The commercial requirements of users of electricity are best served by distributing electricity at high voltage and low amperage and by changing the same current into low voltage and high amperage by means of transformer placed on a pole, or better in a vault, before the electricity enters the building.

A transformer (Fig. 92) consists of three parts: (1) the primary coil, which is the wire which connects with the alternating current from the supply lines; (2) a core of iron; (3) and a secondary coil or wire in which is generated an electromotive force by the change of magnetism in the core which it surrounds.