First slide

Tangent Galvanometer

Question

A microammeter has a resistance of 100 $Ω$ and a full scale range of 50 $μ$ A. It can be used as a voltmeter or as a higher range ammeter provided a resistance is added to it. Pick the correct range and resistance combination(s).

Moderate

A

50 V range with 10 K $Ω$ resistance in series
B

10 V range with 200 k $Ω$ resistance in series
C

5 mA range with 1 $Ω$ resistance in parallel
D

10 mA range with 1 $Ω$ resistance in parallel

Solution

We know that ${\dot{I}}_{g} = V / (R + G)$
For $a, 50 \times 10^{- 6} = 50 / (R + 100)$
$R = (10^{6} - 100) Ω$
For $b, 50 \times 10^{- 6} = 10 / (R + 100)$
$∴ R = 200 \times 10^{3} Ω$
Further $i_{g} = {S / (S + G)} i$
For $c, 50 \times 10^{- 6} = {S / (S + 100)} 5 \times 10^{- 3}$
Solving, we get $S = 100 / 99 \approx 1 Ω$

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