Low-expansion alloy identification in metrology: “Invar” is a famous iron–nickel alloy used where minimal thermal expansion is required (e.g., measuring tapes, pendulum rods). Its typical nickel content by percentage is closest to:

Introduction / Context:
Invar is widely referenced in materials and metrology because of its exceptionally low coefficient of thermal expansion. Knowing its nominal composition helps engineers select it for precision instruments and dimensional stability applications.

Given Data / Assumptions:

• We identify the characteristic nickel content of Invar.
• No derivations are required; this is a property/composition recall question.

Concept / Approach:
Classic Invar is approximately Fe–36% Ni (balance iron). This composition minimises thermal expansion near room temperature due to magnetic and lattice effects. Variants (Invar 36) are standard in precision equipment, bimetal thermostats (with partners), and LNG tank membranes.

Step-by-Step Solution:

Verification / Alternative check:
Materials datasheets list Invar 36 as 36% Ni, demonstrating the stable, low thermal expansion coefficient (~1–1.5 × 10^-6 per °C near ambient).

Why Other Options Are Wrong:

• 12%/24%/30% nickel: compositions of other Fe–Ni alloys that do not exhibit the same ultra-low expansion as classic Invar.

Common Pitfalls:

• Confusing Invar (low expansion) with Monel (Cu–Ni) or Kovar (Fe–Ni–Co with glass-matching expansion).

Final Answer:
36% nickel