Power transformer

A transformer trades voltage for current without moving parts, using nothing but two coils sharing the same iron core. The first coil's alternating current builds a churning magnetic field in the iron; that field threads through the second coil and induces a voltage there. The ratio of turns sets the ratio of voltages — wind twice as many turns on the output side and you double the voltage while halving the current, like a lever that trades a long gentle push for a short forceful one.

Transformers are why the grid works at all. Power lost in a wire is I²R, so to ship gigawatts across hundreds of kilometres you step the voltage up to 230,000–765,000 V (tiny current, tiny losses), then step it back down to safe levels near homes. Only AC can be transformed this easily and cheaply — which is the deep reason Tesla's AC beat Edison's DC for the grid. A modern power transformer can be the size of a house and run at over 99.5% efficiency.