*A One Quarter Kilowatt Closed Core Transformer.* The simplest form of a closed core transformer consists of two independent coils of insulated wire wound upon an iron ring. When an alternating current is passed through one of the coils, known as the primary, it generates a magnetic flux in the iron core. The lines of force passing through the core induce in the secondary coil an electromotive force the magnitude of which is in nearly the same ratio to the primary inducing electromotive force as the number of turns of wire in the secondary is to the number of turns in the primary. For example, if it is desired to raise the potential of the no volt alternating current to 22,000 volts, the number of turns in the secondary of the transformer must be at least 200 times the number in the primary.
A circular ring of iron wire presents several theoretical advantages as a transformer core but it would be difficult to form and afterwards place the windings in position. The core is therefore usually in the form of a hollow rectangle, built up of very thin sheets or laminations of soft iron carefully insulated from one another by a coat of varnish. If the core were solid or the separate laminations not insulated from one another, heavy currents, known as eddy currents, would be set up in the iron and cause heating. A considerable loss in the efficiency of the transformer would also result.
One half of both the secondary and the primary windings of a properly designed transformer are placed on opposite sides or "legs" of the core in order to reduce the magnetic leakage and increase the efficiency. The only difficulty involved in such construction is the proper insulation of the primary from the secondary, but if careful attention is given to this point no difficulty will be experienced.
*Core.*—The dimensions and method of assembling the core laminations are indicated by Fig. 44. Long strips are cut from soft Russian or Swedish stovepipe iron. The strips, which are 1 3/4 inches wide are then cut up into short lengths, one half of which are 7 3/4 inches long and the other half 4 3/4 inches. Enough are cut to form two piles of each size 1 3/4 inches high when compressed. The completed core will then form a hollow rectangle 9 1/2 x 6 1/2 x 1 3/4 inches.