Understanding Resistor Material Properties

The question asks to identify a specific copper-based alloy used for making standard resistors. The key properties that make an alloy suitable for this purpose are its high resistivity and a low temperature coefficient.

Key Properties for Resistor Alloys

• High Resistivity: This property means the material resists the flow of electric current effectively. A higher resistivity allows for a resistor to have a significant resistance value even with a small physical size.
• Low Temperature Coefficient: This indicates that the material's resistance changes very little with fluctuations in temperature. For standard resistors, stability is crucial, so the resistance should remain consistent regardless of heating during operation or ambient temperature changes.
• Copper-Based Alloy: The question specifically mentions a copper base, guiding the selection process.

Analysis of Options

Alnico Alloy Characteristics

Alnico is an alloy primarily composed of aluminum, nickel, cobalt, and iron. While it has magnetic properties, it is not copper-based and is not typically used for making standard resistors due to its specific characteristics not aligning with the required high resistivity and low temperature coefficient for resistors.

Manganin: The Preferred Resistor Material

Manganin is a copper-based alloy, typically containing around 84% copper, 12% manganese, and 4% nickel. This specific composition gives Manganin:

• High Electrical Resistivity: It offers significantly higher resistance compared to pure copper.
• Very Low Temperature Coefficient: Its resistance remains remarkably stable over a range of temperatures. This is its most significant advantage for precision applications.

Because of these properties, Manganin is widely used in the construction of standard resistors, particularly for applications requiring high accuracy and stability, such as measuring instruments and shunts.

Tungsten Metal Properties

Tungsten is a metal known for its extremely high melting point and strength. It is used in applications like incandescent light bulb filaments. While it has relatively high resistivity compared to copper, its temperature coefficient is significant, and it's not a copper-based alloy, making it unsuitable for the specific requirements of standard resistors mentioned in the question.

Brass Alloy Composition and Use

Brass is an alloy of copper and zinc. It is valued for its workability, strength, and resistance to corrosion. However, brass has relatively low resistivity compared to alloys like Manganin and its temperature coefficient is not as low, making it inappropriate for high-precision standard resistors.

Conclusion

Based on the analysis, Manganin perfectly fits the description of a copper-based alloy with high resistivity and a low temperature coefficient, making it the preferred material for standard resistors needing stability and accuracy.