Resistors and Resistor Color Codes

Unit Of Resistance Is The Ohm
Picture of Georg Ohm who lived from 1789 to 1854 Georg Ohm 1789 - 1854 was a German physicist and mathematician, Ohm did his work on resistance, voltage and current flow in the years 1825 and 1826, and published his results in 1827. He was able to show that "electromotive force" acting between the extremities of any part of a circuit is the "product of the strength of the current", and the resistance of that part of the circuit". That discovery became known as "Ohms Law" E=IxR and the standard unit of resistance became the ohm.
ohms law A resistor (R) is a current limiting, power dissipating, device designed to limit the flow of electrons to a known controlled value. Because they dissipate power as heat, resistors also have a wattage rating.
Resistors come in all sizes and shapes, but as far as the leaded types go (ones with wire leads, not surface mount) the most common by far is the 1/4 watt metal film type.
1/4 watt resistor To the right is a picture of several small resistors all of the same wattage (1/4 watt) but different values of resistance.
Below is a picture of resistors all of the same value of resistance, but each with different power ratings.
resistors have different power ratings The largest one in this picture has a rating of 10 watts. For many years now resistors of the metal film type have been very popular because of their temperature stability and low noise. The older resistors were made of carbon and their resistance would tend to increase over the years due to a number of reasons, but the main one being moisture absorption.
When you come across a older carbon resistor be sure to measure it's value with an ohmmeter to be sure it's still what it says it is.
So how do you know what the resistance of a resistor is?
The value of resistance is color coded on the unit. Colored bands are used to denote the ohmic value.
resistor color code bands
    A- First significant number

    B- Second significant number

    C- Decimal multiplier

    D- Tolerance in percent.

Here is the resistor color code chart for the bands. These colors have been used since the beginning of electronics, over a hundred years!

Resistor Color Code Chart

Let's look at an example below to see how this all works.
1/2 watt 
carbon resistor The first band (A) is brown so looking at the chart above we see that it equals 1. The next band (B) is black and equals 0. So far we have 10. The next band (C) is orange which equals 3 so we multiply 10 by 3 = 1000. The resistors value is 10 x 1000 = 10,000 ohms.
Another way to look at the multiplier is to simply add that many zeros. Since orange is three, we add three zeros to 10 and get 10,000. The last band (D) is gold which means this resistor has a tolerance of 5% of its rated value.

1/2 watt carbon resistor One more example. Look at the resistor on the right and see if you can tell it's value. We have orange, white, red and a silver tolerance band. Orange=3, White=9, Red=2. So it's 39 with two zeros or 3900 ohms. Silver=10% tolerance.
By the way, both these resistors are the older carbon type. So I would check them with an ohm meter before use, especially the higher resistance one of 10K ohms.
Just a few more things about resistors.
There's a type of resistor that is variable so the user can set the value. Its real name is potentiometer or "pot" as it is called for short. (No you can't smoke it -- but I have seen them smoke!) You and I know it as the old fashioned volume control.
Now for a little basic electronics
Basic Series DC Circuit The device on the left is, yes, the battery (duh!) and the resistor is on the right labeled (R). The connecting wires are just straight lines. Remember these two symbols, the battery and the resistor.

By the way, as you know batteries are made up of cells, the total voltage of the battery is dependent on the total number of cells. That is why the schematic symbol is drawn the way it is.

Now if each cell of this battery has a voltage of 2.4 volts, what would the total voltage be across this battery? (look at the number of cells in the drawing) What would be the voltage across the resistor? (Answer: Total battery voltage is 12 volts. The second answer is same as the battery voltage. 12 volts.)