Wheatstone bridge and strain gauges

The typical Wheatstone bridge arrangement as shown above contains a power source, a galvanometer (G), two resistors of known resistance (R₁, R₂), a variable resistor (R₄) and an unknown resistance, which is the one to be measured (R₃). The connection across CD containing the galvanometer is known as the bridge.

This circuit is sensitive to changes in the ratio of resistances across pairs of resistors. When the voltages at C and D are equal, the ratios of resistances equal each other (R₁/R₂ = R₃/R₄), no current will flow through the galvanometer and the bridge is balanced.

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In the circuit above, R₃ is the unknown resistance to be measured. The ratio of resistors R₁ and R₂ (ACB) is known. If the bridge is unbalanced, there will be a voltage present across CD and current will flow through the galvanometer. By altering the resistance of the variable resistor R₄ until the ratio of resistance across the limb ADB equals that of ACB, the bridge can be balanced, and no current flows across CD. By knowing the resistance required at R₄ to balance the bridge, R₃ can be calculated by using the equation above.

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