MANOMETER, or PRESSURE GAUGE, is an instrument for measuring the hydrostatic pressure exerted by gases, of steam in a steam-boiler.
The simplest and at the same time most accurate form of manometer is that known as the "mercury manometer," MANOMETER-PRESSURE-GAUGE sometimes also called the "free-air manometer," and represented in fig. 1. It consists essentially of two vertical communicating form bore, is fixed hermetically in the neck of a large wrought-iron cylinder C, its lower end dipping below the surface of mercury contained in the cylinder. The other tube, EF, is attached at its lower end to the with the vessel the pressure in which is to be ascertained. Usually the tube Et', the cross pipe D, and the space above the mercury in C are filled with water. At first the tube ED' is left open to the atmosphere, and the height of the mercury in AB noted. When EF is then put in communication with the vessel in which the pressure (above atmo.
rises the pressure is deduced. For accurate work corrections must be made for the fall of the mercury in C as it rises in AB, and for the temperature and the height of the barometer at the time of the experiment.
The great drawback to the employment of the simple mercury manometer for measuring very great pressures is the mechanical difficulty of obtaining a sufficiently long column of mercury. E. H. Al. Amagat, however, has lately (1880) worked with a column one-fifth of a mile high. His experiments were undertaken to find out how the various gases, nitrogen, oxygen, air, hydrogen, &c., departed from Mariotte's law when subjected to enormous pressures. At the bottom of a coal-mine at Verpilleux, near St Etienne, which had a depth of 327 metres, was placed the glass manometer tube containing the compressed gas, while the mercury tube (made of steel) extended up the shaft, being gradually built up in sections. See Nature, vol. xxii. pp. 62, 63. By means of Amagat's tables of the volume and corresponding pressure of the several gases, and with special forms of manometer to suit par ticular circumstances, accurate and delicate measures of enormous pressures can now be obtained. Professor Tait, for instance, has recently applied these tables along with a manometer of his own devising for testing the behaviour of the thermometers supplied to the " Challenger " expedition under a pressure of as much as 10 tons to the square inch.
" Reg,nault's manometer" is shown in fig. 2. AB is a strong metal tube, closed at the lower end, and carrying at the upper a bent pipe for admitting the compressed gas and a stop-cock R pierced with holes in a T form. DE and FG are two graduated glass tubes communicating at their, lower ends by a narrow passage in the metal block to which they are hermetically fixed. DE and AB also comhaving radial holes at right angles to each other, DE can communicate either with AB or with the atmosphere at 0 ; and by the stop-cock R" it can communicate either with FG or with the open air. The three tubes are surrounded by a cylinder MM' containing water to keep the temperacocks R and R' are then turned as in fig. 2a, so that AB and DE communicate with each other. Part of the com 67 pressed gas flows over into DE, and the mercury in FG rises. By manipulating the stop-cock R" as shown in fig. 2b, part of the mercury is allowed to run out of DE till a conveniently measurable difference of level between the mercury surfaces in DE and FG is attained.
Let be this differmice. Also let x be the pressure of the gas originally filling the volume V of AB, V' the additional volume occupied by the expanded gas, and H be the height of the hero-meter at the time ; then we have by Mariotte's law xV = (V + V')(H + h), from which x=VV(H-F/i) (1) V' is determined by weighing the mercury required to fill the space it occupies, and V can be calculated from (1) when AB is filled with dry air at.pressure H.
In Regnault's apparatus the length of AB and DE was 1 metre, the diameter of AB 5 mm. and of DE 20 mm. The section of DE was thus sixteen times that of AB, and in this way a very great pressure could be measured by a comparatively small difference of level between the mercury surfaces in PC and DE. The instrument, however, is subject to errors, arising chiefly from the difficulty of measuring accurately the volumes V and V'.
The "compressed air manometer" (fig. 3) consists of a strong graduated glass tube of uniform narrow bore, closed 67 at the top and fixed hermetically into the neck of a wide iron cylinder. The tube contains dry air, and its lower end dips below the surface of mercury contained in the cylinder. Attached to the side of the cylinder is a tube A, with a stop-cock, to afford communication with the vessel the pressure in which is to be measured. When the manometer is attached to the vessel containing compressed gas the mercury rises in the glass tube till the presheight of the mercury column above the level of the mercury in the cylinder is equal to the pressure on the surface of mercury in the cylinder.
"Desgoffe's manometer" depends upon the same principle as the hydraulic press, and can be employed to measure the enormous pressure reached in the cylinder of that instrument. It is represented in perspective in fig. 4 and in section in fig. 5. V is a strong circular iron vessel, in which moves up and down for a short distance a flat piston D attached to a cylindrical plug T. The lower part of V contains mercury which has free communication with a graduated vertical glass tube AB fixed hermetically into the side of V. Above the mercury in V is placed a thin layer of water, and above that is stretched a thin membrane of india-rubber bolted down water-tight by an iron ring. The chamber C contains a cavity in which the plug T moves water-tight- By means of the tube t the instrument can be put in communication with the vessel containing the fluid whose pressure is to be measured. The compressed fluid acting upon T depresses the piston D and causes the mercury to rise in AB.
Let p be the pressure of the fluid per unit of area, s the area of 67 T, and S the area of ll ; also let P be the pressure per unit of area as recorded by the height of the mercury in AB. Thus evidently we have ps - PS , or 2) - .
Hence by making S very great and s very small a very great pressure can be measured by a comparatively short column of mercury in AB. As part of the pressure p is employed to stretch the iudia-rubber membrane, the ratio S : s should be made very great, so that D will only sink a very shortdistance. Cailletet, who employed this manometer in his experiments on the compressibility of fluids, had it so arranged that (neglecting the stretching of the indiarubber) the mercury in AB rose 4'3 metres while the piston in D sunk only one-eighth of a millimetre.
Metallic manometers depend on the principle exemplified in the aneroid barometer. Suppose a long tube, preferably of elliptic section, and having thin walls of elastic material, to be closed at one end and either bent or coiled up in the form of a spiral. Let the open end be attached to an apparatus whereby the pressure inside the tube can be either increased or diminished. If the pressure inside the tube be made greater than that outside, the tube has a tendency to straighten or uncoil itself, but if the pressure outside be greater than that inside the tube has a tendency to bend or coil itself up farther. Fig. 6 represents an early form of metallic mano 67 meter made on this principle by Bourdon, the first to construct such instruments. A metallic tube ab,closed at b, is coiled in a spiral and rigidly attached at the open end a to a tube with stop-cock In, whereby it can communicate with the compression apparatus. A light index e is attached to b and moves over a graduated scale. The scale is graduated by applying known pressures inside the tube. This form of manometer is very convenient for rough practical work, but has no pretensions to scientific accuracy, as changes of temperature affect the elasticity of the tube in a way which is difficult to discover and allow for. Various forms of metallic manometers have been recently invented, the best-known of which are perhaps those of Bourdon and Schiffer, in which the index is moved by a train of wheels actuated by the free end of the elastic tube.
Air-pump Manometer. - For measuring pressures less than that of the atmosphere, as in the receiver of an air-pump, a special form of mercury manometer is employed, consisting of a glass U tube with each leg over 30 inches long and half filled with mercury. One leg communicates by an air-tight communication with the receiver of the air-pump, and the other is left open. As the exhaustion proceeds, the mercury falls in the open leg and rises in the other.
When only considerable degrees of exhaustion are to be measured, the instrument takes the form of a short U tube closed at one end and open at the other, and has its closed leg completely filled with mercury, the mercury being held up by the atmospheric pressure. The whole is enclosed in a wide glass tube closed at the top and hermetically fixed at the lower end to a brass piece, provided with a stop-cock, whereby it can be screwed on to the sole plate of the air-pump. The difference of level in the two legs gives the degree of exhaustion obtained.
See Ganot's Physics; Wiillner's Lehrbuch der Experimentalphysik; Amagat in Annales de Chemie et de Physique, March 1880; Report of .11. M. S. Cluilleuper, in regard to pressure corrections supplied by thermometers, by Professor Tait. (J. BL.)