Invar tape material in baseline measurement: the nickel content of the nickel–iron alloy (Invar) used for precision tapes is typically closest to which percentage?

Introduction / Context:
Invar tapes are used for high-precision baseline measurements because their thermal expansion is extremely low, minimizing temperature-induced length errors.

Given Data / Assumptions:

• Invar is a nickel–iron alloy engineered for near-zero thermal expansion.
• Typical composition is narrowly controlled to achieve this property.

Concept / Approach:
Near-zero thermal expansion occurs when nickel content is about 36%, with iron comprising the balance. This composition keeps the coefficient of thermal expansion around 0.7 × 10^-6 per °C, far below that of steel.

Step-by-Step Solution:
Recall definition of Invar and its target thermal property.Match the composition that delivers this behavior → ~36% Ni.

Verification / Alternative check:
Materials references for metrology equipment list Invar 36 (UNS K93600) as the standard alloy for tapes and rods.

Why Other Options Are Wrong:

• 24%, 40%, or 60% nickel do not yield the characteristic near-zero expansion needed for surveying tapes.

Common Pitfalls:

• Confusing Invar with stainless or Monel alloys that have higher expansion.

Final Answer:
36%