How does a transformer change voltage with no moving parts?

A transformer changes voltage using only wire and iron, with nothing that moves. It has two coils of wire wound around the same iron core. When alternating current flows in the first coil, it makes a magnetic field in the core that rises, falls and reverses with the current. That changing field cuts through the second coil and induces a voltage in it, even though the two circuits are not connected by any metal. Michael Faraday's law of induction, discovered in 1831, describes exactly this effect.

The voltage that comes out depends on the number of turns. In an ideal transformer, each winding gets a voltage proportional to its number of turns, so the voltage ratio equals the turns ratio. A secondary coil with ten times the turns gives ten times the voltage, and the current falls by the same factor. That is how a power station steps up for the long journey and how a pole-top box steps down for your home. Real distribution transformers are about 98 to 99 percent efficient.

There is a catch that shaped history: a transformer only works with a changing field. It changes voltage in alternating current circuits but cannot pass direct current. That is one reason the grid is alternating current.

The story is human. In 1884, three Hungarian engineers at the Ganz Works, Zipernowsky, Bláthy and Déri, decided that the open-core devices of the day were impracticable because they could not reliably regulate voltage. Their closed-core design was 3.4 times more efficient and finally made it feasible to light homes, businesses and public spaces from a shared supply.