An instrument to determine the pressure exerted in hydrostatic or pneumatic machines, as the hydrostatic press, air pump, and steam engine. When the pressure exerted is less than the pressure of the atmosphere, as in the condenser of a steam engine, the gauge is usually termed a barometer, and consists simply of a barometer tube, the lower end of which plunges in a cup containing mercury open to the atmosphere, whilst the upper end communicates with the condenser, and the degree of exhaustion or vacuum, as it is usually termed, is measured by the altitude of the column of mercury in the tube above the surface of the metal in the cup. When the pressure exerted does not exceed two or three times the force of the atmosphere, it may be measured by means of an inverted syphon or bent tube of wrought iron containing a portion of mercury; one leg of the tube communicates with the vessel in which the pressure is exerted, and the other leg is open to the atmosphere, and contains a float by which the rise of the mercury is indicated; but when the pressure is very great, a tube of sufficient length to support the corresponding column of mercury would be extremely inconvenient, and it is usual then to measure the pressure by the compression of a volume of air contained in a glass tube, the upper end of which is sealed, whilst the communication of the air in the tube with the chamber in which the pressure is exerted is intercepted by a quantity of mercury in the lower part of the tube, and the pressure in the chamber acting upon the surface of the mercury exposed to it causes the mercury to rise in the tube, and to compress the air therein into a smaller space, which space will be inversely, as the pressure exerted.
The figure on page 617 represents a pressure gauge upon this principle, described in No. LXIII. of the Philosophical Magazine, by Mr. H. Russell. The gauge consists of a glass tube, sealed at one end, with a ball blown very near the other, leaving only as much beyond the ball as may be necessary for connecting it with the pipe leading from the vessel containing the condensed steam or gas, or other elastic vapour. This ball, when the tube is filled with air, and subject only to atmospheric pressure, should be about three quarters full of mercury, and the whole capacity need not exceed that of the tube more than as two to one. That the divisions in the scale may be in geometrical progression, the tube is placed m a horizontal position; this renders the instrument altogether so simple in appearance, that persons totally unacquainted with instruments of this description, may at once be brought to understand its nature, and be able to affirm with confidence the degree of pressure to which it is subject. To determine the degree of pressure at any point, ascertain the distance from the sealed end of the tube, and by that measure divide the length contained between the sealed end and the bulb, the quotient will be the number of atmospheres.
Thus in general terms, where T represents the whole tube, P the part into which the column of air is compressed, and A the number of atmospheres, we have T/P=A. Thus, suppose the tube 8 feet long, and the column of air compressed into half that length, then w have 3/4=2 atmospheres. If this column be again compressed into half its volume, it will be represented by 3/2=4 atmospheres; - if, again, compressed into half its volume, we have 3/1=8 atmospheres; - if, again,
(8 feet=96 inches) 96/6= 16 atmospheres; - and lastly, 96/3 =32 atmospheres. In the above figure the mercury chamber is blown in the tube itself, so that in this plan we have no joints whatever to make in the instrument; and being placed in a horizontal position at a convenient distance from the floor, all parts of the scale may be examined with equal facility. For the internal diameter of the tube, perhaps one-sixteenth of an inch will be found preferable.