First slide

Electrical measuring instruments

Question

A potentiometer wire AB is 100 cm long and has a total resistance of 10 ohm. If the galvanometer shows zero deflection at the position C, then value of unknown resistance R is 2N $Ω$ , then N is.

Moderate

Solution

$\begin{array}{l} Potential Gradient K = \frac{10}{100 (cm)} p o t e n t i a l g r a d i e n t = \frac{E_{0}}{L} W H E R E E_{0} = 10 v o l t \\ A l s o, V_{A C} = (\frac{5}{R + r}) R T e r m i n a l p o t e n t i a l d i f f e r e n c e = \frac{E R}{R + r} \\ ∴ K \times 40 = (\frac{5}{R + 1}) R \\ \Rightarrow R = 4 Ω \end{array}$
Thus, $N = 2$ .

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