160 TRANSFORMERS. [Exp. 



which is the joint resistance of the primary and secondary in 

 terms of the primary: 



The value thus determined will be used for comparison with the 

 value (R = Wc--I z ) determined from the copper loss in the 

 short-circuit test. 



PART III. SHORT-CIRCUIT TEST. 



17. Method of Test. For the short-circuit test, the sec- 

 ondary (low-potential side) of the transformer is short circuited 



and the primary (high- 

 ly potential side) is supplied 

 1 1 with a small difference of 

 <?- potential, just sufficient to 

 .j " cause the full-load or de- 



FIG. 6. Connections for short-circuit test sired Current to flow. This 

 for copper loss and impedance voltage. 



is rarely more than 5 or 6 



per cent, of normal voltage. Instruments* are connected in the 

 primary, as shown in Fig. 6. The current might be measured 

 by an ammeter in either circuit, but it is betterf to have the 

 ammeter on the primary side with the voltmeter and wattmeter. 



* ( I7a). The most important reading to have correct is that of the 

 ammeter, since the wattmeter reading varies as P and the voltmeter read- 

 ing varies as I, and all results are calculated for values of current. For 

 this reason it is well to place the ammeter directly in the primary circuit, 

 as in Fig. 6, in which case no correction is necessary. If the ammeter is 

 connected in the supply line, as in Fig. I, and the instruments are read 

 simultaneously, a small error is introduced (tending, in this case, to favor 

 the transformer) unless a correction is applied. See Appendix III., Exp. 

 5-A. Connected as in Fig. 6, the wattmeter usually needs no correction ; 

 but for the accurate measurement of small power the method of con- 

 nection shown in Fig. 7, Exp. 5-A, should be used and a correction 

 applied. 



For selection of instruments, see 44. 



t (J7b). It is important for accuracy to have the short circuit of the 

 secondary as " short " as possible, i. e., with practically zero impedance 



