358 PRINCIPLES OF ELECTRICAL DESIGN 



we have given it here. It is usually permissible to assume that, 

 if the difference in temperature between ingoing and outgoing air 

 is 18C., the actual temperature rise of the heated surfaces as 

 measured by thermometer, will not exceed 50C.; but, unless 

 every part of the machine is carefully designed, excessive local 

 heating may result. The temperature of the copper in the slots 

 might ordinarily be from 15 to 25C. higher than that of the 

 iron from which it is separated by layers of insulating material; 

 but should this insulation be very thick, and the cooling ducts 

 of insufficient section or improperly located, very much higher 

 internal temperatures may be reached. 







FACULTY OF APPLIED SCiLNCE. 

 Da* 



