570 ELECTRICAL MEASUREMENTS 



are always of like polarity. Thus in Fig. 351 A the -f and - 

 signs indicate the polarities at some particular instant. The 

 primary and secondary currents are shown diagrammatically as 

 flowing in opposite directions. 



The + on the primary is the terminal at which the cur- 

 rent enters and the + on the secondary is the terminal by 

 which the current leaves the transformer to enter the external 

 circuit. 



Some manufacturers cross the secondary connections giving 

 the relative polarity shown in Fig. 35 IB. 



A simple method of testing the polarity is as follows : connect 

 a direct-current voltmeter to one of the windings (Fig. 35 1C) 

 noting which terminal is connected to the + post of the volt- 

 meter. Touch for an instant the terminals of a dry cell to the 

 terminals of the other winding, making the polarity such that 

 the meter reads up the scale on closing the circuit. The two cor- 

 responding + terminals will be the carbon of the cell and the 

 + post of the voltmeter. A very small current should be used, 

 otherwise the iron may be left in a highly magnetized condition 

 and the ratio and the phase angle of the transformer altered from 

 their normal values. 



When stating the conditions under which instrument trans- 

 formers are used (especially current transformers) it is highly 

 desirable that the inductance and resistance of the external sec- 

 ondary circuits be specified, for in that case there can be no mis- 

 understanding of the conditions under which the transformer is 

 operating. 



If the conditions are such that the readings of the 5-ampere 

 wattmeters and ammeters, which are commonly used in the sec- 

 ondary circuits of current transformers, are in the lower parts of 

 the scales there is a temptation to substitute lower range instru- 

 ments (for instance, 3 amperes) in order to obtain a good scale 

 reading. It must not be forgotten that such instruments will 

 heavily tax the current transformer and alter both the ratio and 

 the phase angle, for the burden placed on the transformer by the 

 3-ampere equipment is such that the voltage at the transformer 

 terminals must be increased to approximately 2. 8 times its original 

 value. 



