This instrument is used to check the correctness of polarity markings on transformers. In operation, the two leads carrying the test current are connected to one of the transformer windings. The other two leads from the indicating meter are connected to the other winding. When the push button is pressed, DC current from a small dry cell flows through the transformer winding. The momentary induced voltage in the other winding causes the center-zero meter to deflect in one direction. Right side deflection indicate correct polarity, left side deflection stands reverse polarity
 
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