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contained in the coil. The larger z, and 

 the smaller n, may be the length of wire, 

 and therefore its resistance. On purely 

 theoretical grounds it would thus seem 

 that a small core strongly magnetized, 

 and as high a frequency as the alternator 

 can be constructed to give, would pro- 

 duce the best results. In practice, how- 

 ever, this conclusion is wrong, and the 

 heating of the iron core in consequence 

 of hysteresis must be taken into consider- 

 ation. Since a transformer is au 

 apparatus at rest, and is frequently 

 placed in a position where the cooling 

 effect of the air is small, the ratio 

 between heat generated and available 

 cooling surface must be very different 

 from that permissible in a dynamo, and 

 it is this ratio which is the true limit to 

 the output of a transformer. The arma- 

 ture of a dynamo is not subjected to 

 excessive heating if a cooling surface of 

 about 1 square inch per watt be provided. 

 In a transformer, the amount of cooling 

 surface should be at least three times, 

 and if possible, four or five times as great, 



