Understanding Voltmeter Range Extension

To extend the range of a voltmeter, a high resistance is connected in series with the voltmeter. This series resistance is often called a multiplier. The purpose of the multiplier is to limit the current flowing through the voltmeter's internal movement, allowing it to measure higher voltages without exceeding its full-scale current limit.

Properties of Material for Voltmeter Multiplier

The material used for making the multiplier resistor must have specific properties to ensure accurate and stable voltage measurements. Key requirements include:

• Low Temperature Coefficient of Resistance: The resistance should change very little with variations in temperature. This prevents the voltmeter's calibration from drifting due to ambient temperature fluctuations or self-heating.
• High Resistivity: A material with high resistivity allows for a relatively compact resistor to achieve the required high resistance value.
• Stability: The resistance value should remain stable over time and under various operating conditions.

Analyzing the Options

Let's consider the given material options:

• Copper: Copper is a good conductor with low resistivity. However, it has a relatively high temperature coefficient of resistance, making it unsuitable for precision resistors like voltmeter multipliers.
• Steel: Steel is an alloy primarily of iron. It also has a significant temperature coefficient of resistance and is not typically used for precision resistance applications.
• Aluminium: Aluminium is another good conductor like copper, with low resistivity and a relatively high temperature coefficient of resistance. Not suitable for voltmeter multipliers.
• Manganin: Manganin is an alloy consisting mainly of copper, manganese, and nickel. It is specifically known for its very low temperature coefficient of resistance and high stability over a wide range of temperatures. It also has a relatively high resistivity compared to pure copper. These properties make Manganin an ideal material for manufacturing precision resistors, including the multiplier resistors used to extend the range of voltmeters.

Why Manganin is Used

Due to its superior properties, particularly its low temperature coefficient of resistance and high stability, Manganin is the material of choice for constructing precision resistors, shunts for ammeters, and multiplier resistors for voltmeters. Using Manganin ensures that the resistance value of the multiplier remains nearly constant despite temperature changes, thus maintaining the accuracy of the voltmeter measurement over its extended range.

Therefore, Manganin is the material used as a series resistor for range extension of a voltmeter.