Question

An ammeter and a milliammeter are converted from the same galvanometer. Out of the two, which current measuring instrument has a higher resistance?

Answer 1

Hint: We only need to apply the concept of shunt resistance. It is a major point in the case of galvanometer and ammeter. As required here we need ohm’s law:
 $ S = \dfrac{{{I_g}}}{{I - {I_g}}}G $
Here, $ {I_g} $ is the current through galvanometer,
 $ S $ is the shunt resistance
 $ I $ is the current in the circuit,

Complete step by step answer
We already know that a resistor having a very low value of resistance is called shunt resistance.
And, $ S = \dfrac{{{I_g}}}{{I - {I_g}}}G $
The value of $ {I_g} $ is very small.
So, $ S \approx \dfrac{{{I_g}}}{I}G $
Clearly, smaller the value of range, larger is the shunt resistance. Obviously,
milliammeter will have a larger shunt resistance and hence it will have a higher resistance
 $ \dfrac{1}{{{R_A}}} = \dfrac{1}{G} + \dfrac{1}{S} $
This means that higher the value of shunt resistance, higher the resistance for the galvanometer.

Additional Information
In order for an ammeter to measure a device's current, it must be connected in series to that device. This is necessary because objects in series experience the same current. An ammeter is placed in series to measure current. All of the current in this circuit flows through the meter. A shunt resistance should be connected in parallel to the galvanometer so as to keep its resistance low. Such low resistance galvanometer (ammeter) is used in series with the circuit to measure the strength of current through the circuit.

Note
A galvanometer can detect only small currents. Thus, to measure large currents it is converted into an ammeter. It can be converted into an ammeter by connecting a low resistance called shunt resistance in parallel to the galvanometer. A voltmeter is a device used to measure the potential difference between two points in a circuit.