Barometer
BAROMETER (from Gr. "baros", pressure, and "metron", measure), an instrument by which the weight or pressure of the atmosphere is measured. The ordinary or mercurial barometer consists of a tube about 36 in. long, hermetically closed at the upper end and containing mercury. In the "cistern barometer" the tube is placed with its open end in a basin of mercury, and the atmospheric pressure is measured by the difference of the heights of the mercury in the tube and the cistern. In the "siphon barometer" the cistern is dispensed with, the tube being bent round upon itself at its lower end; the reading is taken of the difference in the levels of the mercury in the two limbs. The "aneroid" barometer (from the Gr. "nêros", wet) employs no liquid, but depends upon the changes in volume experienced by an exhausted metallic chamber under varying pressures. "Baroscopes" simply indicate variations in the atmospheric pressure, without supplying quantitative data. "Barographs" are barometers which automatically record any variations in pressure.
Philosophers prior to Galileo had endeavoured to explain the action of a suction pump by postulating a principle that "Nature abhorred a vacuum." When Galileo observed that a common suction pump could not raise water to a greater height than about 32 ft. he considered that the "abhorrence" was limited to 32 ft., and commended the matter to the attention of his pupil Evangelista Torricelli. Torricelli perceived a ready explanation of the observed phenomenon if only it could be proved that the atmosphere had weight, and the pressure which it exerted was equal to that of a 32-ft. column of water. He proved this to be the correct explanation by reasoning as follows: - If the atmosphere supports 32 feet of water, then it should also support a column of about 2 ft. of mercury, for this liquid is about 13 times heavier than water. This he proved in the following manner. He selected a glass tube about a quarter of an inch in diameter and 4 ft. long, and hermetically sealed one of its ends; he then filled it with mercury and, applying his finger to the open end, inverted it in a basin containing mercury. The mercury instantly sank to nearly 30 in. above the surface of the mercury in the basin, leaving in the top of the tube an apparent vacuum, which is now called the Torricellian vacuum; this experiment is sometimes known as the Torricellian experiment. Torricelli's views rapidly gained ground, notwithstanding the objections of certain philosophers. Valuable confirmation was afforded by the variation of the barometric column at different elevations. René Descartes and Blaise Pascal predicted a fall in the height when the barometer was carried to the top of a mountain, since, the pressure of the atmosphere being diminished, it necessarily followed that the column of mercury sustained by the atmosphere would be diminished also. This was experimentally observed by Pascal's brother-in-law, Florin Périer (1605-1672), who measured the height of the mercury column at various altitudes on the Puy de Dôme. Pascal himself tried the experiment at several towers in Paris, - Notre Dame, St Jacques de la Boucherie, etc. The results of his researches were embodied in his treatises De l'équilibre des liqueurs and De la pesanteur de la masse d'air, which were written before 1651, but were not published till 1663 after his death. Corroboration was also afforded by Marin Mersenne and Christiaan Huygens. It was not long before it was discovered that the height of the column varied at the same place, and that a rise or fall was accompanied by meteorological changes. The instrument thus came to be used as a means of predicting the weather, and it was frequently known as the weather-glass. The relation of the barometric pressure to the weather is mentioned by Robert Boyle, who expressed the opinion that it is exceedingly difficult to draw any correct conclusions. Edmund Halley, Leibnitz, Jean André Deluc (1727-1817) and many others investigated this subject, giving rules for predicting the weather and attempting explanations for the phenomena. Since the height of the barometric column varies with the elevation of the station at which it is observed, it follows that observations of the barometer afford a means for measuring altitudes. The early experiments of Pascal were developed by Edmund Halley, Edme Mariotte, J. Cassini, D. Bernoulli, and more especially by Deluc in his Recherches sur les modifications de l'atmosphère (1772), which contains a full account of the early history of the barometer and its applications. More highly mathematical investigations have been given by Laplace, and also by Richard Ruhlmann (Barometrischen Hohenmessung., Leipzig, 1870). The modern aspects of the relation between atmospheric pressure and the weather and altitudes are treated in the article Meteorology.
Many attempts have been made by which the variation in the height of the mercury column could be magnified, and so more exact measurements taken. It is not possible to enumerate in this article the many devices which have been proposed; and the reader is referred to Charles Hutton's Mathematical and Philosophical Dictionary (1815), William Ellis's paper on the history of the barometer in the Quarterly Journal of the Royal Meteorological Society, vol. xii. (1886), and E. Gerland and F. Traumüller's Geschichte der physikalischen Experimentierkunst (1899). Descartes suggested a method which Huygens put into practice. The barometer tube was expanded into a cylindrical vessel at the top, and into this chamber a fine tube partly filled with water was inserted. A slight motion of the mercury occasioned a larger displacement of the water, and hence the changes in the barometric pressure were more readily detected and estimated. But the instrument failed as all water-barometers do, for the gases dissolved in the water coupled with its high vapour tension destroy its efficacy. The substitution of methyl salicylate for the water has been attended with success. Its low vapour tension (Sir William Ramsay and Sydney Young give no value below 70° C.), its low specific gravity (1.18 at 10° C.), its freedom from viscosity, have contributed to its successful use. In the form patented by C. O. Bartrum it is claimed that readings to .001 of an inch of mercury can be taken without the use of a vernier.
The diagonal barometer, in which the upper part of the tube is inclined to the lower part, was suggested by Bernardo Ramazzini (1633-1714), and also by Sir Samuel Morland (or Moreland). This form has many defects, and even when the tube is bent through 45° the readings are only increased in the ratio of 7 to 5. The wheel barometer of Dr R. Hooke, and the steel-yard barometer, endeavour to magnify the oscillation of the mercury column by means of a float resting on the surface of the mercury in the cistern; the motion of the float due to any alteration in the level of the mercury being rendered apparent by a change in the position of the wheel or steel-yard. The pendant barometer of G. Amontons, invented in 1695, consists of a funnel-shaped tube, which is hung vertically with the wide end downwards and closed in at the upper end. The tube contains mercury which adjusts itself in the tube so that the length of the column balances the atmospheric pressure. The instability of this instrument is obvious, for any jar would cause the mercury to leave the tube.
The Siphon Barometer (fig. 1) consists of a tube bent in the form of a siphon, and is of the same diameter throughout. A graduated scale passes along the whole length of the tube, and the height of the barometer is ascertained by taking the difference of the readings of the upper and lower limbs respectively. This instrument may also be read by bringing the zero-point of the graduated scale to the level of the surface of the lower limb by means of a screw, and reading off the height at once from the surface of the upper limb. This barometer requires no correction for errors of capillarity or capacity. Since, however, impurities are contracted by the mercury in the lower limb, which is usually in open contact with the air, the satisfactory working of the instrument comes soon to be seriously interfered with.
Fig. 2 shows the Cistern Barometer in its essential and simplest form. This barometer is subject to two kinds of error, the one arising from capillarity, and the other from changes in the level of the surface of the cistern as the mercury rises and falls in the tube, the latter being technically called the error of capacity. If a glass tube of small bore be plunged into a vessel containing mercury, it will be observed that the level of the mercury in the tube is not in the line of that of the mercury in the vessel, but somewhat below it, and that the surface is convex. The capillary depression is inversely proportional to the diameter of the tube. In standard barometers, the tube is about an inch in diameter, and the error due to capillarity is less than .001 of an inch. Since capillarity depresses the height of the column, cistern barometers require an addition to be made to the observed height, in order to give the true pressure, the amount depending, of course, on the diameter of the tube.
The error of capacity arises in this way. The height of the barometer is the perpendicular distance between the surface of the mercury in the cistern and the upper surface of the mercurial column. Now, when the barometer falls from 30 to 29 inches, an inch of mercury must flow out of the tube and pass into the cistern, thus raising the cistern level; and, on the other hand, when the barometer rises, mercury must flow out of the cistern into the tube, thus lowering the level of the mercury in the cistern. Since the scales of barometers are usually engraved on their brass cases, which are fixed (and, consequently, the zero-point from which the scale is graduated is also fixed), it follows that, from the incessant changes in the level of the cistern, the readings would be sometimes too high and sometimes too low, if no provision were made against this source of error.
A simple way of correcting the error of capacity is - to ascertain (1) the neutral point of the instrument, or that height at which the zero of the scale is exactly at the height of the surface of the cistern, and (2) the rate of error as the barometer rises or falls above this point, and then apply a correction proportional to this rate. The instrument in which the error of capacity is satisfactorily (indeed, entirely) got rid of is Fortin's Barometer. Fig. 3 shows how this is effected. The upper part of the cistern is formed of a glass cylinder, through which the level of the mercury may be seen. The bottom is made like a bag, of flexible leather, against which a screw works. At the top of the interior of the cistern is a small piece of ivory, the point of which coincides with the zero of the scale. By means of the screw, which acts on the flexible cistern bottom, the level of the mercury can be raised or depressed so as to bring the ivory point exactly to the surface of the mercury in the cistern. In some barometers the cistern is fixed, and the ivory point is brought to the level of the mercury in the cistern by raising or depressing the scale.
In constructing the best barometers three materials are employed, viz.: - (1) brass, for the case, on which the scale is engraved; (2) glass, for the tube containing the mercury; and (3) the mercury itself. It is evident that if the coefficient of expansion of mercury and brass were the same, the height of the mercury as indicated by the brass scale would be the true height of the mercurial column. But this is not the case, the coefficient of expansion for mercury being considerably greater than that for brass. The result is that if a barometer stand at 30 in. when the temperature of the whole instrument, mercury and brass, is 32°, it will no longer stand at 30 in. if the temperature be raised to 69°; in fact, it will then stand at 30.1 in. This increase in the height of the column by the tenth of an inch is not due to any increase of pressure, but altogether to the greater expansion of the mercury at the higher temperature, as compared with the expansion of the brass case with the engraved scale by which the height is measured. In order, therefore, to compare with each other with exactness barometric observations made at different temperatures, it is necessary to reduce them to the heights at which they would stand at some uniform temperature. The temperature to which such observations are reduced is 32° Fahr. or 0° cent.
If English units be used (Fahrenheit degrees and inches), this correction is given by the formula
x = -H .09T - 2.56 ,
1000
in the centigrade-centimetre system the correction is .0001614 HT (H being the observed height and T the observed temperature). Devices have been invented which determine these corrections mechanically, and hence obviate the necessity of applying the above formula, or of referring to tables in which these corrections for any height of the column and any temperature are given.
The standard temperature of the English yard being 62° and not 32°, it will be found in working out the corrections from the above formula that the temperature of no correction is not 32° but 28.5°. If the scale be engraved on the glass tube, or if the instrument be furnished with a glass scale or with a wooden scale, different corrections are required. These may be worked out from the above formula by substituting for the coefficient of the expansion of brass that of glass, which is assumed to be 0.00000498, or that of wood, which is assumed to be 0. Wood, however, should not be used, its expansion with temperature being unsteady, as well as uncertain.
If the brass scale be attached to a wooden frame and be free to move up and down the frame, as is the case with many siphon barometers, the corrections for brass scales are to be used, since the zero-point of the scale is brought to the level of the lower limb; but if the brass scale be fixed to a wooden frame, the corrections for brass scales are only applicable provided the zero of the scale be fixed at (or nearly at) the zero line of the column, and be free to expand upwards. In siphon barometers, with which an observation is made from two readings on the scale, the scale must be free to expand in one direction. Again, if only the upper part of the scale, say from 27 to 31 in., be screwed to a wooden frame, it is evident that not the corrections for brass scales, but those for wooden scales must be used. No account need be taken of the expansion of the glass tube containing the mercury, it being evident that no correction for this expansion is required in the case of any barometer the height of which is measured from the surface of the mercury in the cistern.
In fixing a barometer for observation, it is indispensable that it be hung in a perpendicular position, seeing that it is the perpendicular distance between the surface of the mercury in the cistern and the top of the column which is the true height of the barometer. The surface of the mercury column is convex, and in noting the height of the barometer, it is not the chord of the curve, but its tangent which is taken. This is done by setting the straight lower edge of the vernier, an appendage with which the barometer is furnished, as a tangent to the curve. The vernier is made to slide up and down the scale, and by it the height of the barometer may be read true to 0.002 or even to 0.001 in.
It is essential that the barometer is at the temperature shown by the attached thermometer. No observation can be regarded as good if the thermometer indicates a temperature differing from that of the whole instrument by more than a degree. For every degree of temperature the attached thermometer differs from the barometer, the observation will be faulty to the extent of about 0.003 in., which in discussions of diurnal range, etc., is a serious amount.
Before being used, barometers should be thoroughly examined as to the state of the mercury, the size of cistern (so as to admit of low readings), and their agreement with some known standard instrument at different points of the scale. The pressure of the atmosphere is not expressed by the weight of the mercury sustained in the tube by it, but by the perpendicular height of the column. Thus, when the height of the column is 30 in., it is not said that the atmospheric pressure is 14.7 lb on the square inch, or the weight of the mercury filling a tube at that height whose transverse section equals a square inch, but that it is 30 in., meaning that the pressure will sustain a column of mercury of that height.
It is essential in gasometry to fix upon some standard pressure to which all measurements can be reduced. The height of the standard mercury column commonly used is 76 cms. (29.922 in.) of pure mercury at 0°; this is near the average height of the barometer. Since the actual force exerted by the atmosphere varies with the intensity of gravity, and therefore with the position on the earth's surface, a place must be specified in defining the standard pressure. This may be avoided by expressing the force as the pressure in dynes due to a column of mercury, one square centimetre in section, which is supported by the atmosphere. If H cms. be the height at 0°, and g the value of gravity, the pressure is 13.596 Hg dynes (13.596 being the density of mercury). At Greenwich, where g = 981.17, the standard pressure at 0° is 1,013,800 dynes. At Paris the pressure is 1,013,600 dynes. The closeness of this unit to a mega-dyne (a million dynes) has led to the suggestion that a mega-dyne per square centimetre should be adopted as the standard pressure, and it has been adopted by some modern writers on account of its convenience of calculation and independence of locality.
The height of the barometer is expressed in English inches in England and America, but the metric system is used in all scientific work excepting in meteorology. In France and most European countries, the height is given in millimetres, a millimetre being the thousandth part of a metre, which equals 39.37079 English inches. Up to 1869 the barometer was given in half-lines in Russia, which, equalling the twentieth of an English inch, were readily reduced to English inches by dividing by 20. The metric barometric scale is now used in Russia. In a few European countries the French or Paris line, equalling 0.088814 in., is sometimes used. The English measure of length being a standard at 62° Fahr., the old French measure at 61.2°, and the metric scale at 32°, it is necessary, before comparing observations made with the three barometers, to reduce them to the same temperature, so as to neutralize the inequalities arising from the expansion of the scales by heat.
The sympiezometer was invented in 1818 by Adie of Edinburgh. It is a revived form of Hooke's marine barometer. It consists of a glass tube, with a small chamber at the top and an open cistern below. The upper part of the tube is filled with air, and the lower part and cistern with glycerin. When atmospheric pressure is increased, the air is compressed by the rising of the fluid; but when it is diminished the fluid falls, and the contained air expands. To correct for the error arising from the increased pressure of the contained air when its temperature varies, a thermometer and sliding-scale are added, so that the instrument may be adjusted to the temperature at each observation. It is a sensitive instrument, and well suited for rough purposes at sea and for travelling, but not for exact observation. It has long been superseded by the Aneroid, which far exceeds it in handiness.
Aneroid Barometer. - Much obscurity surrounds the invention of barometers in which variations in pressure are rendered apparent by the alteration in the volume of an elastic chamber. The credit of the invention is usually given to Lucien Vidie, who patented his instrument in 1845, but similar instruments were in use much earlier. Thus in 1799 Nicolas Jacques Conté (1755-1805), director of the aerostatical school at Meudon, and a man of many parts - a chemist, mechanician and painter, - devised an instrument in which the lid of the metal chamber was supported by internal springs; this instrument was employed during the Egyptian campaign for measuring the altitudes of the war-balloons. Although Vidie patented his device in 1845, the commercial manufacture of aneroids only followed after E. Bourdon's patent of the metallic manometer in 1849, when Bourdon and Richard placed about 10,000 aneroids on the market. The production was stopped by an action taken by Vidie against Bourdon for infringing the former's patent, and in 1858 Vidie obtained 25,000 francs (£1000) damages.
Fig. 4 represents the internal construction, as seen when the face is removed, but with the hand still attached, of an aneroid which differs only slightly from Vidie's form. a is a flat circular metallic box, having its upper and under surfaces corrugated in concentric circles. This box or chamber being partially exhausted of air, through the short tube b, which is subsequently made air-tight by soldering, constitutes a spring, which is affected by every variation of pressure in the external atmosphere, the corrugations on its surface increasing its elasticity. At the centre of the upper surface of the exhausted chamber there is a solid cylindrical projection x, to the top of which the principal lever cde is attached. This lever rests partly on a spiral spring at d; it is also supported by two vertical pins, with perfect freedom of motion. The end e of the lever is attached to a second or small lever f, from which a chain g extends to h, where it works on a drum attached to the axis of the hand, connected with a hair spring at h, changing the motion from vertical to horizontal, and regulating the hand, the attachments of which are made to the metallic plate i. The motion originates in the corrugated elastic box a, the surface of which is depressed or elevated as the weight of the atmosphere is increased or diminished, and this motion is communicated through the levers to the axis of the hand at h. The spiral spring on which the lever rests at d is intended to compensate for the effects of alterations of temperature. The actual movement at the centre of the exhausted box, whence the indications emanate, is very slight, but by the action of the levers is multiplied 657 times at the point of the hand, so that a movement of the 220th part of an inch in the box carries the point of the hand through three inches on the dial. The effect of this combination is to multiply the smallest degrees of atmospheric pressure, so as to render them sensible on the index. Vidie's instrument has been improved by Vaudet and Hulot. Eugène Bourdon's aneroid depends on the same principle. The aneroid requires, however, to be repeatedly compared with a mercurial barometer, being liable to changes from the elasticity of the metal chamber changing, or from changes in the system of levers which work the pointer. Though aneroids are constructed showing great accuracy in their indications, yet none can lay any claim to the exactness of mercurial barometers. The mechanism is liable to get fouled and otherwise go out of order, so that they may change 0.300 in. in a few weeks, or even indicate pressure so inaccurately and so irregularly that no confidence can be placed in them for even a few days, if the means of comparing them with a mercurial barometer be not at hand.
The mercurial barometer can be made self-registering by concentrating the rays from a source of light by a lens, so that they strike the top of the mercurial column, and having a sheet of sensitized paper attached to a frame and placed behind a screen, with a narrow vertical slit in the line of the rays. The mercury being opaque throws a part of the paper in the shade, while above the mercury the rays from the lamp pass unobstructed to the paper. The paper being carried steadily round on a drum at a given rate per hour, the height of the column of mercury is photographed continuously on the paper. From the photograph the height of the barometer at any instant may be taken. The principle of the aneroid barometer has been applied to the construction of barographs. The lever attached to the collapsible chamber terminates in an ink-fed style which records the pressure of the atmosphere on a moving ribbon. In all continuously registering barometers, however, it is necessary, as a check, to make eye-observations with a mercury standard barometer hanging near the registering barometer from four to eight times daily.
See Marvin, Barometers and the Measurement of Atmospheric Pressure (1901); and C. Abbe, Meteorological Apparatus (1888). Reference may also be made to B. Stewart and W. W. H. Gee, Practical Physics (vol. i. 1901), for the construction of standard barometers, their corrections and method of reading.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
