So we've learned that materials such as copper, aluminum, and gold are good conductors. Other materials, such as paper, glass, and rubber are good insulators. This means that electrons don't flow through them very easily. Between good conductors, like copper and silver, and insulators are conductors that are "resistant" to electrical current. The unit of resistance is the ohm and 1 ohm is considered the resistance of round copper wire, 0.001" diameter, 0.88" (22.35 mm) long at 32 deg F (0 deg C).

The Ohm

Resistors

Imagine now two lengths of wire put together. The combined wire would have a resistance of 2 ohms. The most basic electronic component, and the first we will learn about, the resistor, works this way when placed in a linear arrangement. For instance, for a linear arrangement of R1 (100 ohms), R2 (70 ohms), and R3 (80 ohms) the combined resistance will be 250 ohms (R1 + R2 + R3). Resistors can also be placed "parallel" to each other, and we will learn more about that later.

Resistors are two terminal electronics components made of resistive wire and various compounds and films.

The resistance value of a resistor can be found using an ohmmeter, which is a function of a multimeter. This is done by turning off power to the circuit (never test resistance on a live circuit), and touching the leads of the multimeter to either side of the resistor. Most resistors are also marked with colored bands, as in the photograph above.

Resistor color coding

Band A is the first digit in the value of the resistor
Band B is the second digit in the value of the resistor
Band C is the decimal multiplier (x10 to the power of n)
Band D is the tolerance band in %, with no color meaning 20%

The color system works as follows, for the first three bands:
Black = 0
Brown = 1
Red = 2
Orange = 3
Yellow = 4
Green = 5
Blue = 6
Violet =7
Grey = 8
White = 9

For the third band, two more values are possible
Gold means the value of n = -1
Silver means the value of n = -2

So now we'll try a few examples. If a resistor's color bands are Brown, Black, and Red, the value of the resistor is 1 (brown), 0 (black), followed by 2 zeroes (red). A 1,000 ohm resistor.
If a resistor's color bands are Green, Orange, and Brown, the value of the resistor is 5 (green), 3 (orange), followed by 1 zero (brown). A 530 ohm resistor.
Try to figure out the value of the resistor in the photograph below using the first three bands.

The fourth band is the resistor's tolerance band. The tolerance band's values are as follows:
Brown = ±1%
Red = ±2%
Green = ±0.5%
Blue = ±0.25%
Violet = ±0.1%
Gray = ±0.05%
Gold = ±5%
Silver =±10%
None =±20%

The resistor in the photograph above has a gold tolerance band, therefore, it has a tolerance of 5%. Resistor tolerance is pretty important to understand, because if you need a 4900 Ù resistor, you can use a 5000 Ù with a gold tolerance band, but not a 5000 Ù with a brown tolerance band. A red tolerance band would be at exactly the lowest range of the resistor, and not a wise choice, although it would work at least temporarily. It is not always possible to find a resistor with the exact resistance that you need, therefore manufacturers overlap the range of their resistors.

Resistors with a tolerance below 1% have 5 bands. In this case the first 3 bands are the significant digits, the fourth is the multiplier, and the fifth is the tolerance band. Some older resistors have five bands. These are just like normal 4 band resistors, with a gold or silver tolerance band, and a fifth band which signifies the temperature coefficient. I will explain temperature coefficients if anyone asks.

New resistors have a numerical label, similar to the color coded system. The first two digits are the significant digits and the third digit is the multiplier. For instance, a 243 resistor is 24 kÙ (kiloohms). Resistances less than 100 Ù are written 100 (10 Ù), 220 (22 Ù) etc. or simply in 2 digits to prevent mistakes, e.g. 10, 22. Resistances less than 10 Ù are written 0R22, 4R7, etc. in which case the R signifies a decimal point (0.22 Ù, 4.7 Ù). Numerical resistors marked 0 are approximately 0 Ù. The resistors in most of your equipment will probably be color coded.

Next up: Ohm's Law and Potentiometers