Concepts/Formula used:

Ohm’s Law:

Potential Difference (V) across a resistor of resistance R when current I passes through it is given by Ohm’s law:

Ammeter:

It consists of a galvanometer coil in parallel with a stunt resistance.

Kirchhoff’s junction rule:

The sum of currents entering a junction is equal to the sum of currents leaving it.

The given voltmeter looks like this:

The maximum potential difference that can be measured is ,and let the current through the voltmeter for maximum deflection be .

Note that the coil (r) and the other resistor (R) are in series.

(Note that 1kΩ = 1000Ω )

Now, using Ohm’s law,

The ammeter we want looks like the following diagram:

The maximum current that can be measured is .

From our previous calculations we know that the current through the coil for maximum deflection is .

Note that the shunt resistance (r_s) and the coil (r) are parallel in an ammeter.

By Kirchhoff’s junction rule,

Using Ohm’s law, we have

and

As r_s and r are in parallel, the potential difference across them is the same.

Hence, the shunt resistance is 0.251Ω.