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Joined 2026-09-21 Message
Incoming links: Mercury Barometers and Manometers by W. G. Brombacher and others
D. P. Johnson 2026-10-31
Fortin barometer 2026-10-31
An adjustable-cistern barometer associated with Jean Nicolas Fortin. Restoring the cistern surface to a fixed index makes the lower mercury level reproducible before the upper level is read.[ref]
J. L. Cross 2026-10-31
Mercury-barometer temperature correction 2026-10-31
A measured mercury column represents pressure through its actual mass density and local gravitational acceleration. An equivalent reference column therefore obeys . If the indicated scale length expands as and mercury volume as , the first-order thermal factor multiplying the scale reading is . These factors describe a pressure-unit reduction, not a physical change in local gravity.[ref]
Mercury Barometers and Manometers by W. G. Brombacher and others. §§9–10 and equation45
A mercury barometer balances atmospheric pressure against the hydrostatic head between the open cistern surface and the mercury surface in a closed tube. Writing for the pressure above the column, the ideal relation isThe relevant is the vertical difference between the two mercury levels. It is not simply the length of filled glass, and the top space need not be an absolutely perfect vacuum.
A cistern barometer needs the following corrections or controlled adjustments:
- Cistern level and scale zero. When mercury rises in a fixed tube, the reservoir level falls. A fixed-zero scale must allow for both motions. For uniform tube area and effective cistern free area , volume conservation gives , so . In a Fortin barometer, an adjustable cistern brings its surface to a fixed index before the column is read.
- Temperature. Both mercury density and scale length change. To express a corrected physical height as an equivalent height at reference density and standard gravity , use . If a scale marked at the reference temperature expands with coefficient , while mercury has volume-expansion coefficient , a reading at temperature difference has the first-order thermal factor . For an ordinary metal scale warmer than its reference, the mercury-density effect dominates and the thermal correction reduces the reading.
- Gravity. The same physical mercury height represents a different pressure where differs. Reduction to a conventional mercury pressure unit includes ; this does not assert that local gravity has physically changed.
- Capillarity, scale and sighting. Correct the capillary depression and calibrated scale/index errors, read the proper part of the meniscus, avoid parallax, and set the instrument vertical. Mercury contamination and changes in the meniscus can compromise a previously determined capillary correction.
- Top-space pressure. Trapped gas and mercury vapour add . A significant trapped-air error cannot be cured by pretending the top space is empty.
The observational precautions are also described in the Guide to Meteorological Instruments and Methods of Observation by the World Meteorological Organization.[ref]
Guide to Meteorological Instruments and Methods of Observation by the World Meteorological Organization. Part I, chapter 3, mercury barometers
The quantitative corrections are separated in Mercury Barometers and Manometers by W. G. Brombacher and others, particularly the cistern, temperature and gravity discussions. Reduction of a weather-station pressure to sea level is a separate atmospheric comparison, not an intrinsic correction to the instrument.[ref]Mercury Barometers and Manometers by W. G. Brombacher and others. §§5.1–5.3, fixed and adjustable cisterns; §§8–10, capillarity, temperature and gravity; equation 45
W. G. Brombacher 2026-10-31