Question

A voltmeter of resistance $1000$ ohm gives full scale deflection when a current of $100mA$ flows through it. The shunt resistance required across it to enable it to be used as an ammeter reading $1A$ 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
$V=I_{g}R=100\times 10^{-3}\times 10^3=100V$ ... (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
$100=I_8\left(\frac{Rr}{R+r}\right)$
$100=1\times \left(\frac{1000r}{1000+r}\right)$
$10^5+100r=1000r$
$10^5=(1000-100)r$
$10^5=900r$
$\frac{100000}{900}=r$
$r\approx 111\Omega$