An ammeter is a crucial instrument used to measure electric current flowing through a circuit. Sometimes, the current to be measured is higher than the maximum current the ammeter can safely measure. In such cases, it becomes necessary to extend the range of the ammeter to allow it to measure larger currents without damage.

Ammeter Range Extension

To safely extend the range of an ammeter, a specific component is used that allows most of the high current to bypass the ammeter's internal coil, while only a small, measurable portion passes through the ammeter itself. This component must be connected in a way that divides the total current. Given the options, let's explore which one is suitable for this purpose.

Shunt Resistor: Extending Ammeter Range

A shunt resistor, often simply called a shunt, is a very low resistance wire or conductor connected in parallel with the ammeter (or galvanometer which is the core of an ammeter). Here's why a shunt is used to extend the range of an ammeter:

• Parallel Connection: When a shunt is connected in parallel, it provides an alternative path for the electric current. According to Kirchhoff's current law, the total current divides between the ammeter and the shunt.
• Low Resistance: Since the shunt has very low resistance, the majority of the high current flows through the shunt, bypassing the sensitive coil of the ammeter. Only a small, proportional current flows through the ammeter, which is within its measurable limit.
• Protection: This arrangement protects the ammeter from being damaged by large currents, effectively allowing it to measure currents much higher than its original full-scale deflection current.

The value of the shunt resistance (\(R_{sh}\)) required to extend the range can be calculated using the formula:

\[ R_{sh} = \frac{I_g R_g}{I - I_g} \]

Where:

• \(I_g\) is the full-scale deflection current of the galvanometer (ammeter's internal coil).
• \(R_g\) is the resistance of the galvanometer (ammeter's internal resistance).
• \(I\) is the new desired full-scale range of the ammeter.

Multiplier Explained: Voltmeter Range Extension

A multiplier is a high resistance connected in series with a galvanometer. Multipliers are specifically used for the following purposes:

• To convert a galvanometer into a voltmeter.
• To extend the range of a voltmeter.

When a multiplier is connected in series, it increases the total resistance of the voltmeter circuit. This allows the voltmeter to measure higher potential differences (voltages) across a circuit, as a larger voltage drop occurs across the series combination. It is important to note that a multiplier is NOT used to extend the range of an ammeter because connecting a high resistance in series with an ammeter would significantly increase its overall resistance, which is undesirable for an ammeter (an ideal ammeter should have zero resistance) and would impede the current flow being measured.

Shunt vs. Multiplier: Key Differences

Understanding the distinct roles of a shunt and a multiplier is crucial for instrumentation:

Conclusion on Ammeter Range

Based on the principles of electrical circuits and instrument design, only a shunt resistor can be used to effectively and safely extend the range of an ammeter. A multiplier, on the other hand, serves a completely different purpose related to voltmeters. Therefore, the correct choice is "Only shunt".