The Standard Transformer
The transformer is an efficient electrical device for converting between two different voltages. Different components and devices need different voltages, so a electrical component is needed to handle the task. The transformer handles this task thanks to magnetic induction. There are two coils, and each has a number of loops of wire. The voltage will change with the ratio of the two numbers of loops. For example, if you are given a transformer with coils of 50 turns and 100 turns, then a 5 volt alternating current at the 50 turn coil will produce a 100/50 * 5 volt = 10 volt alternating current. Conversely, 5 volts at the 100 turn coil will only produce 2.5 volts in the 50 turn coil.
Tapping the Transformer
A tap in a transformer is a wire that is attached to a coil part way. That is between one wire of a coil and the tap, there are a reduced number of turns. The result of the tap is a way to select voltage from the coil. Back to our example, suppose our 50 turn coil has a tap at the tenth coil. Then if we have a 10 volt input to the 100 turn coil, then 5 volts are available across the entire 50 turn coil. Between the tap and the closer end of the coil, there is a voltage of 10/100 10 volts = 1 volt. And between the other end of the coil and the tap is 40/100 10 volts = 4 volts. As a result, the tapped coil will provide 1, 4 and 5 volts, depending on which two ends you choose.
Control With the Taps
The tapped transformer provides a good deal of control over the voltage that is obtained from a transformer. Systems exist which can automatically choose which tap to use in order to get a certain voltage. Audio transformers also benefit from taps as different taps will have different impedances.