INSTRUMENT "TRANSFORMERS 



563 



shows this to be the case. They are much more permanent than 

 the instruments they actuate. When used for voltage measure- 

 ments, only the ratio of transformation is important, and this 

 should be constant under the varying operating conditions. The 

 line voltage is given by 



V = (Ratio) X (Reading of voltmeter). 



For 2,300-volt circuit. For 13,000-volt circuit. 



FIG. 344. Potential transformers, General Electric Co. Note stars 

 on base of left-hand transformer indicating polarity. 



Load 



FIG. 345. Showing manner of using potential transformer. 



Switchboard voltmeters are graduated so that the line voltage 

 is read directly from the dial. If the ratio is not constant, the 

 combination of transformer and voltmeter may be calibrated as 

 a unit. 



Current Transformers. The current transformer is used in 

 cases where very large alternating currents must be measured 

 and also where the current coils of instruments must be isolated 

 from high-voltage lines. 



