Mercury-in-glass thermometer principle: The basic operating principle of a mercury-in-glass thermometer is primarily based on which property change of mercury with temperature?

Introduction / Context:
Liquid-in-glass thermometers are among the oldest and most reliable temperature indicators. Mercury is often used because it does not wet glass, expands uniformly over a wide range, and is visible against the scale. This question focuses on the fundamental physical effect that enables the reading.

Given Data / Assumptions:

• A sealed capillary and bulb contain mercury.
• Pressure is nearly constant (atmospheric or sealed reference) during normal operation.
• Glass tube has a narrow bore to magnify displacement per degree.

Concept / Approach:
As temperature rises, mercury’s volume increases. Because the bulb and capillary confine the liquid, expansion forces the mercury column to climb in the narrow tube. The scale is calibrated so the height corresponds to temperature. The mechanism relies on volumetric expansion—not pressure change, conductivity, or electrical effects.

Step-by-Step Solution:

Verification / Alternative check:
Consistency of readings across instruments arises from well-characterized β for mercury and standardized stem dimensions; corrections for emergent stem are sometimes applied.

Why Other Options Are Wrong:

• Pressure and thermal conductivity changes are not the measurement basis.
• Differential linear expansion describes bimetallic thermometers, not mercury types.
• Electrical resistance change underlies RTDs, not liquid-in-glass devices.

Common Pitfalls:
Ignoring stem correction when part of the mercury column is outside the bath; errors grow at higher temperatures.

Final Answer:
Volumetric expansion