Polarity detector is a device used to detect polarity of an applied voltage. One can determine if the voltage that's present in a given line is positive or negative. The polarity tester has a circuit diagram as shown. When we connect a pair of wires one at point A and other at B, if the line at A has a greater sufficient positive voltage than B then the lamp connected in the circuit glows. Else A has negative potential compared to B. If the lamp does not glow even if lines are swapped than it indicates the lines are dead, that is they have no voltage in them.