Why is it that while using a moving coil galvanometer as a voltmeter, a high resistance in series is required whereas in an ammeter a shunt is used?

The use of a moving coil galvanometer as a voltmeter or ammeter involves different configurations due to the nature of the measurements being taken.

Understanding the Role of Resistance

When using a galvanometer as a voltmeter, it measures the potential difference across two points. To ensure accurate readings without drawing too much current, a high resistance is placed in series with the galvanometer. This high resistance limits the current flowing through the device, allowing it to measure voltage without affecting the circuit significantly.

Why High Resistance for Voltmeters?

• Minimizes Current Draw: A voltmeter should not alter the circuit it is measuring. High resistance ensures that only a small amount of current flows through the galvanometer.
• Protects the Galvanometer: Excessive current can damage the sensitive coil of the galvanometer.

Using a Shunt in Ammeters

In contrast, when a galvanometer is used as an ammeter, it measures the current flowing through a circuit. To handle larger currents without damaging the galvanometer, a shunt resistor is connected in parallel. This allows most of the current to bypass the galvanometer while still enabling it to measure a fraction of the total current.

Benefits of Using a Shunt

• Protects the Device: The shunt prevents excessive current from passing through the galvanometer, safeguarding it from damage.
• Accurate Current Measurement: The shunt allows the galvanometer to measure high currents accurately by only requiring a small portion of the total current to pass through it.

In summary, high resistance is essential for voltmeters to limit current and protect the device, while a shunt is used in ammeters to allow most current to bypass the galvanometer, ensuring accurate measurements without damage.