. A text book of physics, for the use of students of science and engineering . erable, mercury may be used. Theabsolute pressure inside the vessel may be calculated by addingalgebraically the barometer reading to the reading of the pressure gauge, first dividing the latterby 13-59 if water has beenused in the gauge. Pressure gauges for indicat-ing high pressures, such asthat in a steam boiler, areusually of the Bourdon action in these gaugesdepends on the tendencywhich, a curved, partiallyflattened tube has to becomestraight when subjected tointernal pressure. Expt. 89.—Bourdon action
. A text book of physics, for the use of students of science and engineering . erable, mercury may be used. Theabsolute pressure inside the vessel may be calculated by addingalgebraically the barometer reading to the reading of the pressure gauge, first dividing the latterby 13-59 if water has beenused in the gauge. Pressure gauges for indicat-ing high pressures, such asthat in a steam boiler, areusually of the Bourdon action in these gaugesdepends on the tendencywhich, a curved, partiallyflattened tube has to becomestraight when subjected tointernal pressure. Expt. 89.—Bourdon a piece of rubber tubeabout a yard long to a watertap ; close the outer end by aclip, or a piece of glass rod;bend the tube into a curve lyingon the table. Quickly open the water tap, when it will be found thatthe rubber tube, which has been slightly flattened by the bending, willdistinctly show movement in the attempt to become straight. The interior parts of a Bourdon pressure gauge are shown in A is a flattened tube of hard, solid-drawn phosphor bronze. Fig. 393.—Interior parts of a steam pressuregauge. 396 HEAT chap. secured to a bracket B, which has passages in it forming the steaminlet to the tube. The free end of the tube is closed, and is connectedby means of a short link C to a small-toothed sector D. The sectorgears with a pinion on the spindle E, which carries a pointer travellingover a scale of pressures marked on the outside of the case. Bourdon gauges show the difference between the pressure insidethe vessel and the pressure of the atmosphere. If the pressureinside the vessel is lower than that of the atmosphere, the gauge iscalled a vacuum gauge, and the dial is graduated in inches or centi-metres of mercury so as to facilitate the process of calculating theabsolute pressure. The absolute pressure is obtained by deductingthe vacuum gauge reading from the observed barometric example, if a vacuum gauge reads 72 cm. at a time when the
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