5-A] STUDY AND OPERATION. H3 



together, the voltage measured across the two primary and secondary 

 leads on the opposite side will be the sum of the voltages of the two 

 windings. 



29. Polarity ; Direct Current Method. The alternating current 

 method is usually preferred, but sometimes the following method will 

 be found convenient. The primary is supplied with a direct current 

 sufficient to give a reading on a direct current voltmeter connected 

 to the primary terminals. The voltmeter terminals are then con- 

 nected to what are supposed to be corresponding terminals of the 

 secondary. If, when the primary circuit is closed,* the voltmeter 

 needle is thrown in the same direction as the preceding reading, the 

 voltmeter has been connected to the secondary terminals correspond-, 

 ing to primary terminals; *'. e., the voltmeter lead from the primary 

 terminal (') or X is connected to the secondary terminal to be marked 

 (') or X- If tne voltmeter needle is thrown in the opposite direction, 

 the reverse is true. 



30. Potential Ratio. Where one transformer alone is to be tested, 

 the transformer should be supplied with any convenient voltage and 

 the voltage of each circuit measured either by two voltmeters, one of 

 which has been calibrated in terms of the other, or by one voltmeter 

 reading direct on the low potential side and with a multiplier on the 

 high potential side.f 



When one transformer has been tested in this manner, or a small 

 potential transformer of accurate ratio is available, two transformers 

 can be run in parallel from the same circuit and their secondary 

 voltages on open circuit compared by readings taken with one volt- 

 meter or with two voltmeters whose relative calibration is known. 



If the secondaries of two similar transformers are connected in 

 series and in opposition, any difference will be shown by a voltmeter 

 connected across the two. 



31. Current Ratio. For commercial testing of the ratio of a 

 transformer, test may be made by comparison of primary and 

 secondary currents instead of voltages. The secondary circuit is 

 short-circuited through an ammeter of low resistance and the 



*The current should be small so as not to injure the voltmeter by 

 slamming the needle when the circuit is made and broken. 



fit is not necessary to run the transformer at full rated potential. 

 When high potentials are used, due caution should be observed. 



