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Rh with a kind of iron shell. This type is called a "shell transformer." In either type the primary and secondary coils are placed as close: together as is compatible with an efficient insulation between them. This is done in order that the secondary winding may be traversed by as near as possible the whole flux which passes through the primary winding, and thus the ratio of transformation is very nearly equal to the ratio between the number of turns in the two windings.

The efficiency of transformers is remarkably high. Under efficiency must be understood the ratio of the power received by the primary circuit to the power given out by the secondary circuit. No machine can have an efficiency of unity; the output must always be smaller than the input, but the difference in the case of a transformer is much smaller than in the case of a dynamo of equal power. Even a small transformer of but a few KW. power may have as much as 90 per cent. efficiency, whilst large transformers of 1000 KW. may reach 98 or 98½ per cent.

Alternating currents are produced by machines of the type represented by Fig. 15. Such a machine is simply an implement for converting mechanical power into the electric