Transformer

A transformer is a passive device which can change the voltage of an AC current which is fed into it; the output can be either higher or lower voltage, depending on the details. (Of course, conservation of energy dictates that if the output is higher voltage, it will be a smaller current, and vice versa.)

Transformers also isolate the input and output circuits; i.e. the input voltage is measured across the two input leads; and similarly the output across the two output leads. However, unless there is some other connection between the two sides, neither lead on either side has any voltage relative to either on the other.

(It is possible to have a transformer which has the same output voltage as the input; these are called 'isolation' transformers, since it is the isolation property of a transformer which is being sought in this case.)

A transformer consists of two inductors wrapped around a 'core' - an element through which magnetic fields can easily pass. (The core is usually a loop, so that the field bin it ends around and joins itself.) When a current flows into the first inductor, it creates a magnetic field, which flows around the core, and induces a current in the second inductor.

If the output inductor has more loops than the input, the output voltage will be higher, and vice versa.