122 TREATISE ON ALTERNATING CURRENTS. 



75. Third Method. This method, due to Mr. Mordey, 

 consists in running the transformer at the load at which it is to be 

 tested until its temperature, as determined by a thermometer 

 placed inside it, is constant. The alternating current is now re- 

 placed by a steady current, having such a value as to maintain the 

 same steady temperature. The total losses are then given by the 

 product of the steady current in amperes and the volts between 

 the terminals of the transformer. 



76. Fourth Method. This is a method by which the 

 efficiency can be determined by calculation from a knowledge of 

 the primary E.M.S., applied P.D., the resistances of the primary 

 and secondary windings, the frequency, the dimensions of the 

 core, the thickness of the core stampings, and the number of turns 

 on the primary coil. 



The resistances of the coils may be calculated by determining 

 the lengths and cross-sections of the wire composing the two coils 

 respectively. 



CALCULATION OF HYSTERESIS Loss. 



77. To calculate the hysteresis loss, we must first determine 

 the maximum induction in the core, and then refer to Table IV., 

 Chap. XL, giving the loss in watts per cubic centimetre at that 

 induction. 



Let e be the E.M.S. applied primary P.D. 

 A the cr-oss-section of the core. 

 NI the number of primary turns. 

 B the maximum induction. 

 n the frequency. 



Then 



^! 



= -JQB vei r nearl y 



since this gives the counter E.M.F. in the primary circuit; 

 therefore 







Substituting the known values of e, n, N, and A, we obtain the 

 value of B. Referring now to Table IV., Chap. XL, we find the 



