Introduction:
A moving coil galvanometer is an instrument used to measure small electric currents. It consists of a coil of wire suspended between the poles of a permanent magnet. When a current flows through the coil, it experiences a torque due to the interaction between the magnetic field of the coil and the magnetic field of the permanent magnet. The deflection of the coil is proportional to the current passing through it.

Conversion into a voltmeter:
To convert a moving coil galvanometer into a voltmeter, a resistor is introduced in series with the galvanometer. This resistor is called the shunt resistor. The purpose of the shunt resistor is to divert most of the current away from the galvanometer coil, allowing it to measure a voltage instead of a current.

Reason for introducing a resistance of large value in series:
The resistance of the shunt resistor is chosen to be much smaller than the resistance of the galvanometer coil. This is because the shunt resistor is connected in parallel with the coil, and according to Ohm's Law, the majority of the current will flow through the path of least resistance.

By introducing a resistance of large value in series with the galvanometer, the overall resistance of the circuit increases. This causes the current passing through the galvanometer to decrease, as the voltage is divided between the shunt resistor and the galvanometer coil.

Importance of a large resistance value:
1. Sensitivity: The sensitivity of the voltmeter depends on the deflection of the coil. By introducing a resistance of large value in series, the current passing through the coil is reduced, resulting in a smaller deflection. This allows the voltmeter to measure smaller voltages accurately.

2. Protection: The large resistance value of the shunt resistor helps to protect the galvanometer coil from damage. Since most of the current flows through the shunt resistor, only a small fraction passes through the galvanometer coil. This prevents excessive current from flowing through the delicate coil, which could lead to overheating and damage.

3. Range: By selecting different values for the shunt resistor, the voltmeter can be calibrated to measure different voltage ranges. A larger resistance value will allow the voltmeter to measure higher voltages, while a smaller resistance value will allow it to measure lower voltages.

Conclusion:
In conclusion, the conversion of a moving coil galvanometer into a voltmeter is done by introducing a resistance of large value in series. This helps to increase the overall resistance of the circuit, reduce the current passing through the galvanometer coil, improve sensitivity, protect the coil from damage, and provide a wide range of voltage measurements.