A resistor is a two-terminal passive electronic component which implements electrical resistance as a circuit element. When a voltage V is applied across the terminals of a resistor, a current I will flow through the resistor in direct proportion to that voltage. This constant of proportionality is called conductance, G. The reciprocal of the conductance is known as the resistance R, since, with a given voltage V, a larger value of R further "resists" the flow of current I as given by Ohm's law:
Click the title above to view the Resistor schematic symbols of electrical & electronic circuit diagram - resistor, potentiometer, variable resistor.

Click the title above to view the brief history of the resistor's color coding.

In the very early days resistors used to resemble what looked like sub-miniature rheostats, a sort of ceramic tube, with legs like sharpened solder tags welded close to the ends of the tube. When soldered in place they stood about an eighth of an inch (3.175mm) above the circuit board. The whole body of the resistor was dipped in turquoise coloured paint, and the value was identified by a wonderful combination of dots, spots and numbers, which half the time missed the printing machine by miles! As carbon film and carbon composition resistors became more popular, coloured rings or bands around the whole body became 'the norm' for identification.

Count the number of color bands on the resistor. For resistors with a tolerance anywhere from 5% to 10%, there will be four bands. For resistors with a tolerance anywhere from 1% to 2%, there will be five bands. For four band resistors, the first two bands are the value digits, the third band is the multiplier and the fourth is the tolerance. For five band resistors, the first three bands are the value digits, the fourth is the multiplier and the fifth is the tolerance.
Image from www.williamson-labs

Here's one mnemonic "Bright Boys Rave Over Young Girls But Veto Getting Wed. Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White <=> 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
Alternatively, most of the colors are those from the traditional rainbow. Black is 0 (as in 'nothing'), Brown is 1, then Red through Violet, and finally Gray and White are 8 and 9.

When an electrical current passes through a resistor, electrical energy is lost by the resistor in the form of heat and the greater this current flow the hotter the resistor will get. Resistors are rated by the value of their resistance and the power in watts that they can safely dissipate based mainly upon their size. Every resistor has a maximum power rating which is determined by its physical size as generally, the greater its surface area the more power it can dissipate safely into the ambient air or into a heatsink.

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