Question

Constantan wire is used for making standard resistance, because it has

• A. Negligible temperature coefficient of resistance
• B. Low specific resistance
• C. High specific resistance
• D. High melting point

Answer 1

The Correct Option is A

To understand why constantan wire is used for making standard resistances, we need to examine its key electrical properties.

1. Understanding Standard Resistance Requirements:
Standard resistors need to maintain a consistent resistance value over time and under varying conditions, particularly temperature changes.

2. Properties of Constantan:
Constantan (a copper-nickel alloy) has several important characteristics:
- Negligible temperature coefficient of resistance (α ≈ ±0.00002/°C)
- Moderate specific resistance (about 49 µΩ·cm)
- Good corrosion resistance
- Reasonable melting point (~1210°C)

3. Why Temperature Coefficient Matters Most:
For standard resistances, the most critical property is:
- Minimal change in resistance with temperature (low α)
This ensures the resistance value remains stable during use.

4. Why Other Options Are Less Important:
- Low specific resistance would require longer wires
- High specific resistance isn't necessary for standard resistors
- Melting point is secondary to resistance stability

5. Comparison with Other Materials:
Unlike copper (which has α ≈ 0.004/°C) or other alloys, constantan's near-zero temperature coefficient makes it ideal for precision applications.

Final Answer:
Constantan wire is used for making standard resistances because it has a negligible temperature coefficient of resistance.