Many boys possessing electrical toys and apparatus operating upon direct current only, have bemoaned the fact that the lighting system in their town furnished alternating current. Very often in the case of small cities or towns one power-house furnishes the current for several communities and the energy has to be carried a considerable distance. Alternating current is then usually employed. [Illustration: Fig. 176.—Alternating Current System for Light and Power.] The illustration shows the general method of arranging such a system. A large dynamo located at the power-house generates alternating current. The alternating current passes into a "step-up" transformer which raises the potential to 2,200 volts (approximately). It is then possible to use much smaller line wires, and to transmit the energy with smaller loss than if the current were sent out at the ordinary dynamo voltage. The current passes over the wires at this high voltage, but wherever connection is established with a house or other building, the "service" wires which supply the house are not connected directly to the line wires, but to a a "step-down" transformer which lowers the potential of the current flowing into the house to about 110 volts. In larger cities where the demand for current in a given area is much greater than that in a small town, a somewhat different method of distributing the energy is employed. [Illustration: Fig. 177.—Motor Generator Set for changing Alternating Current to Direct Current.] The alternating current generated by the huge dynamos at the "central" station is passed into a set of transformers which in some cases raise the potential as high as five or six thousand volts. The current is then sent out over cables or "feeders" to various "sub" stations, or "converter" stations, located in various parts of the city. Here the current is first sent through a set of step-down transformers which reduce the potential to the approximate value originally generated by the dynamos. It then passes into the "rotary converters" which change the alternating current into direct current after which it is sent by underground cables direct to the consumers in the neighborhood. A transformer in its simplest form consists of two independent coils of wire wound upon an iron ring. When an alternating current is passed through one of the coils, known as the primary, it produces a magnetic field which induces a current of electricity in the other, or secondary, coil. The potential or voltage of the current in the secondary is in nearly the same ratio to the potential of the current passed into the primary as the number of turns in the secondary is to the number of turns in the primary. [Illustration: Fig. 178.—Step-Up Transformer.]