Mercury-in-glass thermometers The operating principle of a mercury-in-glass thermometer is based primarily on which physical effect?

Introduction / Context:
Mercury-in-glass thermometers are classic devices used for precise temperature measurement over a wide range. Despite the proliferation of electronic sensors, their fundamental operating principle remains a foundational concept in instrumentation courses and laboratory practice.

Given Data / Assumptions:

• The thermometer comprises a capillary tube and a bulb filled with mercury.
• The scale is calibrated against fixed points or reference instruments.
• Ambient pressure effects on the column inside the sealed capillary are negligible compared to thermal expansion of mercury.

Concept / Approach:
Temperature rise causes the volume of mercury to increase significantly more than the glass envelope. The difference in volumetric expansion forces the mercury column to rise in the fine capillary, which is read against a calibrated scale. This is distinct from “pressure-rise” devices (e.g., gas-filled or vapor-pressure thermometers) and from “linear expansion” of solids (e.g., bimetallic strips) that do not rely on a fluid volume change inside a capillary.

Step-by-Step Solution:

Verification / Alternative check:
Comparison with vapor-pressure thermometers confirms that mercury-in-glass does not rely on bulk pressure change but on column displacement arising from volume difference.

Why Other Options Are Wrong:

• Pressure rise with temperature: Characteristic of gas/vapor-pressure types, not mercury-column thermometers.
• Linear expansion: Refers to solids like bimetallic elements.

Common Pitfalls:
Confusing “capillary pressure” with the principle of operation; capillary action helps column stability but the measurement basis is volumetric expansion.

Final Answer:
Volumetric expansion.