5-B] TEST BY LOSSES. J 59 



PART II. RESISTANCE MEASUREMENTS. 



15. Data. The primary and secondary resistances are meas- 

 ured by means of a bridge or by direct current, fall-of -potential 

 method (17, Exp. i-A). Disconnect the voltmeter before the 

 current is thrown off, to avoid damage by inductive kick. Avoid 

 heating the coils and so causing their resistance to increase; the 

 testing current should not exceed 25 per cent, of the full-load 

 current for the coil, or should not be long continued. The range 

 of the ammeter to be used is thus determined from the known 

 value of full-load current. 



The range of voltmeter is found by assuming an approxi- 

 mate value for resistance drop; thus, if the resistance drop 

 were one per cent, in primary or secondary for full-load current, 

 this would be 10 volts in a i,ooo-volt coil and I volt in a 100- 

 volt coil. If only one fourth of full-load current were used toi 

 testing, the voltage readings would be 2.5 and 0.25 volts, re- 

 spectively. 



The resistance measurements by direct current are to be 

 used as a check and for comparison with the results obtained 

 in the short-circuit test. 



Temperature conditions should be taken account of (22). 



1 6. Equivalent Primary Resistance. From the measured 

 values of R^^ and R 2 , compute the equivalent resistance* R, 



* ( i6a). The equivalent resistance R must have such a value that 



Dividing by I* and writing the ratio of turns (5"i -h 5" 2 ) in place of the ratio 

 of currents (7 2 -=-/i), we have 



It is obvious, also, that R = (copper loss) -f- /i 2 . 



Any resistance in the secondary, either within the transformer or in the 

 external circuit, has the same effect as though it were multiplied by the 

 square of the ratio of turns and placed in the primary circuit. It may 

 be noted here that the same is true of reactance. 



