__________ wire is never used for making the heating element.

ANS:C - Invar

Invar is a nickel-iron alloy known for its low coefficient of thermal expansion (CTE), which means it expands and contracts very little with changes in temperature. This unique property makes Invar valuable in applications where dimensional stability over a wide temperature range is critical. Invar typically contains around 36% nickel and 64% iron, although variations in composition exist depending on the specific application requirements. The name "Invar" is derived from "invariable," reflecting its stable dimensions across a range of temperatures. Key characteristics and properties of Invar include:

1. Low Thermal Expansion: Invar has one of the lowest coefficients of thermal expansion among all known materials. This means it exhibits minimal dimensional changes when subjected to temperature variations, making it suitable for precision instruments and devices where dimensional stability is crucial.
2. High Strength: Invar maintains good mechanical strength and toughness, allowing it to withstand mechanical stresses and loads in various applications.
3. Magnetic Properties: Invar is ferromagnetic, meaning it can be magnetized in the presence of a magnetic field. However, its magnetic properties can vary depending on its composition and processing.
4. Resistance to Corrosion: Invar exhibits good resistance to corrosion in many environments, although it may be susceptible to certain forms of corrosion under specific conditions.
5. Applications: Invar is commonly used in applications such as precision instruments, scientific equipment, aerospace components, and specialized machinery where dimensional stability is critical. Examples include precision measuring devices, optical instruments, bimetallic strips, and aerospace components subjected to extreme temperature changes.

Overall, Invar's unique combination of low thermal expansion, high strength, and other desirable properties make it a valuable material in various engineering and manufacturing applications, particularly those requiring precise dimensional control and stability over a wide temperature range.