The Wheatstone Bridge consists of two resistive potential dividers connected to a common voltage source. If one potential divider has resistances **R**_{1} and **R**_{2} in series and the other potential divider has resistances **R**_{3} and **R**_{4} in series, with **R**_{1} and **R**_{3} connected to one side of the voltage source and **R**_{2} and **R**_{4} connected to the other side of the voltage source, then at the balance point where the two resistively divided voltages are equal:
**R**_{1} / R_{2} = R_{3} / R_{4}
If the value of resistance **R**_{4} is unknown and the values of resistances **R**_{3}, **R**_{2} and **R**_{1} at the balance point are known, then:
**R**_{4} = R_{3}R_{2} / R_{1} |