Bimetallic thermometers — low-expansion alloy: Name the iron–nickel alloy (~36% Ni) universally used for the low-expansion strip in bimetallic thermometers.

Introduction / Context:
Bimetallic thermometers rely on differential thermal expansion of two bonded metals to convert temperature into pointer motion. One alloy is typically chosen for exceptionally low thermal expansion to maximize sensitivity and linearity of the composite strip.

Given Data / Assumptions:

• Target alloy contains roughly 36% nickel in iron.
• Application is the low-expansion leg in a bimetal.
• Need: dimensional stability with temperature changes.

Concept / Approach:
Invar (Fe–Ni ~36% Ni) has a very low coefficient of thermal expansion, about one twentieth that of common structural metals. In a bimetallic pair, Invar is bonded to a higher-expansion alloy so that temperature changes cause curvature and pointer movement.

Step-by-Step Solution:
Identify alloy with 36% Ni: Invar fits the composition.Match property: lowest practical expansion among the listed choices.Conclude: Invar is the correct low-expansion alloy used.

Verification / Alternative check:
Instrumentation catalogs and materials handbooks list Invar as the standard for precision bimetal strips, pendulums, and metrological components.

Why Other Options Are Wrong:
Constantan: Cu–Ni alloy used for resistance measurement (strain gauges, thermocouple negatives), not ultra-low expansion. Chromel/Alumel: nickel-based thermocouple materials for type K, selected for emf generation, not for low expansion.

Common Pitfalls:
Confusing electrical-thermocouple alloys with mechanical low-expansion alloys.

Final Answer:
Invar