Question

What is constantan wire?

Answer 1

Recall that constantan is a copper-nickel alloy and can be easily drawn into wires. Additionally, it has a negligible temperature coefficient of resistance and is resistant to atmospheric corrosion. To this end, determine the applications for which constantan may be utilized owing to its previously-mentioned properties and arrive at an appropriate conclusion about the kind of wire it serves to be.

Complete answer:
A constantan wire is a wire made up of copper-nickel alloy. It usually consists of 55% copper and 45% nickel. It has a number of special characteristics due to which it is known as the most versatile alloy available.
It has a high specific thermal resistance due to which it has good insulating properties and lowers heat loss.
It has a negligible temperature coefficient of resistance ($\pm 0.00002\;K^{-1}$) due to which the variation of its resistivity is constant over a wide range of temperatures.
It offers good ductility due to which it can be easily drawn into wires since it easily stretches upon application of tensile strain. It possesses a tensile strength of about 450MPa before it breaks.
It is resistant to atmospheric corrosion and can be easily soldered, welded and moulded.
Since the resistance of constantan does not change with temperature, it is used for the purpose of making thermocouples and other such appliances that require practically no resistance variation with temperature. Since it has a high melting point $(1210^{\circ}C)$ and has one of the highest resistivities among all alloys, it is used for DC current shunts where the constantan wire is placed in series with the load in a DC circuit which is already carrying a high current. It can easily measure the voltage drop of the current flowing through it. Therefore, a constantan wire is essentially an electrical resistance wire that can be used for heating.
Thus, constantan is used in building chrome-constantan or iron-constantan thermocouples for temperature measurements. Due to its high specific thermal resistance, good ductility and high electrical resistivity, it is also an ideal alloy for heavy-duty industrial rheostats and wires.

Note:
Constantan may have versatile uses but it does have some negative impacts on human health. Exposure to the nickel in constantan is a source of carcinogen. Hypersensitivity to nickel may cause allergies, asthma or inflammatory reactions. The dust powder of the alloy may cause abrasive irritation to the eyes and respiratory discomforts, and ingestion may cause low oral toxicity or nausea. Thus, certain precautions need to be taken for the storage of constantan. It should be stored in a cool, dry area in a tightly sealed container, and it should always be handled using gloves, avoiding any direct contact.