current, and resistance through an object is given by a simple equation, derived from and often confused with Ohm's Law:
where V is the voltage across the object in volts, I is the current through the object in amperes, and R is the resistance in ohms. If V and I have a linear relationship -- that is, R is constant -- along a range of values, the material of the object is said to be ohmic over that range. An ideal resistor has a fixed resistance across all frequencies and amplitudes of voltage or current.
Superconducting materials at very low temperatures have zero resistance. Insulators (such as air, diamond, or other non-conducting materials) may have extremely high (but not infinite) resistance, but break down and admit the flow of current under sufficiently high voltage.
The resistance of a component can be calculated from its physical characteristics. Resistance is proportional to the length of the resistor and to the material's resistivity, a physical property of the molecules that comprise the resistor, and inversely proportional to cross-sectional area.
Standard resistors are sold in values from a few milliohms to about a gigohm; only a limited range of values called preferred values are available. In practice, the discrete component sold as a "resistor" is not a perfect resistance, as defined above. Resistors are often marked with their tolerance (maximum expected variation from the marked resistance). On color coded resistors a leftmost silver band denotes 10% tolerance, a gold band 5% tolerance, and a red band 2% tolerance. Closer tolerance resistors, called precision resistors, are also available.
A resistor has a maximum working voltage and current above which the resistance may change (drastically, in some cases) or the resistor may be physically damaged (burn up, for instance). Although some resistors have specified voltage and current ratings, most are rated with a maximum power which is determined by the physical size. Common power ratings for carbon composition and metal-film resistors are 1/8 watt, 1/4 watt, and 1/2 watt. Metal-film resistors are more stable than carbon resistors against temperature changes and age. Larger resistors are able to dissipate more heat because of their larger surface area. Wire-wound and sand-filled resistors are used when a high power rating is required, such as 20 watts.
Variable resistor The variable resistor is a resistor whose value can be adjusted by a mechanical movement, for example by being turned by hand.
A potentiometer is a common form of variable resistor. In this form, a movable contact called the wiper travels the length of a fixed resistor, both ends of the resistor as well as the wiper having electrical connections. This provides a variable ratio of resistances, as well as either an increasing or decreasing resistor. The resistive element may be resistance wire, carbon particles in plastic or a ceramic/metal mixture.
A rheostat is a variable resistor with two terminals, one fixed and one sliding. It is often used with high currents.
Variable resistors can be cheap single-turn types or multi-turn types with a helical element. Some even have a mechanical display to count the turns.
Although rotary potentiometers are more common, linear potentiometers also exist.
One popular form of rotary potentiometer is called a "string pot." It's a multi-turn potentiometer with an attached reel of wire.
Potentiometers are available in both linear and logarithmic forms. A "linear pot" has a resistive element of constant cross-section, resulting in a device where the resistance between the wiper and one end terminal is proportional to the distance between them. A "log pot" has a resistive element that either tapers in a controlled way from one end to the other, or is made from a material whose resistivity varies from one end to the other. This results in a device where the distance between the wiper and one end terminal is proportional to the logarithm of the resistance between them. The "log pot" is used as the volume control in audio amplifiers, where it is also called an "audio pot", because the amplitude response of the human ear is also logarithmic. It ensures that, on a volume control marked 0 to 10, for example, a setting of 5 sounds half as loud as a setting of 10.