First slide

Electrical measuring instruments

Question

The error in measurement of unknown resistance of X is minimum in a meter bridge when l=70 cm, where l is the distance of null point from one end. If $X = [\frac{l}{(A - l)}] R$ find the value of A, where R is known resistance.

Moderate

A

35 cm
B

105 cm
C

140 cm
D

210 cm

Solution

$\begin{array}{l} X = [\frac{l}{(A - l)}] R \\ \frac{Δ X}{X} = \frac{Δ l}{l} - \frac{Δ l}{(A - l)} = 0 \\ l = A - l \\ or A = 2 l = 140 cm \end{array}$

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