156 COMMERCIAL ELECTRICAL TESTING 



former coils; this being generally the proper amount for 

 two or four transformers. The ammeter should not 

 read below the center of the scale, nor should the current 

 be sufficient to appreciably heat the windings while 

 taking resistance. For very low voltage secondary 

 windings, use about 40 amperes, as this current usually 

 gives sufficient drop to be read on the voltmeter. The 

 drop lines must not include the resistance of any tempo- 

 rary connections. Adjust the resistance in the box until 

 the reading comes to about the middle of the scale of the 

 voltmeter. Considerable time will be saved by short 

 circuiting the secondary while the primary is being 

 measured, and by short circuiting the primary while the 

 secondary is being measured. In measuring secondary 

 resistances, especially when low, the contacts for the 

 voltmeter leads should be carefully cleaned with sand- 

 paper. 



Take three readings on each coil, holding about the 

 same current. It is far better to allow the ammeter to 

 vary slightly, than to try to hold exactly the same 

 reading, as the observer is likely to be prejudiced. In 

 entering readings, always record the constants of the 

 meters, the voltmeter resistance, the resistance of the 

 drop lines, the resistance in the box and the temperatures 

 of the coils. If the transformers have more than one 

 primary and one secondary coil, a clear sketch should be 

 made and the coils so marked as to prevent confusion. 

 In recording results, the value of the unit deflection 

 should be noted and readings pointed off accordingly. 

 Readings should be taken as rapidly as is consistent with 

 accuracy. The method of calculating rise in temperature 

 by increase of resistance is explained under heat runs. 



Polarity 



The polarity test is taken, since it affords the only 

 means of readily determining the connections required 

 for transformers in banks; for instance, several trans- 

 formers in parallel. When transformers are connected 

 for measurement of resistance, the polarity test can 

 readily be made with a special voltmeter. Select one 

 transformer as a standard; when several are in test 



