Solveeit Logo
Login

Question

Question: How can a galvanometer be converted into an ammeter?...

How can a galvanometer be converted into an ammeter?

Explanation

Solution

The deflection of the galvanometer is proportional to the current flowing through it. Therefore, it must allow a maximum flow of current through it. To do so, its effective resistance should be very low.

Complete step by step answer:
We know that a galvanometer is used to detect very small current passing through it if any. The coil in the galvanometer is suspended between the strong shoe magnets and when the current passes through the coil, it shows deflection. We know that the deflection of the galvanometer is proportional to the current flowing through it.
To convert the galvanometer into the ammeter, it should fulfill the following conditions:
Since it measures only a small value of current, its effective current capacity should be increased.
The effective resistance of the galvanometer should be decreased since the maximum current will flow through the galvanometer if and only if the resistance is low.
To do so, a low resistance is connected across the galvanometer known as shunt resistance as shown in the figure below.

When the current I pass through the above circuit, it divides at the junction A. We know that the voltage remains constant in parallel arrangement, therefore,
Vg=VS{V_g} = {V_S}
⇒IgRg=(I−Ig)S\Rightarrow {I_g}{R_g} = \left( {I - {I_g}} \right)S
⇒Ig=SS+RgI\Rightarrow {I_g} = \dfrac{S}{{S + {R_g}}}I
Therefore, we can say that the current flowing through the galvanometer is proportional to the current flowing through the circuit. We know that the deflection of the galvanometer is proportional to the current flowing through it. Therefore, the deflection of the galvanometer is also proportional to the current passing through the circuit.

Note:
To measure the current, the ammeter must allow all the incoming current to pass through it. Therefore, the resistance of the ammeter is very low. The major advantage of a galvanometer used as an ammeter is it increases the range of an ammeter. Also, the shunt resistance connected across the galvanometer protects the galvanometer from damage.