Bimetallic thermometer sensitivity: two strips with different linear expansion are bonded. Which material below has the highest linear thermal expansion coefficient and thus is commonly paired against low-expansion alloys?

Introduction / Context:Bimetallic thermometers rely on differential thermal expansion. When heated, the high-expansion strip elongates more than the low-expansion strip, causing curvature that drives a pointer. Material choice maximizes sensitivity while preserving mechanical stability.

Given Data / Assumptions:

• Candidate materials: nickel, brass, chromel (Ni–Cr), invar (Fe–Ni low-expansion alloy), stainless steel.
• We compare typical room-temperature linear expansion coefficients.

Concept / Approach:Typical coefficients (approximate, 10^−6 / °C): brass ~19, stainless ~17, nickel ~13, chromel ~14, invar ~1.2. Thus, brass exhibits the largest expansion among the listed candidates and is often used as the high-expansion member paired with a low-expansion alloy such as invar to generate significant curvature per degree of temperature change.

Step-by-Step Solution:List approximate coefficients for each material.Identify the maximum among the options.Select “Brass” as the high-expansion strip.

Verification / Alternative check:Materials handbooks show brass near 18–20 × 10^−6 / °C; invar near 1–2 × 10^−6 / °C; the differential is what drives the bimetal action.

Why Other Options Are Wrong:Nickel/chromel: Moderate expansion, lower than brass.Invar: Extremely low expansion (chosen as the low-expansion partner).Stainless: High but still below brass in most grades.

Common Pitfalls:Confusing bimetal selection with electrical properties; expansion coefficient, not resistivity, governs mechanical deflection.

Final Answer:Brass