Transformers have two sets of wires wound around an iron core, and they use electromagnetic induction to increase or decrease electrical current. The primary wires magnetize the iron after the application of current, and the secondary wires use the magnetic induction to create an output current. The increase or decrease of output current depends on how many times the input and output wires wind around the core.
All transformers operate on the same basic principle, no matter their size. An iron core shaped like a picture frame has two sets of insulated wires wound around the opposing sides. The primary winding creates a magnetic field in the iron, and the secondary winding uses the magnetic field to create an output current. If the primary winding has 100 turns and the secondary has 200 turns, then the output voltage doubles but the amperage drops by half. Reverse the wiring and the output amperage doubles with the voltage dropping by half. Using a combination of windings, engineers can control how much current is increased or decreased. The large transformers found on utility poles decrease or step down high voltage/low amperage current to more manageable levels for home use. For amplifiers and other electronics, a transformer may increase, or step up voltage as needed.