A.C, as far as the conveyance of electric power to great distances is concerned, because the higher the pressure the smaller the current strength corresponding to a given amount of power, and the smaller the quantity of metal required in the transmission line. But if high pressure is an economic necessity as regards transmission of power, it is an objection as regards the utilisation of power. We must therefore transform at the place of utilisation the small current of high pressure into a large current of low pressure. This is done by an apparatus called the "transformer." The principle, on which the transformer works, is illustrated in Fig. 22. The high pressure current is delivered at the terminals T1 T1. To these is connected a coil of many turns of wire wound on an iron core C. On the same core is placed a second coil of fewer turns of stouter wire, and the consuming devices (lamps or motors) are connected to the terminals T2 T2 of this coil. The high-pressure current passing through the winding of the primary coil P, magnetises the iron core, and since the secondary coil C is encircling this core also, it is traversed by the flux of force produced by the primary current. Thus by electromagnetic induction
