Question

Why is constantan or manganin used for making standard resistors?

Answer 1

Constantan and manganin are used for making standard resistors for a specific combination of four key properties that make them ideal for precision applications.

Here’s a breakdown of why these alloys are the materials of choice:

The Four Key Properties

1. Very Low Temperature Coefficient of Resistance (TCR)

This is the most important reason.

• What it is: The TCR measures how much a material's resistance changes when its temperature changes. A low TCR means the resistance remains stable and "constant" even if the ambient temperature fluctuates.
• Why it matters for a standard resistor: A "standard" resistor is a reference point. Its value must be known and reliable. If its resistance changed every time the lab temperature went up or down by a few degrees, it would be a useless standard.
• How they excel: Both constantan and manganin have a TCR that is very close to zero around room temperature (20-25°C), which is the typical operating environment for these components. The resistance of a copper wire, by contrast, would change significantly over the same temperature range.

2. High Resistivity (ρ)

• What it is: Resistivity is a material's inherent ability to resist the flow of electric current. A high resistivity means the material is a relatively poor conductor.
• Why it matters: To build a resistor with a specific value (e.g., 10 Ω), you need a certain amount of material. If you use a material with very low resistivity (like copper), you would need an impractically long and thin wire to achieve a useful resistance. With high-resistivity alloys like constantan and manganin, you can create resistors of a desired value using a reasonable and manageable length of wire. This makes the final component compact and sturdy.

3. Long-Term Stability

• What it is: The resistance of the material should not change or "drift" over time due to aging, oxidation, or changes in its internal crystalline structure.
• Why it matters: A standard resistor might be used for years or even decades. Its value must remain stable over its entire lifespan to be a reliable reference.
• How they excel: After proper heat treatment (annealing), both alloys exhibit excellent long-term stability. Manganin is particularly renowned for this property.

4. Low Thermoelectric EMF against Copper

• What it is: When two different metals are joined, a small voltage (thermoelectric EMF) can be generated if there is a temperature difference across the junction. This is known as the Seebeck effect.
• Why it matters: Standard resistors are connected into circuits using copper wires. If a significant thermoelectric voltage were generated at the connection points, it would act like a small battery in series with the resistor, introducing errors into the precise voltage and current measurements you are trying to make.
• How they excel: Manganin is especially good in this regard, producing a very small thermoelectric EMF when connected to copper. This minimizes measurement errors, making it the preferred choice for the highest-grade laboratory standards. Constantan has a higher thermoelectric EMF, which is why it's also used to make thermocouples (where you want this effect), but it's still low enough for many resistor applications.

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Comparison: Manganin vs. Constantan

While both are excellent, they have slightly different strengths, which determines their primary use.

| Property | Manganin (Cu-Mn-Ni) | Constantan (Cu-Ni) | Why It Matters |
| :--- | :--- | :--- | :--- |
| TCR | Extremely low, but in a narrow range around room temp. | Very low over a wider temperature range. | Manganin is perfect for stable lab environments. Constantan is more versatile for devices that see temperature swings. |
| Thermoelectric EMF | Extremely low against copper. | Higher than Manganin. | Manganin is the superior choice for high-precision standards where even tiny error voltages are unacceptable. |
| Long-Term Stability | Excellent, considered the gold standard. | Very good. | Manganin is used for primary reference standards that must not drift over many years. |
| Primary Use | Precision standard resistors, potentiometers, Wheatstone bridges. | Strain gauges, shunts, general-purpose precision resistors, thermocouples. | Each alloy is optimized for its best application. |

Conclusion

In short, constantan and manganin are used for standard resistors because their resistance is stable against changes in temperature, they have high enough resistivity to be practical, they don't change value over time, and they don't introduce voltage errors when connected to copper wires.

For the absolute highest level of precision and stability required in a metrology lab, manganin is the preferred material.