Should the pressure within the tube be less than that of the surrounding atmosphere (as is the case when the instrument is SCHÄFFER'S GAUGE. measuring the vacuum in a condenser), then the cross-section of the tube becomes flatter than its normal or ordinary shape, and, consequently, the inclosed end of the tube curves wards, thus moving the pointer, I, in the other direction. It is usual in practice to have separate instruments for recording pressures above and below the atmospheric pressure, and to increase the range of the pointer by the intervention of a toothed lever quadrant fixed between the end of the drag-link and the pointer. This quadrant gears with a small pinnion fixed to the pointer-spindle, and a fine watch-spring, with its inner end attached to this spindle, and its outer end to the case of the instrument, ensures the pointer coming back to zero when the pressure is removed, as well as prevents it lagging behind or sticking. These devices are clearly shown in the above figure, which represents Schäffer's patent, the difference between it and the Bourdon patent being, that the former relies upon the natural elasticity of a concentrically corrugated steel plate placed across the hollow opening in the flange of the pipe, G, which communicates with the boiler or condenser. The centre of this corrugated plate is attached by a clip and rod to the toothed quadrant as shown. When the pressure is greater than that of the atmosphere, the corrugated plate is bulged upwards, and when it is less, it is bulged downwards. These motions, being proportional to the pressures per square inch, are correspondingly indicated on the graduated dial by the pointer. Pressure Pyrometer.-In Lecture IV., under the heading of Pyrometers, we referred to this instrument, which depends for its action upon the fact that the pressure of a gas, generated from a liquid with which it is in direct communication, corresponds to the temperature of the liquid. The name given to it by the makers is the Thalpotasimeter, and it is constructed, as may be seen from the following figures, of a metal stem, containing the liquid, and ending in a Bourdon or Schäffer gauge. The metal stem is shown fixed in the first case into a pipe, and in the second case, into a flue, through which hot gases are passed. Their temperature inside the pipe or the flue is communicated to the stem of the instru ment, and therefore to the liquid within it. If water be placed within the stem, then the pressure (and consequently the temperature) rises in accordance with Regnault's tables (see next Lecture). Instruments filled with ether are made and graduated from 100° to 220° Fah.; those filled with water, from 212° to 680° Fah. ; and those filled with mercury, up to 1400° Fah. LECTURE XI.—QUESTIONS. 1. Describe an experiment for ascertaining approximately the relation between the pressure and temperature of steam at a moderate pressure (say 10 lbs.) above that of the atmosphere. (1887, S. and A. Elemty. Exam.) 2. How would you ascertain the pressure of the vapour of water at a temperature above 212° F.? Describe some method of conducting the experiment. 3. From the table of Regnault's results (Lecture XII.), plot out a curve showing the rise in pressure of steam from 1 lb. to 200 lbs. absolute on the square inch corresponding with increase of temperature. Adopt a scale of I inch = 50 lbs. and I inch = 100° F. 4. Sketch and describe the mercurial pressure gauge which was much used with low-pressure boilers. Mark on your sketch a scale showing the position of the mercury in the tube with a pressure of 8 lbs. per square inch above atmospheric pressure. Height 16.3" = 5. If the specific gravity of mercury be 13'5, calculate how much higher the mercury will stand in one leg than in the other when the pressure of steam is 10 lbs. on the square inch above the atmosphere. Ans. 20.5 inches. 6. Sketch and explain how a barometer gauge is made and fitted to a condenser. If the mercury in an ordinary barometer stands at 30 inches when that in the gauge stands at 26 inches, find the pressure per square inch of the vapour in the condenser. Ans. 1'96 lb. 7. Explain what is meant by the expression "a gauge shows 25 inches of vacuum." If the weather barometer stands at 29 inches, and the condenser gauge at 23 inches, what is the pressure of vapour in the condenser ? Ans. 3.25 lbs. per square inch. 8. If the vacuum gauge shows 26 inches when the weather barometer shows 30°2 inches, what is the pressure of vapour in the condenser, and what would be the same if the gauge fell to 18 inches? Ans. 2'1 lbs. and 5.85 lbs. 9. What effective pressure is obtained on the piston when the steam gauge shows 25 lbs., and the vacuum gauge 22 inches? Ans. 36 lbs. per square inch. 10. What effective pressure is obtained when the steam gauge shows 41 lbs. and the vacuum gauge 27 inches, and what would be the effective pressure if the vacuum fell to 11 inches? Ans. 545 and 46.5 lbs. II. Sketch and describe fully by an index of parts (using the first letters of the names of the parts) a Bourdon's pressure and a Bourdon's vacuum gauge. Account for the peculiar action of the tube. 12. Sketch and describe Schäffer's corrugated plate pressure and vacuum gauge. 13. Upon what principle does the pressure pyrometer or thalpotasimeter depend? Sketch such an instrument, and mention for what purposes it is used. 14. Suppose a Bourdon pressure gauge to be faulty in its graduation: sketch how you would connect it up to a mercurial column to test it. If the specific gravity of mercury be 13'5, calculate what every inch of the column will correspond to in lbs. pressure on the square inch. Ans. 49. 15. What do you understand by the statement that there is a vacuum of 10 lbs. registered on the vacuum gauge of a condenser? Sketch and describe fully some form of gauge for testing the pressure in a condenser. (S. and A. Exam., 1894.) LECTURE XII. CONTENTS.-Pressure and Volume of a Gas-Boyle's Law --Pressure Volume, and Density-Curve of Volumes and Pressures-Table giving the Chief Properties of Saturated Steam-Watt's Diagram of Work, with Example-Area of Diagram is a Measure of the Work done in One Stroke. Pressure and Volume.-We saw in Lecture XI., by the experiment with Marcet's boiler, that the pressure of steam increased with the temperature; we now come to consider the relation which exists between pressure and volume. Boyle's Law. The pressure of a perfect gas at a constant temperature varies inversely as the space it occu 35 30 -25 GT -20 บ = volume. Then pv= constant, since px I V. To illustrate this law the following simple piece of apparatus may be used:It consists of a small metal box, B, to which are attached two glass tubes, GT, one a little more than 35" long, and the other fully 10". A stop-cock, SC, is 10 screwed into the metal box, and the short tube is provided with a screw plug, S P. The whole is fixed to a board, on which there is a graduated scale of inches. 15 S.P. -10 5 -5 0 Mercury is poured into the long tube GT and the screw plug, SP, is taken out until the mercury rises in both tubes to the zero line. The screw plug is then replaced and encloses a column of air Jo" high in the short tube. Supposing the barometer to stand at 30′′, we now continue pouring mercury into the long tube until the level of the mercury in it B GT for Glass tubes. SC SP |