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especially as it is somewhat difficult to 

 determine, in a body of such complicated 

 shape, what is and what is not effective 

 cooling surface. From the table, giving 

 the loss by hysteresis, it is easy to calcu- 

 late for any induction, frequency, and 

 weight of the active iron in a transformer, 

 what the iron heat will be ; and, adding to 

 this the copper heat, the total cooling 

 surface required to keep the apparatus at 

 a safe temperature can at least approxi- 

 mately be determined, or the limit of 

 output for any given transformer can be 

 approximately found. In this connection, 

 it is instructive to compare two similar 

 transformers, one twice the linear dimen- 

 sions of the other. For the purpose of 

 the comparison, it may be assumed that 

 in the small transformer the iron heat is 

 the same as the copper heat, though it is 

 generally greater. If the large transformer 

 be worked at the same induction per unit 

 area of core, its iron heat will be eight 

 times that of the small transformer, or 

 four times its total heat; but as the 

 cooling surface is only four times as 



