70 COMMERCIAL ELECTRICAL TESTING 



may be combined to equal Rs F , likewise ^i and R n 

 to equal Rep. 



The total losses will then be 



2 W = Running light + C 4 2 # 4 +C 4 2 # 5 + C^V L -\-C L 2 Rs r 



To calculate resistances hot when calculating efficien- 

 cies, the temperature should be obtained from the 

 formula : 



r=C&TXrise by thermometer) +25 C. 

 K is the ratio between the rise in temperature by ther~ 

 mometer and that determined by resistance measure- 

 ment. Resistance measurements of temperature have 

 been determined by actual tests on a large number of 

 different armatures and fields. For all armatures, or 

 field spools of revolving field machines, AT =1.25. For 

 stationary ventilated field spools A" =1.7. See Tables 

 VIII and IX, and Fig. 22 for form used in calculating 

 and plotting efficiency. 



