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Carbon composition resistors are used to provide electrical resistance and limit the current in a circuit. The resistance value is measured in ohms, and the amount of current flowing through a circuit can be determined by dividing the known voltage by the ohms of resistance. By controlling the current, the resistor provides a measure of protection for more sensitive components in the circuit.
All resistors are considered to be passive electronic components. This is because a resistor consumes energy but does not produce power gain or its own energy. Instead, it converts the consumed energy into heat and dissipates this heat to the surrounding air.
There have been few changes to the design of carbon composition resistors since the first ones were created in the early 20th century. One of the most noticeable changes is the insulation that now surrounds the carbon core. In early carbon composition resistors, this core was exposed and the leads were soldered directly onto the component. Modern carbon composition resistors have improved upon this design using a ceramic or plastic outer layer to protect the part from damage.
These electronic components were widely used until the 1970s when more reliable, less expensive options began to be mass produced. Since then, the carbon composition resistor has been largely replaced by metal film, foil, and other resistor types. While carbon composition resistors are useful for providing current control in non-critical circuits, this type of resistor is subject to a number of factors that may cause its resistance to vary, including high temperatures, over-voltages, and humidity. All of these factors can create significant changes in the actual resistance provided by the component, making it unsuitable for precision electronic equipment.
The modern carbon composition resistor is available in resistance ratings ranging from mere fractions of an ohm to 22 meg ohms. This wide range of resistance values compared with the small size of the component makes the carbon composition resistor a versatile part for high current electrical designs. When exact current specifications are not required, such as in welding equipment and power supply units, these resistors can provide a long-lasting and reliable solution within a 5% tolerance of the resistance rating.
The resistance value of carbon composition resistors is displayed in a color coded set of painted rings on its outer surface. The first two bands show the first two numbers of the resistor's value. These two numbers are then multiplied by a power of ten, which is designated by the third band. The tolerance, or margin of error, for the resistor is displayed on the fourth band.