Question

A galvanometer having a coil resistance of $60$ shows full-scale deflection when a current of $1.0amp$ passes through it. It can be converted into an ammeter to read current up to $5.0amp$ by

• A. putting in series a resistance of $15$
• B. putting in series a resistance of $240$
• C. putting in parallel a resistance of $15$
• D. putting in parallel a resistance of $240$

Answer 1

Answer: (C)

putting in parallel a resistance of $15$

Given: A galvanometer having a coil resistance of $60$ shows full scale deflection when current of $1.0$ amp passes through it

To find when can it be converted into an ammeter to read current up to $5.0$ amp

Solution:

Galvanometer is a very sensitive instrument therefore it cannot measure heavy currents. In order to convert a Galvanometer into an Ammeter, a very low shunt resistance is connected in parallel to Galvanometer. Value of shunt is so adjusted that most of the current passes through the shunt.

If resistance of galvanometer is $R_g$ and it gives full-scale deflection when current $I_g$ is passed through it. Then,

$V=I_g{R}_g⟹V=1.0A\times 60\Omega ⟹V=60V$

Let a shunt of resistance ($R_s$) is connected in parallel to galvanometer. If total current through the circuit is $I$.

$I=5A=I_s+I_g$

$V=I_s{R}_s=(I-I_g)R_s$

$(I-I_g)R_s=I_g{R}_g$

$(5-1)R_s=60\times 1⟹R_s=15\Omega$

is the resistance that should be connected in parallel to convert it into an ammeter of range $5A$.