Alternating current is best adapted for the transmission of electric power because of the ease with which it can be changed from one voltage to another. This is important because electric energy cannot be transmitted efficiently long distances except at high voltages. Therefore, it is necessary to raise the voltage for transmission and reduce it again where it is used and, at present, no simple method has been devised for changing DC voltages up and down for transmission purposes. A common device for changing AC voltage is known as a transformer.

19. Transformers.- A transformer usually consists of two separate windings on an iron core (See Figure 9) . It was stated at the beginning of this chapter that when a current is passed through a wire a magnetic field is formed around the wire, and if an iron bar is placed in such a field it is magnetized. Magnetic lines of force were described. These lines extend through the iron as well as through the air, and they form in closed loops, parts of the loops being in the iron and parts of them being in the air. If the entire path is in iron, i.e., if we have an iron ring, the number of lines of force are increased because it requires less magnetizing force to produce lines of force in iron than in air. Earlier in this chapter it was stated also that if the lines of force which passed through a loop changed in number, a voltage would be generated in the loop.

Referring to Figure 9 (a), if an alternating voltage is applied across the terminals of coil "A" an alternating current will pass through the coil. The current will set up lines of force through the iron, and the number of lines of force set up will be proportional to the current in the coil at any instant. Since the current is constantly changing, the lines of force will be constantly changing also. Coil "B" is a series of loops connected together, so we should expect to have a voltage generated in coil "B" when an alternating current was flowing in coil "A," and such is the case. If coil "B" is closed through an external circuit similar to that marked "Load," current will flow in the load circuit. Coil "A," or the coil to which the alternating current is supplied from an outside source, is called the primary. Coil "B," the coil in which the voltage is generated by the transformer action, is called the secondary of the transformer. Instead of having only one secondary winding, any number (within practical limits) can be used so as to supply several circuits at different voltages as shown in Figure 9 (b) .

The voltage generated in the secondary winding depends on the ratio of the number of turns of the secondary compared with the number of turns of the primary. If the secondary has twice as many turns as the primary, it will have twice the voltage of the primary, or if it has one-half the number of turns of the primary, it will have only one-half the primary voltage. It is therefore possible to obtain any desired voltage within quite a wide range.

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