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The Wheatstone Bridge is used to find the value of an unknown resistor.
When there is no p.d. across the bridge the we say the Wheatstone Bridge is balanced.
If the value of resistance in the decade box is changed then it follows there is a direct proportional change in the current that will flow across the bridge.
Provided the resistance is not changed too much away from the balance point then it can be written
Further info about the Wheatstone Bridge
BBC Bitesize Higher Physics: Wheatstone Bridge.