Question

Find the unknown resistance AB when the resistance of the remaining arms are BC=120ohms, CD=110ohms, DA=110ohms, AB=?.

Answer 1

If we consider a Wheatstone bridge, and the circuit is balanced, i.e. the potential difference between points B and D is 0, then the unknown resistance is can be found from the relation

$$\frac{R_{AB}}{R_{BC}}=\frac{R_{AD}}{R_{CD}},\\ \space\\ R_{AB}=R_{BC}\cdot\frac{R_{AD}}{R_{CD}}=120\cdot110/110=120\space \Omega.$$