First slide
combination of resistance
Question

The top left and right arms of a Wheatstone bridge have resistances P and Q. The bottom left and right arms have resistances R and S. When R = 12 ohm, the bridge is balanced. Now P and Q are interchanged and R = 27 ohm balances the bridge. The resistance S is 

Moderate
Solution

for the first case PQ=12S----1 for the second case QP =27S----2 multiplying 1 and 2 1=27 x 12S2    solving S =18

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