Barometry and pressure types A mercury barometer is designed to measure which type of pressure directly?

Introduction / Context:
Understanding pressure types—atmospheric, absolute, gauge, and vacuum—is fundamental to instrumentation. Devices differ in their reference: some measure relative to ambient, others to absolute zero. A mercury barometer is a classical instrument whose operation is often misunderstood in this regard.

Given Data / Assumptions:

• The barometer consists of a mercury column with an evacuated space above the column.
• Ambient air pressure acts on the reservoir at the base.
• Temperature and local gravity affect the reading but not the reference concept.

Concept / Approach:
A barometer balances atmospheric pressure against a column of mercury. Because the space above the mercury column is (ideally) vacuum, the column height directly reflects the atmospheric pressure. While atmospheric pressure itself is an absolute pressure, the conventional answer distinguishes the measurand: the device specifically measures the current atmospheric pressure value, not gauge pressure (which is relative to atmosphere on both sides) nor “vacuum” level of an enclosure. Thus, the correct response is atmospheric pressure.

Step-by-Step Solution:

Verification / Alternative check:
Standard meteorology expresses barometric readings in mmHg or hPa as atmospheric pressure.

Why Other Options Are Wrong:

• Gauge: Gauge instruments compare to atmosphere; barometer compares atmosphere to vacuum.
• Vacuum: Barometer does not measure vacuum level inside process vessels.
• Absolute: Although atmospheric pressure is absolute, the accepted identification for this instrument is “atmospheric.”

Common Pitfalls:
Confusing the reference concept with the quantity measured and mixing terminology in exam settings.

Final Answer:
Atmospheric