When a current of 5 A flows through an ammeter, it gives full deflection. The resistance of ammeter is 20 Ω. If the same ammeter is to be used for measuring a maximum current of 10 A, then the ammeter must be connected to a resistance of :

20 Ω in parallel

The correct answer is Option (4) → 20 Ω in parallel

Given,

full deflection current, $I_{full}=5A$

Resistance of the ammeter, $R=20Ω$

Maximum current to be measured, $I_{max}=10A$

$V_{ammeter}=I_{full}×R_{ammeter}$ [By Ohm's law]

$V_{ammeter}=5×20=100V$

Let the Shunt resistor be $R_{shunt}$. The voltage across the shunt resistor must also be 100V when 5A flows through.

$V_{shunt}=R_{shunt}×I_{shunt}$

$⇒R_{shunt}=\frac{100}{5}=20Ω$

Thus,

the shunt resistor must be connected in parallel with the ammeter $20Ω$.