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While "real world" resistors cannot attain this perfect goal, they are designed to present little variation in electrical resistance when subjected to changing temperature and other environmental factors.

Resistors may be fixed or variable. Variable resistors are also called potentiometers or rheostats (see below) and allow the resistance of the device to be altered by turning a shaft or sliding a control.

Some resistors are long and thin, with the actual resisting material in the centre, and a conducting metal leg on each end. This is called an axial package. The photo on the right shows a row of commonly used resistors in a bandolier. Resistors used in computers and other devices are typically much smaller, often in surface-mount ( Surface-mount technology) packages without leads. Larger power resistors come in more sturdy packages designed to dissipate heat efficiently, but they are all basically the same structure.

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 in ohms, an SI unit. A component has a resistance of 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 one coulomb of electrical charge (approximately 6.241506 × 10¹⁸ electrons) per second in the opposite direction. (see: Current)

Any physical object is a kind of resistor. Most metalFor alternative meanings see metal (disambiguation). In chemistry, a metal is an element that readily forms cations and has metallic bonds, and it is sometimes said that it is similar to a cation in a cloud of electrons. The metals are one of the three grs are conductorsIn science and engineering, conductors are materials that contain movable charges of electricity. When an electric potential difference is impressed across separate points on a conductor, an electric current between those points appears in accordance with, and have low resistance to the flow of electricity. The human body, a piece of plasticThe term plastics covers a range of synthetic or semi-synthetic organic condensation or polymerization products that can be molded or extruded into objects or films or fibers. Their name is derived from the fact that in their semi-liquid state they are ma, or even a vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also has a resistance that can be measured. Materials that have very high resistance are called insulatorsInsulators are materials which prevent the flow of heat (thermal insulators) or electrical current (electrical insulators). The opposite of electrical insulators are conductors and semiconductors, which permit the flow of current (Note: a semiconductor is.

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:

V = IR

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.

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 material) and inversely proportional to cross-sectional area. The equation to determine resistance of a section of material is:

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 rightmost silver band denotes 10% tolerance, a gold band 5% tolerance, a red band 2% tolerance, and a brown band 1% 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.

Furthermore, all real resistors also introduce some inductance and capacitance, which change the dynamic behavior of the resistor from the ideal equation.

Resistors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent resistance (R_eq):

The parallel property can be represented in equations by two vertical lines "||" (as in geometry) to simplify equations. For two resistors,

The current through resistors in series stays the same, but the voltage across each resistor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total resistance:

A resistor network that is a combination of parallel and series is broken up into smaller parts that are either one or the other. For instance,