Transformer

A transformer changes the voltage of an alternating current thanks to electromagnetic induction. In an ideal transformer:

$\frac{U_1}{U_2}=\frac{n_1}{n_2}=\frac{I_2}{I_1}$

where $n_1, n_2$ are the numbers of turns of the primary and secondary windings. The ratio $n_1/n_2$ is the turns ratio. When $n_2>n_1$, the transformer steps the voltage up; when $n_2<n_1$ — it steps it down.

Key point: by stepping the voltage up, a transformer steps the current down by the same factor (and vice versa), because the power $P=U\cdot I$ stays constant (in the ideal, lossless case). This is why energy is transmitted over high-voltage lines — a small current means small losses.