Question

A voltmeter of resistance $1000$ ohm gives full scale deflection when a current of $100\, mA$ flows through it. The shunt resistance required across it to enable it to be used as an ammeter reading $1\, A$ at full scale deflections?

• A. $10000$ ohm
• B. $9000$ ohm
• C. $222$ ohm
• D. $111$ ohm

Answer 1

Answer: (D)

As we know
$R = 1000$ ohm
$V = I_{g}R = 100 \times 10^{-3} \times 10^{3} = 100\, V$ ... (i)
Voltmeter is used as ammeter by providing a shunt resistance parallel to it

$\therefore $ Voltage $V = I_{g}\left(\frac{Rr}{R + r}\right)$... (ii)
From (i) and (ii) we get
$100=I_{8}\left(\frac{R r}{R+r}\right)$
$100=1 \times\left(\frac{1000 r }{1000+ r }\right)$
$10^{5}+100 r=1000 r$
$10^{5}=(1000-100) r$
$10^{5}=900 r$
$\frac{100000}{900}=r$
$r \approx 111 \Omega$