Thermometry principles: which thermometer operates on volumetric expansion of a liquid?

Introduction / Context:
Different industrial thermometers rely on distinct physical principles: volumetric expansion, pressure–temperature relations, phase equilibria, or differential solid expansion. Choosing the right device requires understanding the underlying mechanism.

Given Data / Assumptions:

• Mercury-in-glass uses liquid volume expansion.
• Constant-volume gas thermometer measures pressure change at fixed volume.
• Vapor-pressure thermometer uses saturation pressure changes.
• Bimetallic thermometer uses differential linear expansion of two metals.

Concept / Approach:
Liquids generally expand with temperature. In a mercury-in-glass thermometer, the liquid’s volumetric expansion within a narrow capillary converts small volume changes into easily read length changes along a calibrated scale. The other instruments depend on pressure or bending effects, not direct volume indication within a capillary scale.

Step-by-Step Solution:
Identify volumetric expansion thermometer: mercury-in-glass.Differentiate from constant-volume gas: measures pressure, not volume change.Differentiate from vapor-pressure: reads pressure of a volatile liquid–vapor system.Differentiate from bimetallic: relies on differential expansion producing curvature.

Verification / Alternative check:
Calibration curves for mercury-in-glass are based on volumetric expansion coefficient of mercury and glass bulb/capillary geometry; corrections account for glass expansion (emergent stem correction when needed).

Why Other Options Are Wrong:

• Constant-volume gas: pressure-based; not volumetric expansion indication.
• Vapor-pressure: phase-equilibrium pressure; not capillary volume readout.
• Bimetallic: mechanical deflection; no liquid volume measurement.

Common Pitfalls:
Assuming all filled-system thermometers are “volumetric.” Gas and vapor systems primarily measure pressure, even though volume is fixed or phase is present.

Final Answer:
Mercury-in-glass thermometer