Barometer

Barometer, an instrument for the measurement of atmospheric or other gaseous pressure. It is of varied and extensive use in science. Observations of the variations in the atmospheric pressure frequently enable us to make accurate weather forecasts; hence the term weather glass. Heights of mountains can be estimated from the amount of diminution in pressure as one ascends into the rarer regions of the air. Again, many of the physical properties of gases are dependent on the pressure to which they are subjected, thus rendering the accurate measurement of this pressure an essential in the quantitative study of the gases.

These instruments are of two types, the Aneroid and the Torricellian. The former is comparatively new, but is perhaps simpler in principle. It was invented in 1844, and depends for its working on the fact that a closed box from which the air is removed has the tendency to become compressed by the external pressure of the surrounding atmosphere. If made of flexible material, the diminution in volume of the box may be rendered sufficiently great to admit of exact measurement. When the external pressure varies so does the volume of the box, which therefore behaves as a sensitive spring subjected to a varying stress. In practice the Aneroid barometer is made somewhat drum-shaped, the drum membranes being represented by circular discs of thin corrugated steel. The drum is attached to the casing of the instrument by one of these discs; and at the centre of the other a spring is fixed so as to prevent too great a collapse of the box. The slight motions of this spring, when the external pressure varies, are magnified by a light bent lever, which by a simple mechanism actuates the pointer on the dial face. The dial is graduated in inches of mercury, corresponding to the graduation of a Torricellian barometer. The Aneroid has the distinct advantages of lightness, compactness, and durability, but is not capable of such accuracy as may be obtained with the mercurial barometer.

The second type depends on the principle of the gaseous pressure being able to support a definite height of liquid. If a long glass tube closed at one end be filled with mercury, and then turned mouth downwards into a cistern of this liquid, it will be found that a definite length of mercury will still remain in the tube, kept in position by the pressure of the surrounding air on the surface of the liquid in the cistern. If the tube be of a length exceeding 30 inches, an empty space will exist in the upper part of the tube. This is known as the Torricellian vacuum, and the apparatus, provided with a vertical scale, constitutes a Torricellian barometer.

When gas of any kind is introduced into this space a lowering of the mercury column is produced, by reason of the gaseous pressure within partially neutralising the external pressure. Hence the necessity of preserving the vacuum as perfect as possible. The ordinary British standard of atmospheric pressure is that which will balance 30 inches of pure mercury at Greenwich. The metric standard is equivalent to 76 cm. of pure mercury at Paris, i.e. 29.922 inches. It is necessary in exact work to specify the latitude where the barometric height is taken, since the weight due to a given height of mercury varies at different parts of the earth. Slight corrections are also necessary for expansion of the mercury column and of the metal scale, due to temperature changes. Many refinements are introduced in the more accurate instruments, which readily give the barometric height, measured from the mercury level in the cistern, correct to the 1/1000 of an inch.