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A resistor does exactly what it sounds like, it resists the flow of electricity. While they come in many different types, carbon film is the most common. A carbon film resistor is made up of a ceramic core, with a thin carbon film on the outside. The resistance of such a device is varied by cutting the film into a spiral. Resistors of this type tend to have a high capacitance, and medium inductance, with very low temperature stability, having become popular only due to their low manufacturing costs.
Metal film resistors use a metal film as the resistive layer, have lower capacitance, and good temperature stability.
Surface mount resistors are often cerment resistors, and are made up of metal oxides, and ceramics. They have moderate capacitance, and very good temperature stability.
The other most often encountered resistor is the wire wound resistor, and consists of a metal wire wound around a ceramic core. Although they have a high inductance, they can often dissapate more heat than other types of resistors.
Resistance itself is measured in ohms, and this rating is often indicated on the packageing through the use of colored bands. These bands are:
And are arranged in the following fashion: A B C D
The first three are to be read as: (A*10 + B) * 10^C
The fourth band will be either gold, silver, or non existant. This indicates a tolerance of 5%, 10%, or unstated respectively (unstated is typically 20%).
The most important part about resistors, at least initially, is Ohm's law. V = I * R
What this basically means is that the Voltage drop across any resistive element will be proportional to the current flowing through it. Furthermore, given a Voltage of V across a resistive element, of R ohms, the current flowing through the element will be I.