Kelvin Bridge
B) Kelvin Bridge: It is a device used to measure resistance of a galvanometer . The galvanometer whose resistance is to be found is connected in one of the gap of the meter bridge.
Construction: It,s construction is same as compared to meterbridge only thing is the galvanometer is connected in one of the gap instead of connecting it with a jockey. Refer the below figure.
Diagram:
Working: when the potential difference is applied between two ends of the conductor having length 1 metre, the current is divided at junction according to kirchoff's current law. The jockey of the is tapped on the wire in order to adjust both end of the galvanometer at same potential so that the current through galvanometer become zero. Let us consider point L₁ is obtained from left such that at this point galvanometer shows null deflection. The value of length is noted and L₂ is obtained using mathematical calculation. Now bridge is said to be in balanced condition.
According to wheatstone bridge,
G / R = (ρL₁ / A) / (ρL₂ / A) ----------- ( R= (ρL / A)
G / R = L₁ / L₂
G/ R = L₁ / (100- L₁) --------------(A) ( L₂ = 100 - L₁)
Equation no (A) gives value of unknown resistance .
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