A resistor is one of the most important parts of an electrical circuit. As its name implies, it “resists” the electricity, which courses through its two terminals. The resistor then lowers the voltage to safe levels.
Resistance can be quantified and measured in ohms (which borrows its name from Ohms law, which computes resistance as the difference in voltage divided by the amount of current applied. The greater the Ohm rating, the higher the level of resistance.
What can vary a resistance rating? One of the biggest factors is the way the electrical device is constructed. For example, both carbon composition resistors and carbon film resistors make use of a ceramic cylinder, but the latter has an additional layer of carbon film. Another factor is material. Some use metal film or metal oxide; others use metal wire encased in clay, plastic, or fiberglass.
Resistance levels become particularly crucial when the electrical device must generate high amounts of power or heat. For example, resistors that should withstand high temperature are made with ceramic and metal compounds or composite like cermet. Some use a rare metal called tantalum.
Many resistors have a protective layer of paint, enamel or plastic covering. They are usually color-coded to indicate ohm value. Since even the finest technology can’t completely and precisely control currents, the resistor must also indicate tolerance—or how close it is to its maximum ohm value.
Some resisters have a fixed value; others, called potentiometers, can adjust resistance levels. Some, like varistors, adjust to sudden changes in voltage; others, like thermistors, respond to changes in temperature.