Editing Barometer (section)

=== Mercury barometers ===
A [[Mercury (element)|mercury]] barometer is an instrument used to measure atmospheric pressure in a certain location and has a vertical glass tube closed at the top sitting in an open mercury-filled basin at the bottom. Mercury in the tube adjusts until the weight of it balances the atmospheric force exerted on the reservoir.  High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column.  Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature levels around the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.  The tube has to be at least as long as the amount dipping in the mercury + head space + the maximum length of the column.

[[File:MercuryBarometer.svg|thumb|left|Schematic drawing of a simple mercury barometer with vertical [[Mercury (element)|mercury]] column and reservoir at base]]

Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the [[Earth's atmosphere|atmosphere]].<ref name="http://www.islandnet.com/~see/weather/history/barometerhistory1.htm"/>  He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".<ref>Strangeways, Ian. ''Measuring the Natural Environment''. Cambridge University Press, 2000, p. 92.</ref> Inspired by Torricelli, [[Otto von Guericke]] on 5 December 1660 found that air pressure was unusually low and predicted a storm, which occurred the next day.<ref name="ley196606">{{Cite magazine
 |last=Ley
 |first=Willy
 |date=June 1966
 |title=The Re-Designed Solar System
 |department=For Your Information
 |url=https://archive.org/stream/Galaxy_v24n05_1966-06#page/n93/mode/2up
 |magazine=Galaxy Science Fiction
 |pages=94–106
}}</ref>

[[File:Baro 3.png|thumb|upright|Fortin barometer]]
The mercury barometer's design gives rise to the expression of atmospheric pressure in [[inch of mercury|inches]] or [[Millimeter of mercury|millimeters]] of mercury (mmHg).  A [[torr]] was originally defined as 1 mmHg.  The pressure is quoted as the level of the mercury's height in the vertical column.  Typically, atmospheric pressure is measured between {{convert|26.5|inch}} and {{convert|31.5|inch}} of Hg. One atmosphere (1 atm) is equivalent to {{convert|29.92|inch}} of mercury.

Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers.

In 2007, a [[European Union]] directive was enacted to restrict the use of mercury in new measuring instruments intended for the general public, effectively ending the production of new mercury barometers in Europe. The repair and trade of antiques (produced before late 1957) remained unrestricted.<ref name="eu reuters">{{cite news|url=https://www.reuters.com/article/environmentNews/idUSL0988544920070710|title=EU bans mercury in barometers, thermometers|author=Jones H.|date=10 July 2007|work=Reuters|access-date=12 September 2017}}</ref><ref>{{Cite web| url=https://www.europarl.europa.eu/sides/getDoc.do?type=IM-PRESS&reference=20070706IPR08897&language=EN| title=Ban on sale of mercury measuring instruments - MEPs agree two year exemption for barometers| date=10 July 2007| website=European Parliament| access-date=2021-05-11}}</ref>

==== Fitzroy barometer ====
[[Robert Fitzroy|''Fitzroy'']] barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes.

==== Fortin barometer ====
[[File:Baro 2.png|thumb|upright|Reservoir of a Fortin barometer]]

[[Jean Nicolas Fortin|Fortin]] barometers use a variable displacement mercury cistern, usually constructed with a thumbscrew pressing on a leather diaphragm bottom (V in the diagram).  This compensates for displacement of mercury in the column with varying pressure.  To use a Fortin barometer, the level of mercury is set to zero by using the thumbscrew to make an ivory pointer (O in the diagram) just touch the surface of the mercury.  The pressure is then read on the column by adjusting the [[vernier scale]] so that the mercury just touches the sightline at Z.  Some models also employ a valve for closing the cistern, enabling the mercury column to be forced to the top of the column for transport.  This prevents water-hammer damage to the column in transit.