A transformer combines the two basic principles of magnetism and inductance by placing two coils of wire in close proximity to one another. Here are the principles that the transformer exploits:

• A changing current passing through a wire creates a moving magnetic field around the wire.

• A changing current will be induced in a wire that's exposed to a moving magnetic field.

When a source of AC is connected to one of the coils, that coil creates a magnetic field that expands and collapses in concert with the changing voltage of the AC. In other words, as the voltage increases across the coil, the coil creates an expanding magnetic field. When the voltage reaches its peak and begins to decrease, the magnetic field created around the coil begins to collapse.

The second coil is located within the magnetic field created by the first coil. As the magnetic field expands, it induces current in the second coil. The voltage across the second coil increases as long as the magnetic field expands. When the magnetic field begins to collapse, the voltage across the second coil begins to decrease.

Thus, the current induced in the second coil mirrors the current that is passed through the first coil. A small amount of energy is lost in the process, but if the transformer is well constructed, the strength of the current induced in the second coil is very close to the strength of the current passed through the first coil.

The first coil in a transformer — the one that’s connected to the AC voltage — is called the primary coil. The second coil — the one in which an AC voltage is induced — is called the secondary coil. All transformers have both a primary and a secondary coil.

A transformer whose primary coil has more turns than its secondary coil is called a step-down transformer because it reduces voltage — that is, the voltage at the secondary coil is less than the voltage at the primary coil. Similarly, a transformer that has more turns in the secondary than in the primary is called a step-up transformer because it increases voltage.

Although the voltage increases in a step-up transformer, the current is reduced proportionately. For example, if the primary coil has half as many turns as the secondary coil, the voltage induced in the secondary coil will be twice the voltage that’s applied to the primary coil, but the current that flows through the secondary coil will be half the current flowing through the primary coil.

Similarly, when the voltage decreases in a step-down transformer, the current increases proportionately. Thus, if the voltage is cut in half, the current doubles.

Remember the basic formula for calculating electric power:

P = V I

In other words, power equals voltage times current. A transformer transfers power from the primary coil to the secondary coil. Since the power must stay the same, if the voltage increases, the current must decrease. Likewise, if the voltage decreases, the current must increase.

Transformers are the main reason we use alternating current instead of direct current in large power distribution systems. That’s because when you send large amounts of power over a long distance, it's much more efficient to send the power in the form of high voltage and low current.

Transformers work only with alternating current. That’s because it's the change of the magnetic field created by the primary coil that induces voltage in the secondary coil. To create a changing magnetic field, the voltage applied to the primary coil must be constantly changing. Because DC is a steady, fixed voltage, it creates a fixed magnetic field that won't induce voltage in the secondary coil.