Transformer, in electricity, is a device by means of which the pressure of an alternating current may be increased or reduced, the current strength being correspondingly reduced or increased. It essentially consists of two coils of insulated wire wound on an iron core, usually so arranged that the magnetic circuit is closed. Numerous inventors have designed many different patterns, which, however, principally differ in appearance and in the ease with which they may be constructed. The iron core must be carefully laminated to avoid loss by eddy currents, and as transformers are usually used to reduce a current at 2,000 volts in the mains to a current at 100 volts suitable for use in houses, great care must be expended upon the insulation of one coil from the other. If a current at 2,000 volts is to be converted to one at 100, the primary winding must have twenty times as many convolutions as the secondary, and as the secondary current will be twenty times stronger than the primary, the former main must be of twenty times the section of the latter. A current at 2,000 volts can of course be distributed by a much smaller main conductor than one at 100 volts, and thus a saving both of capital and expenditure by loss in mains may be effected, more especially in supplying a scattered district; but against this must be set the cost of transformers and loss by reason of their want of perfect efficiency.