Barometer

History

Origins

At the time of Galileo, towards 1635, the engineers and fountain-makers of Florence are charged to build gigantic hydraulic installations in the gardens of the palates. They install suction Pompe S but discover with amazement that they are unable to raise the water of more than 18 brews S, that is to say ten meters. Galileo is solicited but he dies in 1642 without to have had time to solve this problem: why can't one aspire water beyond certain height? One found later, in his notes, which he had thought that the air was to have a weight but he had not drawn any conclusion from it. The idea that the liquid is not aspired by the pump but is not driven back towards it by the effect of an external pressure was in total contradiction with the dogmas admitted at that time, which wanted that water rises in the tubes because nature detests the vacuum .

The arrival of mercury

Torricelli succeeds Galileo as physicist at the court of the Duke of Tuscany. Taking again the notes of its predecessor, it makes experiments to prove that the atmospheric pressure is responsible for the rise of water in an empty space. To avoid using columns of Water of ten Meter S height, it with the idea to carry out tests with mercury (quicksilver, quicksilver…) who is 13,6 times more dense. It fills a length tube of it of Verre, stops it with the finger and turns over it on a basin filled, him also, of mercury. It observes that the tube is emptied only partially in the basin and that there remains there always a mercury column of approximately 76 cm height, whatever the depression of the tube in the basin. It from of deduced that the Pression of the Air on the surface of the basin counterbalances the weight of the mercury column and that it is it which makes it possible to make assemble water in the pumps a height of approximately 10 m, but not more. Thus Torricelli invents the barometer in 1643. He also notices that the height of mercury in the tube varies during the Temps (that which runs out) and which a fall generally precedes a period bad time (rain).

The open tank is however not very practical if one wants to transport the instrument. Various solutions are imagined, one builds for example porous leather tanks fixed at the tube and containing an minor amount of mercury. Sir Robert Boyle imagines to fold up the barometric tube upwards, which gives the “tube siphon” still used today.

The French physicist Rene Descartes (1596-1650) improves the system of Torricelli by adding a paper graduation. He is the first to put forward the idea that the atmospheric pressure must decrease with altitude.

The cister barometer is directly deduced from the tube of Torricelli. Without adapted device, the precise reading height of the mercury column is not very easy. One thus laid out above the basin a screw with two pointed ends, the inferior just coming tangenter the free face from metal in the basin. Using a Cathetometer, one comes to measure the difference in height between the higher point of the screw and the free face in the tube. The length of the screw, measured once and for all, is added to the indication of the cathetometer and one thus obtains the height of the mercury column.

Blaise Pascal and atmospheric pressure

The atmospheric pressure constrained mercury to be gone up in the tube on a column of approximately 76 cm of height but it is not sufficient to fill the vacuum which is formed in the upper part.

In the years 1640, one of the questions most discussed among the scientists is: does the air have a Poids?

Blaise Pascal, scientist early but also excellent experimenter, has just invented at 22 years a calculating machine. He remakes the experiment of Torricelli and thinks, like Descartes, which if the air has a weight, then mercury must less higher assemble in the tube if the experiment in altitude is made. It is well what it checks, but with a too weak precision, at the top of the Tour Saint-Jacob with Paris (52 m). Thanks to his/her brother-in-law who lives with the foot of the Puy de Dôme, the September 19th 1648, it remakes the experiment at various altitudes and notes that indeed, the height of mercury decreases well as one rises. The word “barometer” appears a few years later, created by the Irish physicist and chemist Robert Boyle ( barometer , 1665-1666). It is formed on the Greek baros (weight, gravity). But it will be necessary to await the middle of the 19th century so that the manufacturers of instruments, the opticians, the clock and watch makers, start to produce barometers, with fine scientists initially, then with fine servants. From 1870 the graduations are accompanied by weather indications (“good weather”, “variable”…). The denomination “barometer” is essential in France only after the publication in 1676 of the Essai on the nature of the air by Edme Mariotte.

Later, one will give the name of Pascal (without capital letter) to the international unit of pressure, which is worth a newton per square meter.

The chance can bring to a discovery. In 1675, the Picardy abbot, transporting night a mercury barometer, makes curious discovered. With each abrupt movement of metal, a bluish gleam illuminates the tube. This phenomenon is studied inter alia by a pupil of Robert Boyle, Francis Hauksbee. Naturally, no satisfactory explanation is found at the time but thus begin the first research on the electric shocks in rarefied gases… It is known now that the friction mercury on glass is the cause of this Luminescence.

The mercury barometer

The tube of Torricelli, baptized thereafter barometer, is a tube out of U related to a graduation of reference making it possible to measure the difference in level between the two free faces of mercury. The mercury barometer presents many disadvantages:

the tube of glass is cumbersome and fragile;
mercury is a expensive and toxic metal (nowadays it is prohibited, rightly, for many applications like the clinical thermometers);
the very strong surface Tension of mercury makes its face free convex and makes that in the narrow tubes, the level of mercury is established a little below its theoretical value; it is thus necessary not only to make one tangential aiming but also to correct the value obtained according to the diameter of the tube;
another correction must be practiced according to the temperature, to compensate for the dilation of metal and thus the variation of density which accompanies it, this is why very good barometer is associated with a Thermomètre and the adequate tables of correction.

Although the origin is discussed by it, one allots to the Dutch physicist Christian Huygens an important improvement of the tube of Torricelli, in 1672. A tube out of U contains mercury like previously and a zone of vacuum on the closed side, but the open branch contains a nonvolatile liquid moreover weak density whose level depends on that of mercury. Descartes had already produced apparatuses of this kind. By choosing the sections of the tubes suitably, one can thus obtain an amplification of about 10, which makes the reading much easier and precise. This technique makes it possible moreover to avoid the slow Oxydation mercury by the Oxygène of the air.

The first wheel-barometer was built in 1663 by the English astronomer Robert Hooke. A float resting on mercury follows the variations of the level and actuates a needle which indicates the pressure on a dial. The reading is easier and more precise than with the barometer of Torricelli but, according to Privat-Deschanel and Focillon, “the wheel-barometer is always a rather coarse apparatus, whatever the luxury of its presentation”.

In the siphon barometers built on the model imagined by Louis Joseph Gay-Lussac, the short branch with the same section that the long branch, from which it is separated by a very fine tube intended to prevent the air from penetrating in the vacuum chamber. The opening O lets pass the air but it is sufficiently small to prevent mercury from easily leaving. Bunten added there a reserve of guard CD intended to trap the bubbles of air which could, by accident, to cross the siphon.

French Jean Fortin (1750 - 1831) produced a transportable mercury barometer which bears its name. In order to decrease the volume of mercury in the lower basin and to facilitate the reading, Fortin imagined, in collaboration with the mechanic Ernst, a system of leather screw and membrane making it possible to bring the free face to the level of a fixed reference mark height compared to the tube. A cursor related to this one allows the direct measurement height of the barometric column. One will note the design of the tripod, whose folded up branches constitute protections for the tube of glass.

It is at the 18th century that the first barometers of marine to mercury appeared. Their development was slowed down by the sailors themselves, very attached to the ancestral methods of forecast of the time.

The British admiral Fitzroy had the idea, in 1858, to equip all the fishing ports with a barometer.

Barometers with water

According to a document of 1619, a Dutchman, Gijsbrecht de Donckere, would have invented a barometer with water. The air locked up in part of the apparatus dilates or contracts according to the pressure which it undergoes, producing a relatively important variation of level in the fine tube connected to the free air. Johann Wolfgang von Goethe, about 1792-93, would have reinvented an apparatus of this type, starting from the principles of Torricelli. When the atmospheric pressure increases, the level of the liquid in the tube goes down. Conversely, when the pressure drops, there is less of support on water and the liquid goes up.

The indications of the barometers with water are obviously very related to the temperature, and one does not serve any more these apparatuses but at decorative ends.

Barometers with gas

The Eco-Celli barometer is an instrument whose precision can be compared with that of a barometer of Torricelli. Its operation is completely different since it does not contain mercury. Like the barometers with water, this instrument measures the atmospheric pressure thanks to the compressibility of a volume of Gaz locked up which is compressed or slackens according to the atmospheric pressure. The volume of gas also depends on the room temperature and it is thus necessary to make a correction. This one is carried out very simply by moving the scale of a cursor until the metal index is on the same level as the blue liquid of the thermometer. Compared to a simple mercury barometer, the Eco-Celli barometer allows an amplification of 4 times, which makes the reading more precise and especially easier.

The barometer invented by the Britannique Alexandre Adie in 1818 is definitely smaller than a barometer of Torricelli. It is composed of two elements, a U-shaped tube (red liquid) and a thermometer (blue liquid) which are put in parallel. A decrease in pressure makes assemble the red liquid of the barometer and a rise reduces it. The thermometer makes it possible to make the corrections necessary.

Aneroid barometers

The aneroid barometer was developed by the French Lucien Vidie who deposited the patent in 1844 of it (in collaboration with Antoine Redier, inventor of the Réveille-matin). The walls of a air space capsule, known as “Capsule of Vidie” are maintained drawn aside by a spring. The atmospheric pressure presses more or less on the box (capsule) aneroid and thus makes turn the needle on the dial, thanks to a mechanism of precision.

The idea was taken up by Eugene Bourdon in 1849 which used the deformation that a air space flattened tube under the effect of the variations of the external pressure undergoes. “This pretty barometer of cabinet could not replace the mercury barometer in the observations of precision: but, associated with this barometer, it can render great services in the scientific excursions” (Privat-Deschanel and Focillon).

The principle of this apparatus had been proposed in 1700 by the German scientist Gottfried Wilhelm Leibniz; the great merit of Vidie was to transform it into a practical and not very expensive object. The aneroid barometer is less precise than the mercury barometer but it n the other hand makes it possible to manufacture compact instruments, much more robust and easily transportable, especially at sea.

Barographs

The oldest system of recording barometer was invented by the English Moreland in 1670 but it is the capsule of Vidie which is the “engine” of the majority of the current apparatuses. To obtain a more important displacement and efforts one uses a stacking of capsules, generally five. The recording barometers are still called barographs. Much is presented like objects “luxury” in a box glazed to the amounts mahogany tree or of another invaluable wood but there exist also models much more rustic. In the more recent barographs, the capsule is replaced by a piezoresistive sensor and the drum by a screen LCD.

Recent evolutions

In 1989, Casio put on the market the first wrist watch provided with a function barometer, inaugurating a series of multifunction watches intended to the hikers (with Altimètre) and for the plungers (with Manomètre).

Scientific data on the atmospheric pressure

The atmospheric pressure can be expressed in millimetres of mercury (mm Hg) or by using the usual units of pressure. Rather than the millibar (mb), it is to better use the hectopascal (hPa); these two units are strictly equivalent but the second is a multiple of the legal unit.

The pressure decreases when one rises, not in a linear way, but less and less quickly. It also depends on the profile of temperature which reigns above the place where it is measured. In the meteorological observations, one generally indicates two values: pressure at the station, measured in situ by a barometer well calibrated, and the pressure reduced to the sea level or PNM , i.e. that which would reign theoretically, at the same place, with altitude zero of reference (the sea level is not very easy to define…).

The formula below makes it possible to evaluate the reduced pressure. It was established for an atmospheric temperature of 288 Kelvins, that is to say 15° Celsius. If the temperature is appreciably different, the reduction will comprise a considerable error. See on this subject the article on the Atmospheric pressure.

$p_ {red} = p_ {ABS} - 1013,25 \ left 1 - \ left (\ frac {288-0,0065 H} {288} \ right) ^ {5,255} \ right$

p_abs = absolute pressure

p_red = pressure reduced to the sea level

H = altitude above the sea level

It is always useful to have orders of magnitude. At low altitude, if one goes up 10 m, the pressure lowers approximately 1,25 hPa.

A barometer, whatever it is, always gives the pressure which corresponds to altitude where it is. The atmospheric pressure given by the weather stations is always brought back to the sea level, in order to have a point of reference.

Pressure reduced to the sea level, or PNM, is calculated thanks to the following formula:

$p_ {1} = p_ {2} .e ^ {\ frac {G Z_2} {RT}}$

p₁ = pressure reduced to the sea level

p₂ = pressure of the station in hPa

z₂ = altitude of the station in meter

T = (T₂ + T₁)/2 in Kelvin

T₁ = 288,15 - 0,0016 Z₂ average temperature with the sea level adjusted with altitude The temperature of the atmosphere decreases by 6,5°C by km or 0,0065°C/m

T₂ = average temperature of the station on 12:00 in Kelvin or (T_max+T_min) /2

G = 9,80665 acceleration due to gravity

R = 287,08 constant of the dry air R = R*/My R* = constant of perfect gases = 8,314 J K^-1 mole^-1

My = molecular mass of the dry air = 28,9644 G mole^-1

E = 2,71828…

Approximately, at low altitude, the pressure decreases by 1hPa when one goes up 8,3 m or increases 1 hPa when one goes down from 8,3 Mr.

Is the barometer an instrument of forecast of time?

At a given place, the indication given by a barometer varies continuously, in a very fast way under the action of the wind, especially if it blows in gusts, but also later on (a few minutes, a few hours or in a daily way) under the effect of other causes related to various weather or climatic phenomena.

It is generally not possible to make a very good forecast starting from a simple reading of barometer in a given place. However, it is good to know that the approach of a depression or a barometric Creux results in a tendency of pressure to the fall over one period of about 3 to 12 noon. The value and the speed of the decrease in pressure are valid indicators of the intensity of the atmospheric disturbance which approaches.

In the absence of modern weather forecasting, or in supplement of those, an advised observer can manage to make a short-term forecast of a certain value by taking account of local climatology, the winds, the clouds and the tendency of pressure.

The role of the barometer in the history of meteorology

Although several other measuring instruments (Thermomètre, Hygromètre, Anémomètre, Girouette, to name only them) had a role to play in the scientific genesis of meteorology, it is clear that the barometer is of very special importance. The barometer measures a mechanical property of the atmosphere, the pressure, which, contrary to the wind, the temperature, or even to moisture, generally escapes our directions. As of its invention, the scientists suspected the importance of the pressure like weather parameter, but driving progress with a real comprehension was slow. One sometimes gave to the reading of the barometer an importance badly placed, based on empirical observations of an exactitude which nowadays appears debatable.

Indeed, until the beginning of the 20th century, atmospheric mechanics was still badly included/understood. The Jet-stream, for example, is remained primarily unsuspected until in the years 1940. It is during this time of first half of the century that researchers such as Vilhelm Bjerknes and Carl-Gustav Arvid Rossby gave to meteorology with large scales the conceptual framework that one knows to him today, founded on a solid formalism of mathematical physics. It is that it was difficult, before the multiplication of the bonds of communications, to measure the state of the atmosphere on a scale comparable with that of the weather great systems. The scientists of the 19th century were thus generally reduced by it to try to empirically connect the local fluctuations in pressure with the character of the time and the Vent.

Thus, in 1883, Privat-Deschanel and Focillon give the following indications:

in Paris, the barometer is generally with highest when the wind blows of and with low if it blows of the S, the directions changing somewhat according to the seasons. The variations of atmospheric pressure are not related directly cold and to the rain but as that one is rather related to the wind of the NR and this one with the wind of the S or SO, the observation of the barometer makes it possible to envisage them with a relatively good reliability.
in Pétersbourg (ex Petrograd, then Leningrad, then St-Pétersbourg) it rains indifferently by all the winds, the indications of the barometer are without value.
the great storms are preceded by all the more large lowerings of pressure as one is more far from the equator. At the time of the hurricane which devastated part of Europe, in February 1783, the barometer had dropped abruptly by 0,031 m (heights of mercury) in England, from 0,018 to 0,030 m in France and Germany, of 0,007 m only in Rome.
in the intertropical areas, a variation from 0,001 to 0,002 m is enough to predict a violent one hurricane.
and, notices with good sense: The farmers who may find it beneficial the most to envisage the changes of time, often acquire a great intelligence of the weather signs, and the barometer the horn much less often than the inhabitants of the cities.

These remarks contain some elements of truth, but are not supported by a sufficient comprehension of the subjacent mechanisms. For example, it is correct to say that the great storms are preceded by a decrease in pressure but the relationship to the equator is only one observation, misunderstood, and finally incorrect in the light of current knowledge.
Nowadays, the barometer preserves a fundamental importance among a growing battery of instruments. Measurements of pressure, speed of the wind, of temperature and moisture taken on the surface and in altitude are communicated everywhere in the world. These measurements taken in-situ of course have a great intrinsic value for the observation weather but this value is multiplied when it is considered that they are also used with the calibration and the validation as remote measuring instruments which operate starting from satellites, of planes or terrestrial surface. The barometer thus plays a fundamental role in the explosion in the course of the volume of the data of observation of the Earth by remote measurement.

How to measure the height of a building with a barometer?

A famous history tells various manners of measuring the height of a building with a barometer: while making use like masses of it for a plumbline or as a pendulum which one would measure the Eigen frequency, as masses to measure the drop time, like goods to bribe the guard of the building… The “awaited answer” (measurement of the difference in pressure between bottom and the top) being quoted only in the last.

This history in fact would have been published in the Reader' S Digest in 1958 and it would have been transformed with the wire of time into an anecdote presumedly real and allotted to Niels Bohr, becoming thus a modern legend. One can wonder whether the recourse to this famous person is not a manner of transforming an anecdote amusing into a lampoon against the “rigidity of secondary education” opposed to the “creativity”.

See too

Hydrostatic
Atmospheric pressure, variation of the pressure according to altitude

External bonds

Glassmaking of art of Soisy/School: *Le site South-eastern Weather: : The Weather Page of the South-east of Jean-Marie Muggianu (barometers): *Humor: The levelling of a building with a barometer: (many pages Internet, of which)

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