Manufactured circuit components containing definite amounts of resistance are called resistors. In simplest terms, resistors resist, or oppose, the flow of electrons through a circuit. Every material offers some resistance, or opposition, to the flow of electric current through it. Good conductors, such as copper, silver, and aluminum, offer very little resistance. Poor conductors, or insulators, such as glass, wood, and paper, offer a high resistance to current flow.

Resistance is measured in ohms. One ohm is the resistance of a circuit or circuit element that permits a steady current of 1 ampere (1 coulomb per second) to flow when a steady EMF of 1 volt is applied to the circuit.

Resistors are manufactured in a variety of shapes and sizes from a variety of materials. Carbon or composite resistors are usually round, cylindrical, or flat. Wire-wound resistors look like a coiled wire with an attached armature (for adjustment). Wire-wound resistors may be barrel-shaped or flattened cylinders.

The current-carrying capacity of a resistor is rated according to the amount of heat it can safely release in a given amount of time. Therefore, a resistor cannot be used in a circuit where current causes heat to build up faster than the resistor can dissipate it. Since heat is a form of energy, the heat-releasing rate of a resistor is measured in energy units called watts.

Composition resistors are usually rated at 1/4, 1/3, 1/2, 1, and 2 watts. The power ratings of wire-wound resistors are larger, usually ranging from 5 to 200 watts. Higher wattage resistors require larger physical size to dissipate heat quickly.

Carbon or composite resistors have a fixed rating. Wire-wound resistors may be fixed or variable. An example of a variable-rating resistor is one used in a radio's volume control.

The value in ohms and percent of tolerance is normally stamped on the body of wire-wound resistors. Carbon or composition resistors come in a variety of base (body) colors, with tan/brown the most common. Color-coded bands show ohm value and tolerance percentage on carbon or composition resistors.

Because of chemical or manufacturing fluctuations, resistors often contain imperfections. This means a resistor's resistance value is never precise. A resistor will rarely measure the exact number of ohms specified by its label or color codes. The amount it will vary is called tolerance. In complex equipment, such as computers, tolerance is very significant.

Tolerance value can be either greater than or less than the calculated resistance value. Resistor tolerance is given as a percentage value, which indicates the amount that a resistor may vary above or below its specified value. Standard tolerances for carbon or composition resistors are 5, 10, and 20 percent. Wire-wound resistors may have tolerances as low as 1 or 2 percent.

Manufacturers indicate the tolerance, or range of accuracy of a resistor's resistance, with a fourth color band. A gold band indicates the resistor's resistance is within 5% of the value calculated from the resistor's color bands. A silver band indicates tolerance within 10%. If the resistor's tolerance value is within 20%, the resistor will not have a tolerance band.

Resistor tolerance is not an indication of poor manufacturing. Closer tolerances can be achieved, but at greater expense. A resistor with a 20 percent tolerance costs less to produce than one with a 10 percent tolerance.