A voltmeter of resistance 1000 Ω gives full scale deflection when a current of 100 mA flow through it. The shunt resistance required across it to enable it to be used as an ammeter reading 1 A at full scale deflection is
Text Solution
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As we know
$R = 1000 \text{ ohm}$
$V = I_g R = 100 \times 10^{-3} \times 10^{3} = 100\,V \ldots (i)$
Voltmeter is used as ammeter by providing a shunt resistance parallel to it
$\therefore \text{Voltage } V = I_g \left(\frac{R r}{R + r}\right) \ldots \text{(ii)}.$
From (i) and (ii) we get
$100 = I_g\left(\frac{Rr}{R + r}\right)$
$100 = 1 \times \left(\frac{1000r}{1000 + r}\right)$
$10^{5} + 100r = 1000r$
$10^{5} = (1000 - 100)r$
$10^{5} = 900r$
$\frac{100000}{900} = r$
$r \approx 111\Omega$
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