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Barometer

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BAROMETER. The earliest forms of barometer were devel oped from the well known experiment performed in 1643, by Tor ricelli, who, conceiving the idea that the atmosphere had weight, used a mercury column long known, after him, as the "Torricelli tube," for the purpose of demonstrating the existence and magni tude of the atmospheric pressure.

Fundamental Principles.

In the measurement of pressure differences the fundamental idea is to balance this difference against a column of liquid whose weight is known in terms of its height, density and the value of gravity acting upon it. The prin ciple is familiar, and sufficiently well indicated in the diagrams, figs. 1, 2 and 3, in which the weight of the liquid column be tween the levels A and B balances the differences of air pressures on the two liquid surfaces A and B.

By closing the end of the tube above A in fig. I, and completely exhausting the space in the tube above A of all gas and vapour (except the vapour of the liquid), the liquid column can be used to measure the pressure of the air at the level B, and the manom eter of fig. 1 becomes the ba rometer of fig. 2. The present forms of liquid barometer for meas uring atmospheric pressures are based on the principle of fig. 2 or fig. 3.

Mercury is practically the only liquid that can be used con veniently in barometers which measure atmospheric pressures. Owing to its high specific gravity the barometric column is not unduly long. The vapour pressure of mercury is so small that its effect can ordinarily be neglected in the measurement of atmos pheric pressures.

There is an alternative type of barometer, called the aneroid (or non-liquid) barometer, which is dependent on the elastic properties of a thin flexible-walled evacuated capsule in combina tion with a stiff spring. The credit for the invention of the aneroid is usually given to Vidi, who patented his instrument in 1845, but similar instruments were in use much earlier. This instrument is not susceptible of the same high order of accuracy as a mercurial barometer, but it offers the advantage of considerably greater portability.

Units of Measurement of Pressure.

Until comparatively recently it was the usual practice to graduate barometers in inches or millimetres, or in both units. There is, however, a third unit which is now frequently employed in the case of barometers used for meteorological work. This unit is the bar, defined as the pres sure of one million dynes per sq.cm. The bar is a unit of pressure, but it appears on the scale of a meteorological barometer as a pseudo-unit of length, in such a way that the barometric height, when measured in this unit and corrected to suitable standard con ditions, is ultimately expressed in true units of pressure.

It sometimes becomes necessary, e.g., in gasometry, to reduce experimental results so as to relate to some standard pressure, and for this purpose the pressure of a standard atmosphere has been defined as the pressure due to the weight of 76omm. of mercury at o°C. under standard gravity.

The expression "standard atmosphere" is to some extent ambig uous. While this definition holds good generally for physical and chemical purposes, the meteorologist is inclined, at least for some purposes, to regard his standard atmosphere as i bar, which is approximately 7 5omm. of mercury. However, among American chemists and physicists the bar is defined as i,000,000 times smaller than this value.

Chief Types of Mercurial Barometer.

The different types of mercurial barometer vary largely with the method employed to measure the difference in height between the levels, A and B, shown in figs. 2 and 3. Both these levels change with variation of pressure. One of the earliest ideas was to make the level B (fig. 3) in the reservoir adjustable to the zero of a fixed scale which is used to give a direct reading of the level of A above B. A serv iceable barometer of this type was devised early in the 19th cen tury and is known as Fortin's barometer. It is still in ordinary use. Alternatively the scale itself may be made adjustable so that its zero is raised or lowered to the level B of the mercury in the reservoir, the final reading of the barometric height being taken at A as before. This type, an example of which is Newman's barometer, introduced in 184o, is not very extensively used.

A third type of instrument, introduced at a somewhat later date, avoids the direct use of a scale zero and gives a reading of the barometric height by means of a single setting. This type is known as a Kew pattern barometer, and makes use of the fact that changes in pressure produce proportionate changes in the level A. By using a scale with an appropriate contracted spacing, the instrument can be compensated so as to allow for the changes in level B of the mercury in the reservoir. This general type of instrument was origi nally devised for use at sea, but it is now also in frequent service on land.

The Fortin Barometer.

A general view of a modern Fortin barometer is given in fig. 4. The distinctive features of this type of instrument is Fortin's arrange ment for controlling the lower level of the mercury by means of a leather bag which forms the lower part of the reservoir of the barometer. Incidentally by this means, the mercury can be made to fill the tube and cistern of the instrument if desired, thus making the barometer easily portable. Fig. 5 illustrates the present form of de sign of a Fortin cistern, from which the mercury has been removed. The normal procedure in reading this instrument is to use the adjusting screw at the lower end of the barometer in order to raise the mercury in the reservoir so as just to touch an ivory point which is fixed to the ceiling of the cistern and serves as the zero of the barometer. The barometric height is then determined by reading the level of the highest part of the mercury column by means of the scale and vernier. The temperature of the barometer should also be recorded. Provision is made for ensuring a complete and quick communication of the external atmosphere with the interior of the cistern. The glass barometer tube, necessarily closed at the upper end, which is usually about 33in. above the fiducial point, is in England usually made of lead glass. It is surrounded by a metal sheath, suitably slotted in order to admit of viewing the summit of the mercury column in the glass tube. On this sheath is engraved the scale used for reading the barometer. The slots in the sheath provide a run for the vernier slide used in reading the top of the barometric column. The bore of the tube may vary from iin. to *in. in different instruments.

The length of the graduated scale on the sheath depends on the station at which the barometer is to be used. At sea-level the maximum range of variation in atmospheric pressure is from 3 r • 1 to 27.3in. of mercury. Consequently for use at stations near sea level a nominal working range of about 31 to 27in. (790 to 69omm., or i,o5o to 92o bars) is sufficient. The scale, however, must be subdivided above the upper limit to a tance dependent on the length of the vernier used in reading tions of a scale division. If the barometer is to be used in a mine, the upper limit must be increased, following the increase of pheric pressure with the depth of the mine. The lower limit of the scale is governed by the mum altitude at which the ment is to be used. Fortin type barometers are rarely made to read lower than loin. unless signed specifically for use in mountaineering or high-altitude surveying.

The Kew Pattern Barome ter.—This type of barometer yields direct indications of the pressure by means of a single setting on the summit of the mer cury column. No setting is made in the cistern. If the cistern (see fig. 6) and the glass tube of the barometer are cylindrical, the change in the level of the mer cury in the cistern corresponding to a given pressure change is a definite fraction of the change in level of the summit of the mercury column, the value of this fraction depending on the dimensions of the instrument.

It will readily be seen that the movement in the tube is always smaller than that which would be obtained if the mercury in the cistern were brought to a fiducial point.

Accordingly the scale of the Kew barom eter is contracted, but the amount of this contraction is not large, and a nominal pressure-inch of scale rarely measures less than o•95 true inch. The cistern is usually of cast iron, but stainless steel has recently been tried for this purpose. The lower end of the glass barometer tube is situated as nearly as possible at the centre of the cistern in order that it may be effectively sealed by the mercury in all positions of the instrument, whether erect, horizontal or inverted.

A Kew barometer, unless of exceptional size or design, is usually made portable by carefully tilting it until the mercury fills the tube completely. It may then be trans ported either horizontally, or cistern up ward, the latter condition being preferred.

Mercurial barometers have to be spe cially constructed for use on board ship.

As the oscillation of the mercury is a serious obstacle to reading the barometer, the glass tube is con stricted so as to oppose the flow of mercury through it. The amount of constriction is arranged to compromise between the error due to oscillation, or "pumping" as it is technically called, and the error due to the lag of the mercury column in following the variations of atmospheric pressure.

Mercurial marine barometers are usually of the Kew pattern. The glass barometer tube shown in fig. 7 may be regarded as illus trating both a Kew barometer and a marine barometer. In Kew barometers, there is usually a funnel-shaped air-trap A designed so as to collect at A any air that may rise into the barometer tube from the cistern, and to prevent it from reaching the vacuum space above the mercury column. The central portion S of the glass tube is usually of diminished bore in all Kew barometers, whether used on land or at sea. In marine barometers the bore is so diminished as to constitute a constriction. In land barometers this diminution is made in order to economize mercury, and should not be such as to impede the flow of the mercury considerably, or render the baro metric column sluggish in taking up equi librium with the atmospheric pressure.

Reduction of Barometric Readings to Standard Conditions.—Since the in dications of a mercury barometer are influenced by changes of temperature of the instrument, and of gravity acting on the mercury, corrections are normally made for these changes in order to obtain absolute values of the pressure.

Owing to the relatively high thermal expansibility of mercury the correction for temperature is important, being approxi mately Ain., for a barometer reading 3o in. of mercury, corresponding to a change in temperature of 33°F. There is further a small difference between the tempera ture corrections to the Kew and Fortin types of barometer.

It is usual, in the case of inch and metric barometers, to correct the readings so as to refer to mercury at 3 2 ° F. (o°C.) and at standard gravity, the latter being regarded as the value of gravity at mean sea-level in latitude 45°. If the pressure and temperature are constant, the height of the barometric column varies inversely as the value of gravity at the station where the barometer is read. Full particulars and instructions for the correction of barometer readings to standard conditions will be found in the books of meteorological tables referred to in the bibliography.

A useful variation in the procedure of correction has been made in the case of mercury barometers with scales graduated in mil libars. The gravity correction is expressed in terms of tempera ture in such a way that corresponding to any given station the temperature is found at which the instrument reads absolute pres sures in true millibars. This temperature is called the fiducial temperature, and in practice it is only necessary to apply a cor rection for the departure of the temperature of the barometer at the time of reading from the fiducial temperature.

A mercurial barometer indicates the pressure of the air at the level of its cistern. In meteorological work, such as the mapping of isobars for weather forecasting, it is not the pressure at cistern level that is finally required, but the corresponding atmospheric pressure at mean sea-level at a point vertically below the barom eter. Consequently a correction has to be made for the difference in atmospheric pressure between station-level and sea-level. Accuracy of Mercurial Barometers.—It is not possible within the limits of this article to discuss in detail the different sources of error which may affect the accuracy of a mercury ba rometer. Much depends on the dimensions and type of barometer used. Increased accuracy is usually obtained by increasing the diameter of the glass barometer tube up to certain limits depend ing on the purpose for which the instrument is required. Barom eters with a narrow tube (e.g., less than tin. in internal diameter) can scarcely be called precision instruments, as the amount of capillary depression of the barometric column due to the forces of surface tension acting at the mercury meniscus varies con siderably. Even in tubes of the size ordinarily used some varia tion in accuracy may be attributable to variation in capillary de pression.

In

general the Fortin type of barometer has a slight advantage over the Kew pattern in accuracy and permanence. This is es pecially the case with instruments having a tube of small internal diameter. With suitable precautions the Kew barometer can be made to yield an accuracy as great as that of a Fortin instrument of similar dimensions. In many classes of work it is preferable to use a Kew barometer and gain the advantage which this type offers in giving the barometric height by means of a single setting. If continuous accuracy over a period of several years is important, preference should be given to the Fortin type.

The following table indicates the general degree of consistency obtainable from a mercury barometer under good conditions of use: Special accuracy may be obtained by taking particular precau tions in methods of measurement and in the design of the barom eter, particularly in the use of a tube of large diameter. A funda mental standard barometer has recently been constructed at the National Physical Laboratory, Teddington, Middlesex, to measure atmospheric pressures to within o•oimm. of mercury.

Aneroid Barometers.

The aneroid barometer is often used as a substitute for a mercurial barometer under conditions in which the use of the latter is not admissible.

Fig. 8 shows the general arrangements of the mechanism of an aneroid. A corrugated chamber, formed by two thin metal diaphragms, is the fundamental element of an aneroid, and is technically called the vacuum box, since it is usually thoroughly exhausted of air. It is bolted to the base-plate of the instrument (see the sectional view in fig. 9), and connected to a steel spring which controls the stiffness of the combination. This spring is mounted so that it tends to open the box by pulling strongly upwards. A decrease in the atmospheric pressure causes the dia phragm box to expand. Consequently the point at which the box is anchored to the spring will move upwards (fig. 9), relatively to the base-plate of the instrument. The amount of this elastic movement caused by a change in air pressure is relatively small. It is of the order o•oosin. (linear) in an average diaphragm box, r ? cording to the size and range of the movement. A nominal inch of mercury, as shown on the average aneroid scale, usually meas ures at least a linear inch (sometimes more), and corresponds to a magnification of about 200 times the movement of the vacuum box. The aneroid form of barometer can readily be made self--registering, and fig. ro illustrates one of the usual types of barograph. It is a general practice in making barographs to insert a suitable control spring inside each vacuum box, instead of using the external type of spring fitted to indicating aneroids. The main features of the mechanism of the indicating aneroid are retained, except that the metallic chain operating the indicating needle is replaced by a long pen lever, which traces out the record on a uniformly revolving drum driven by clockwork.

The different effects of tem perature changes on the reading of an aneroid barometer are suf ficiently marked to necessitate compensation in aneroids generally. They are twofold in character, for in addition to the thermal expansion of the aneroid mechan ism, particularly the diaphragms and main spring, there is a ther mal change in the elasticity of the material of these parts.

Commercially, an aneroid is called "compensated" if it has some device in it which will make the reading independent of the tem perature at such pressures as occur at sea-level (i.e., at about 3oin.) .

Although an aneroid is sensitive to small changes of pressure, its reading cannot be relied upon to give the absolute value of the corresponding to a change in pressure equal to tin. of mercury. The instrument, however, gives a considerably magnified indica tion of this movement, which is linked up by a system of levers connected with a fine metallic chain which rotates the spindle on which the indicating needle is mounted. Nickel-silver alloy is most frequently employed for the diaphragms of vacuum boxes, but steel has also been used with considerable success.

Aneroid barometers are usually designed to read pressures with a precision which varies from o-oiin. to o-osin. of mercury ac pressure to a precision corresponding to one subdivision of the scale of the instrument (o-oiin. for short. ranges to o•osin. for long ranges).

The utility of the aneroid as an absolute pressure indicator is limited by gradual and appreciable changes in the internal struc ture of the metal of the vacuum box, and for this reason it is highly advisable to compare the reading of an aneroid with that of a reliable mercury barometer at suitable intervals of time in order to avoid errors due to temporary or secular changes in the mechanism. Most important of all the defects to which an aner oid is susceptible is that generally known as "creep," which occurs when an aneroid, which has reached a steady condition at one pressure, is submitted to a different one. Although an aneroid responds readily to a change in pressure, the diaphragms do not remain steady at the new pressure, but show a further small but gradual change in the same direction, in the course of time, while the same pressure is maintained.

Practical Uses of the Barometer.

A good mercurial barom eter is an essential part of the equipment of most laboratories owing to the number of different scientific measurements which are dependent on a knowledge of the atmospheric pressure. Sys tematic use of barometers and other instruments is made on a wide scale by the Meteorological Office, which is responsible for the collection of information required for weather forecasting. Barometric readings are regularly taken on land and at sea over a wide area of the earth's surface in order to map the distribution of atmospheric pressure near the earth's surface.

A considerable amount of work has been done with aneroid ba rometers in the investigation of the meteorology of the upper atmosphere. Apart from the measurement of pressure, there is also a wide field for the use of a barometer in the determination of heights. Barometers are very sensitive to a change in height.

A change in altitude of i,000ft., measured from sea-level corre sponds approximately to a pressure change of one inch of mercury. The relation between height and pressure depends to a consider able extent on the temperature of the atmosphere, and although barometers are furnished with scales of altitude for the direct measurement of differences of height, such scales cannot be re garded as accurate under widely different atmospheric conditions. In general they represent some conventional relation between height and pressure, which needs further correction if accurate heights are to be obtained. There are a number of different con ventional scales in use in England and other countries. Altimeter aneroids, adjustable so as to measure height above a convenient starting level, are in general use in aircraft.

The following table represents approximate average values of the atmospheric pressure at the respective heights above sea level :—

pressure, mercury, tube, aneroid and instrument