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Wheatstone bridge

 

The Wheatstone Bridge consists of two resistive potential dividers connected to a common voltage source. If one potential divider has resistances R1 and R2 in series and the other potential divider has resistances R3 and R4 in series, with R1 and R3 connected to one side of the voltage source and R2 and R4 connected to the other side of the voltage source, then at the balance point where the two resistively divided voltages are equal:
R1 / R2 = R3 / R4

If the value of resistance R4 is unknown and the values of resistances R3, R2 and R1 at the balance point are known, then:
R4 = R3R2 / R1