barometer

1. An instrument for measuring atmospheric pressure, used especially in weather forecasting.
2. Something that registers or responds to fluctuations; an indicator: Opinion polls serve as a barometer of the public mood.

An absolute pressure gage specifically designed to measure atmospheric pressure. This instrument is a type of manometer with one leg at zero pressure absolute. See also Manometer.

The common meteorological barometer (see illustration) is a liquid-column gage filled with mercury. The top of the column is sealed, and the bottom is open and submerged below the surface of a reservoir of mercury. The atmospheric pressure on the reservoir keeps the mercury at a height proportional to that pressure. An adjustable scale, with a vernier scale, allows a reading of column height. Aneroid barometers using metallic diaphragm elements are usually less accurate, though often more sensitive, devices, and not only indicate pressure but may be used to record it. See also Pressure measurement.

Mercury barometer.

An investment instrument whose movements forecast trends.

Investopedia Says:
For example, a barometer stock has a price trend that is indicative of the market. And, the stock market as a whole is said to be a barometer because it can be used to forecast the growth or slowdown in the economy.

Related Links:
The economy has a large impact on the market, so investors should know how to interpret these eleven indicators. Economic Indicators to Know
Investing during an economic downturn simply means changing your focus. Discover the benefits of defensive stocks. Cyclical Versus Non-Cyclical Stocks

Selective compilation of economic and market data designed to represent larger trends. Consumer spending, housing starts, and interest rates are barometers used in economic forecasting. The Dow Jones Industrial Average and the Standard & Poor's 500 Stock Index are prominent stock market barometers. The Dow Jones Utility Average is a barometer of market trends in the utility industry.

A barometer stock has a price movement pattern that reflects the market as a whole, thus serving as a market indicator. General Motors, for example, is considered a barometer stock.

The mercury-in-glass barometer reflects atmospheric pressure which pushes an exposed column of mercury up an upright glass tube with an end which has been partly evacuated, and sealed. This response may be expressed as:

p = g.ρh

For more information on barometer, visit Britannica.com.

The mercury barometer had its origins in the investigations being made in Italy during the early seventeenth century to discover why it was impossible to build a suction pump to raise water higher than about thirty feet (10 m). Once it was found that the height attainable was related to the density of the liquid, the experimenters exchanged their cumbersome metal tubes filled with water for shorter glass tubes with the heaviest fluid available—mercury—which was mined in Tuscany. The results of numerous experiments undertaken in Rome, Florence, and elsewhere were widely circulated and discussed.

The first apparatus generally accepted as a barometer was that set up in Florence in 1644 by Evangelista Torricelli (1608–1647), a mathematician and physicist. Torricelli filled a glass tube with mercury, sealed it at one end, and inverted it with its open end in a dish of mercury. The level always fell a short way down the tube, then settled at a height of about thirty inches. He concluded correctly that the mercury column was sustained by the weight of the air pressing on the open surface of mercury, and further experiments convinced him that the space above the mercury in the tube was a vacuum. He noted that the level rose and fell with changing temperature, but he was unable to use his apparatus to measure variations in the weight of the atmosphere because he had not foreseen that temperature would affect the level of the mercury.

News of this experiment circulated quickly among European scientists, who hastened to replicate the experiment. Torricelli's conclusions were not universally accepted because some disputed whether the air had weight, while both Aristotle and the Catholic Church denied the possibility of a vacuum. In France, the philosopher René Descartes (1596–1650) seems to have been the first person, probably in 1647, to attach a graduated scale to the tube so that he could record any changes attributable to the weather. At around this time Duke Ferdinand II of Tuscany organized the first short-lived meteorological network among scientists in other Italian cities, gathering observations of pressure, temperature, humidity, wind direction, and state of the sky.

Descartes, the Minim friar Marin Mersenne (1588–1648), an important nexus for scientific communications, and physicist Blaise Pascal (1623–1662) also discussed whether the mercury column would be shorter if the experiment was performed at the top of a mountain where, presumably, the atmosphere weighed less. Around 1648 Pascal's brother-in-law Florin Perier (1605–1672) set up a tube at Clermont, where it stood at 26 inches 3½ lines (the French line was one-twelfth of a French inch), and carried another tube to the summit of the Puy de Dôme, where the mercury stood at 23 inches 2 lines.

By 1648, the barometer was serving the three purposes that it continued to serve thereafter: as an apparatus for testing the laws of physics, as an instrument for measuring altitude, and as a weather monitor and, later, prognosticator. The words baroscope and barometer, meaning 'instrument for measuring weight', first used by Robert Boyle in the early 1660s, were soon adopted into the Latin, French, German, and Italian languages.

The Barometer As a Physics Apparatus

Numerous experiments using variations of Torricelli's apparatus were performed by members of the Accademia del Cimento (The Academy of Trial, or Experiment), a group of Florentine virtuosi active from 1657 to 1667, and published in its Saggi di naturali esperienze fatti nell'Accademia del Cimento (Examples of experiments in natural philosophy made by the academy) in 1667. They sought to discover if the space above the mercury was filled with vapor or air diffused through the glass, and what effect different shaped tubes would have if the dish of mercury, or the entire apparatus, was covered. Many of these experiments were inconclusive, the academicians being unable to interpret their findings. With Otto Guericke's invention of the air pump, the barometer served as a means of measuring the strength of the vacuum created for a whole series of related experiments.

A Diversity of Shapes

By 1650, Pascal had probably devised the siphon barometer, which consisted simply of a sealed tube with its open end curved up at the bottom. In 1663, Robert Hooke, demonstrator to the Royal Society, devised the "wheel" barometer, in which a float on the open surface of mercury in a siphon tube was connected to a cord running over a pulley to a counterweight; a pointer on the pulley axle rotated on a large dial, amplifying the small daily variations in height. Many variations of form, usually to enhance portability or to amplify the scale, were proposed in the following century, often by people with no understanding of the glassblower's abilities or the problems of filling such tubes without admitting some air. Among the more practical forms, some of which still survive, were folded, conical, and angled tubes, and tubes with two liquids. By about 1670, the barometer had found its way into wealthier homes and various types could be bought in London and Paris.

In June 1668, Robert Boyle described and illustrated his "portable" siphon, fastened to a board on which a graduated scale was marked, the idea being to send examples to distant places, but he admitted the difficulty of filling such a tube. Credit for the first truly portable barometer is disputed: the barometer maker John Patrick (1654–1730) may have invented the method, and he opposed the patent of 1695 filed by the clockmaker Daniel Quare (1648/9–1724). The tube was sealed into a boxwood cistern with a leather base; a movable plate driven by a screw pressed up on the bag until the mercury filled the tube, after which the instrument could be safely transported. Quare saw this as a means of making domestic barometers in London for sale to provincial customers, but this eminently practical device enabled the subsequent development of mountain and marine barometers.

Mountain and Marine Barometers

The first such measurement in England was probably that made in 1653 by Henry Power, a physician of Halifax, Yorkshire, who reported that the mercury reached only 26 inches at the summit of his local hill. Robert Boyle recognized that, as the mercury fell, even when ascending a church steeple, so it would rise if the barometer was taken down into a mine. In 1672, this observation was confirmed by George Sinclair, a Scottish mining surveyor.

In the early days, explorers and surveyors carried their glass tube, bowl, leather bag of mercury, and graduated rule, and assembled the barometer for each observation, a practice that extended into the eighteenth century, when French academicians sought to measure altitudes of the high Andean peaks, the highest mountains then known. The mathematical formula for the relationship between the altitude and height of mercury was difficult to establish, and astronomer Edmund Halley's 1685 proposal was only the first step on a complex path.

Although the portable domestic barometer became available in the late seventeenth century, the Genevan scientist Jean-André de Luc (1727–1818) was the first to design, around 1750, a robust apparatus consisting of a siphon tube, with thermometers and a plumb-bob, neatly packed in a wooden case. A scale was laid alongside both levels of mercury to measure the distance between that in the tube and that in the open arm. After taking the reading, the tube was tilted until mercury filled it; then, by closing an ivory tap in the siphon and draining off the surplus liquid, the instrument could be carried safely to the next station. The Genevan scientist Horace-Bénédict de Saussure (1740–1799) carried a de Luc barometer to the summit of Mont Blanc, Europe's highest mountain, in 1787.

De Luc's siphons were soon replaced by straight-tube barometers fitted with a leather bag and portable screw, the whole being contained in a slender cylindrical case. In the higher mountains so much mercury descended from the tube, raising the level in the cistern, that the scale alongside the tube became inaccurate. Because the level in the cistern was invisible, a float was inserted in the cistern; as its protruding tip rose against a small graduated scale, the true distance between the two levels could be calculated from this reading.

In his Discourse Concerning the Origins and Properties of Wind (1671), Ralph Bohun (1639–1716) called for the use of a barometer to predict hurricanes, particularly at sea. On board a moving ship, however, the mercury oscillated in the tube and, on occasion, struck the top of the tube and broke the glass. Numerous ineffective designs were proposed in France and England before the London instrument maker Edward Nairne (1725–1806) produced a tube whose central section was constricted to one-twentieth of an inch in diameter. This kept the mercury steady. The barometer, suspended in gimbals, performed satisfactorily on James Cook's second voyage of 1772–1775 and provided the model for marine barometers thereafter.

Meteorology

The height of the mercury column was soon recognized as related to changes in the weather, but the first experimenters were surprised that the mercury fell on rainy days, when they supposed that the water-laden atmosphere was heavier. Soon, however, the correlation between high mercury and fine weather, and between falling or low mercury and rain, encouraged makers to add "Fair," "Changeable," and "Storm" to their scales. Because the mercury expanded and contracted with temperature, small thermometers were put on the frame to correct for this effect.

The barograph, or self-recording barometer, made a late appearance on the architect Sir Christopher Wren's somewhat improbable "Weather Clock." Constructed in 1663, it consisted of several instruments, each of which registered by impressions on a paper chart moved by clockwork. Hooke added a barograph prior to 1681; from the description, he appears to have caused the pulley of a wheel barometer to make similar impressions on the chart. In 1765, the clockmaker Alexander Cumming (1733–1814) constructed a large and elegant continuously recording barograph for King George III (ruled 1760–1820); it was a siphon barometer, the float supporting a light frame carrying a pencil that marked a rotating circular chart. Within a few years similar instruments were being made in France.

Bibliography

Archinard, Margarida. De Luc et la recherche barometrique. Geneva, 1980.

Golinski, Jan. "Barometers of Change: Meteorological Instruments as Machines of Enlightenment." In The Sciences in Enlightened Europe, edited by William Clark, Jan Golinski, and Simon Schaffer (Chapter 3; pp. 69–93). Chicago, 1999.

Mc Connell, Anita. "Origins of the Marine Barometer." Annals of Science. Forthcoming.

Middleton, W. E. Knowles. The Experimenters: A Study of the Accademia del Cimento. Baltimore and London, 1971.

——. The History of the Barometer. Baltimore, 1964; reprint, Trowbridge, U.K., 1994.

—ANITA MCCONNELL

An instrument that measures atmospheric pressure.

Simple and reliable, it’s your best weather forecaster
The simplest, most reliable aid to forecasting the local weather is your barometer. Along with a magnetic compass, it’s the most indispensable of boating instruments.The kind of barometer used on a boat consists of a metal box with a pleated, flexible top. It’s called an aneroid barometer, from the Greek a-, meaning no or not, and neros, meaning wet. That distinguishes it from a mercury barometer, which will not function properly on a bouncy boat.Air is removed from the aneroid box during manufacture, so the position of the top rises and falls according to the pressure of the atmosphere. That’s precisely what you need to know— the pressure of the atmosphere now and several hours, or even days, ago.The accuracy of the readings is not important unless you are sharing your observations with others—say, in a radio net at sea—for plotting. Otherwise, what you need to know is whether the air pressure is rising or falling and how quickly. An old-fashioned recording barograph, with a graph-paper roll wound by clockwork and a moving pen tracing an inked line, provides a clear picture of pressure changes in graphic form. More modern digital barographs provide the same information on the screen of a small handheld instrument powered by batteries.If you don’t have a barograph, you can make do with graph paper. Simply plot the barometric pressure every 4 hours and join the points with a pencil line.In the middle latitudes, a high barometer indicates an air pressure of about 30.50 inches of mercury, or 1,033 millibars (mb). A low barometer reads 29.50 inches, or 999 mb. The average reading at sea level is 29.9 inches (1,013 mb). For reference purposes, 3.4 mb equal 1/10 inch.Here’s what changes in pressure portend:

• steady, persistent decrease in pressure: foul weather is on the way

• steady, persistent increase in pressure: the weather will improve

• pressure remains the same: present conditions likely to continue

• sudden rise or sudden fall: unsettled weather to come

• pressure rises between 4 A.M. and 10 A.M., and between 4 P.M. and 10 P.M.

• pressure falls between 10 A.M. and 4 P.M., and between 10 P.M. and 4 A.M.

n.

An ingenious instrument which indicates what kind of weather we are having.

Schematic drawing of a simple mercury barometer with vertical mercury column and reservoir at base

Goethe's device

A barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.

Contents 1 History 2 Types 2.1 Water-based barometers 2.2 Mercury barometers 2.3 Aneroid barometers 2.4 Barographs 3 Applications 4 Compensations 4.1 Temperature 4.2 Altitude 5 Patents 6 References 7 Further reading 8 See also 9 External links

History

Although Evangelista Torricelli^[1][2][3] is universally credited with inventing the barometer in 1643, two other noteworthy efforts must be cited. Historical documentation also suggests Gasparo Berti, an Italian mathematician and astronomer, built unintentionally water barometer sometime between 1640 and 1643.^[1][4] French scientist and philosopher Rene Descartes described the design of an experiment on atmospheric pressure determination as early as 1631, but there is no evidence that he built a working barometer at that time.^[1]

Types

Water-based barometers

The concept that 'decreasing atmospheric pressure predicts stormy weather' was postulated by Lucien Vidie -- and it is the basis for a weather prediction device called a 'storm glass' or 'Goethe barometer' (who popularized it in Germany). It consists of a glass container with a sealed body, half filled with water. A narrow spout connects to the body below the water level and rises above the water level, where it is open to the atmosphere. When the air pressure is lower than it was at the time the body was sealed, the water level in the spout will rise above the water level in the body; when the air pressure is higher, the water level in the spout will drop below the water level in the body. A variation of this type of barometer can be easily made at home.^[5]

Mercury barometers

A mercury barometer has a glass tube of at least 33 inches (about 84 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury actually creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column 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 at 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.

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 atmosphere^[6]. He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".

The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters (torr): the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is equivalent to about 29.9 inches, or 760 millimeters, of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type normally measure atmospheric pressures between 28 and 31 inches 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. Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes.

On June 5, 2007, a European Union directive was enacted to restrict the sale of mercury, thus effectively ending the production of new mercury barometers in Europe.

Aneroid barometers

See also: Barograph

Old aneroid barometer

Modern aneroid barometer

An aneroid barometer uses a small, flexible metal box called an aneroid cell. This aneroid capsule (cell) is made from an alloy of beryllium and copper.^[7] The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. In addition, the mechanism is made deliberately 'stiff' so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves.

Barographs

A barograph, which records a graph of some atmospheric pressure, uses an aneroid barometer mechanism to move a needle on a smoked foil or to move a pen upon paper, both of which are attached to a drum moved by clockwork.^[8]

Applications

See also: Surface weather analysis

See also: Weather forecasting

Barograph using five stacked aneroid barometer cells.

A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. When used in combination with wind observations, reasonably accurate short term forecasts can be made.^[9] Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, gives a contour map showing areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activities. If the barometer is falling then deteriorating weather or some form of precipitation will fall, however if the barometer is rising then there will be nice weather or no precipitation.

Compensations

Temperature

The density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument. Temperature compensation of an aneroid barometer is accomplished by including a bi-metal element in the mechanical linkages. Aneroid barometers sold for domestic use seldom go to the trouble.

Altitude

As the air pressure will be decreased at altitudes above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is then converted to an equivalent sea-level pressure for purposes of reporting and for adjusting aircraft altimeters (as aircraft may fly between regions of varying normalized atmospheric pressure owing to the presence of weather systems). Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.

Patents

Table of Pneumaticks, 1728 Cyclopaedia

• US patent 2194624, , "Diaphragm pressure gauge having temperature compensating means", granted 1940-03-26 , assigned to Bendix Aviat Corp 
• U.S. Patent 2,472,735 : C. J. Ulrich : "Barometric instrument"
• U.S. Patent 2,691,305 : H. J. Frank : Barometric altimeter"
• U.S. Patent 3,273,398 : D. C. W. T. Sharp : "Aneroid barometer"
• U.S. Patent 3,397,578 : H. A. Klumb : "Motion amplifying mechanism for pressure responsive instrument movement"
• U.S. Patent 3,643,510 : F. Lissau : "Fluid displacement pressure gauges"
• U.S. Patent 4,106,342 : O. S. Sormunen : "Pressure measuring instrument"
• U.S. Patent 4,238,958 : H. Dostmann : "Barometer"
• U.S. Patent 4,327,583 : T. Fijimoto : "Weather forecasting device"

References

Wikimedia Commons has media related to: Barometer

1. ^ ^{a b c} The Invention of the Barometer
2. ^ History of the Barometer
3. ^ Evangelista Torricelli, The Invention of the Barometer
4. ^ Drake, Stillman (1970). "Berti, Gasparo". Dictionary of Scientific Biography. 2. New York: Charles Scribner's Sons. pp. 83-84. ISBN 0684101149. 
5. ^ JetStream. Learning Lesson: Measure the Pressure - The "Wet" Barometer. Retrieved on 2007-05-05.
6. ^ The Invention of the Barometer
7. ^ Enotes.com. How Products Are Made: Aneroid Barometer. Retrieved on 2007-05-05.
8. ^ Glossary of Meteorology. Barograph. Retrieved on 2007-05-05.
9. ^ USA Today. Using winds and a barometer to make forecasts. Retrieved on 2007-05-05.

Further reading

• Middleton, W.E. Knowles. (1964). The history of the barometer. Baltimore: Johns Hopkins Press. New edition (2002), ISBN 0801871549.

See also

• Automated airport weather station
• Barograph
• Bert Bolle’s world record water barometer
• Microbarometer
• Robert FitzRoy's Meteorology
• Storm glass
• Surface weather analysis
• Weather forecasting

External links

Look up barometer in Wiktionary, the free dictionary.

• The largest barometer in the world, Bert Bolle's Water Barometer in the Barometer Tower in the Denmark Visitor Centre, Western Australia
• Various Barometer Designs
• Early use of barometers on surveys
• Wheel and stick barometer images

v • d • e Earth-based meteorological equipment and instrumentation Anemometer · Barograph · Barometer · Ceiling balloon · Ceiling projector · Ceilometer · Dark adaptor goggles · Disdrometer · Field mill · Hygrometer · Ice Accretion Indicator · LIDAR · Lightning detector · Nephelometer · Nephoscope · Pan evaporation · Pyranometer · Radiosonde · Rain gauge · Snow gauge · SODAR · Solarimeter · Sounding rocket · Stevenson screen · Sunshine recorders · Thermograph · Thermometer · Weather balloon · Weather radar · Weather vane · Windsock · Wind profiler

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - barometer, højtryksmåler

Nederlands (Dutch)
barometer

Français (French)
n. - baromètre

Deutsch (German)
n. - Barometer

Ελληνική (Greek)
n. - βαρόμετρο

Italiano (Italian)
barometro

Português (Portuguese)
n. - barômetro (m)

Русский (Russian)
барометр

Español (Spanish)
n. - barómetro

Svenska (Swedish)
n. - barometer

中文（简体）(Chinese (Simplified))
气压计, 晴雨表

中文（繁體）(Chinese (Traditional))
n. - 氣壓計, 晴雨表

한국어 (Korean)
n. - 기압계, 표준

日本語 (Japanese)
n. - 気圧計, 指標

العربيه (Arabic)
‏(الاسم) مقياس, الضغط الجوي, مقياس, تغير ( آراء أو أسعار ألخ)‏

עברית (Hebrew)
n. - ‮ברומטר, מד-כובד‬

If you are unable to view some languages clearly, click here.

To select your translation preferences click here.