Page:Popular Science Monthly Volume 92.djvu/810

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��Popular Science Monthly

��netic action from its contact with D and the current is thus suddenly stopped. This instantly demagnetizes the core and induces in the secondary an E. M. F., which is usually high enough to cause a spark to leap the gap between t and t'. As soon as the core is demagnetized, the

Common magnetic circuit of laminoted iron \

���A.C.6enerctor

��Form of a transformer core that keeps the lines of force in a continuous iron path

spring R, supporting the hammer, re- stores contact with D and the operation is repeated. The condenser, shown in the diagram, is not an essential part of the coil; but when it is introduced, it is found that the length of the spark sent across the air gap is considerably increased.

The commercial transformer is a modi- fled form of the induction coil. The essential difference is that the core in Fig. 3, instead of being straight, is bent into some other form such that the magnetic lines of force have a continuous iron path, instead of being obliged to push out into the air, as in the case of the induction coil. Furthermore, it is always an alternating instead of an intermittent direct current which is impressed on the primary P. Sending such a current through the primary is equivalent to

��Core. Secondory

���Core

��^Primorij

��fig A

��Diagrammatic illustration of two general classes of transformers, a core and a shell type

magnetizing the core first in one direction, then demagnetizing it, then magnetizing it in the oppo.site direction, etc. The re- sult of these changes in the magnetism of the coil is, of course, an induced alter-

��nating current in the secondary coil S.

If there are few turns in the primary and a large number in the secondary, the transformer is called a step-up trans- former, because the voltage produced at the secondary terminals is greater than that impressed at the terminals of the primary, by the ratio of the number of turns of primary and secondary coils. Thus, an induction coil may be said to be of the step-up type. For some uses, how- ever, transformers may be of the step- down type. For example, 2000 volts are impressed at the terminals of the primary, and a lower voltage, say 100 volts, is obtained at the secondary termi- nals. In such a case the primary will have 20 times as many turns as the sec- ondary, and, we call it a 20 : 1 step-down transformer.

Assuming that the losses in the trans-

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���Fig. 5 fig, 6

The usual arrangements of coils and core for a shell type polyphase transformer

former are so small as to be negligible, the same number of magnetic lines of force passes through both primary and secondary coils. Since the E. M. F.'s in the two coils are proportional to the number of lines of force multiplied by the number of turns in the coil, it follows that the E. M. F.'s are directly proportional to the number of turns of wire upon the two coils.

Transformers are divided into two gen- eral classes, namely, core transformers and shell transformers. These two types are illustrated diagrammatically in Fig. 4.

Transformers for two- or three-phase currents can be made by combining two or three single-phase transformers into one piece of apparatus. In certain cases, polyphase transformers are desirable, but general practice is to use two or three separate single-phase transformers for transforming polyphase currents. The usual arrangement of coils and core for

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