Fig. 150 shows about the simplest form of transformer with a solid iron core, on which are wound two coils, the one, P, being the primary, and the other, S, the secondary. Fig. 151 shows the general appearance of one make of transformer. The operation of this apparatus, as already mentioned, is to reduce the high pressure alternating current sent out over the conductors from the dynamo, to a potential at which it can be employed with convenience and safety, for illumination and other purposes. They consist of two or more coils of wire most carefully insulated from one another. A core or magnetic circuit of soft iron, composed of very thin punchings, is then formed around these coils, the purpose of the iron core being to reduce the magnetic resistance and increase the inductive effect. One set of these coils is connected with the primary or high-pressure wires, while the other set, which are called the secondary coils, is connected to the house or low-pressure wires, or wherever the current is required for use. The rapidly alternating current impulses in the primary or high-pressure wires induce secondary currents similar in form but opposite in direction in the secondary coils. These current impulses are of a much lower pressure, depending upon the ratio of the number of turns of wire in the respective coils, it being customary to wind transformers in such a manner as to reduce from 1,000 or 2,000-volt primaries to 50 or 100-volt secondaries, at which voltage the secondary current is perfectly harmless. [Illustration: Fig. 152.] =136. Motor-Dynamos.= Fig. 152. These consist essentially of two belt-type machines on a common base, direct coupled together, one machine acting as a motor to receive current at a certain voltage, and the other acting as a dynamo to give out the current usually at a different voltage. As they transform current from one voltage to another, motor-dynamos are sometimes called Double Field Direct Current Transformers. The larger sizes have three bearings, one bearing being between the two machines, while the smaller sizes have but two bearings, the two armatures being fastened to a common spider. [Illustration: Fig. 153.] _Applications._ The uses to which motor-dynamos are put are very various. They are extensively used in the larger sizes as "Boosters," for giving the necessary extra force on long electric supply circuits to carry the current to the end with the same pressure as that which reaches the ends of the shorter circuits from the station. Motor-dynamos have the advantage over dynamotors, described later, of having the secondary voltage easily and economically varied over wide ranges by means of a regulator in the dynamo field.