Materials engineering—Invar alloy used in thermocouples and instrumentation Invar used with certain thermocouple and precision instruments is an alloy of nickel and which metal?

Introduction / Context:
Invar is a famous low thermal expansion alloy widely used in precision engineering. In temperature measurement systems and certain thermocouple assemblies, support and lead components must maintain dimensional stability across temperature swings to minimize error. Knowing the base metals in Invar helps engineers select compatible materials for instrumentation and thermal systems.

Given Data / Assumptions:

• The question asks about the metallic constituents of Invar.
• Focus is on composition, not on exact percentage tolerances.
• Application context includes thermocouples and precision devices where low expansion is beneficial.

Concept / Approach:
Invar is a nickel–iron alloy specifically formulated to exhibit an extremely low coefficient of thermal expansion near room temperature. This property is a result of a delicate balance in the ferromagnetic structure that compensates normal thermal expansion. Because of this, Invar has been adopted for precision rods, pendulum components, measuring tapes, and structural elements in temperature-sensitive instruments and thermocouple housings or lead frames where dimensional stability matters.

Step-by-Step Solution:

Verification / Alternative check:
Handbooks of materials and instrumentation consistently list Invar as Fe–Ni with low expansion coefficient. Laboratory data and dimensional metrology standards corroborate its use where thermal stability is essential.

Why Other Options Are Wrong:

• Copper: Ni–Cu alloys form cupronickel and Monel type systems with different properties and higher expansion than Invar.
• Chromium: Ni–Cr combinations are common for resistance heating alloys but do not yield the hallmark low expansion of Invar.
• Lead: Ni–Pb combinations are not used for precision low-expansion alloys and are mechanically unsuitable for such roles.

Common Pitfalls:
Confusing Invar with other nickel alloys such as Nichrome or Monel. Also, assuming the presence of chromium due to its popularity in high temperature alloys; Invar targets dimensional stability, not high temperature oxidation resistance.

Final Answer:
iron