86 Mr. J. J. Guest on the Strength of 



quantity being syphoned out, the pressure at 300 lbs. per 

 square inch was determined by the use of a Crosby gauge- 

 tester, so that this is the ultimate basis of the measurement 

 of the fluid pressure. 



31. A new type of Pressure-gauge. — The special gauge 

 (see fig. 3) consisted of a steel tube of oval section, twisted 

 along its axis, so that the tube had a straight axis, but the 

 extremities of the major axes of the sections perpendicular to 

 the tube-axis lie on a pair of helices the angle of which 

 is about 45° to 60°. One end of the tube is coupled to the 

 pressure-system, and the free end is sealed up and a pointer 

 or mirror, preferably the latter, mounted upon it. On the 

 application of internal pressure the tube untwists, and the 

 amount of angular movement is determined by the use of a 

 telescope and scale, or in any other manner. As the strain 

 at any part is small, it is proportional to the pressure ; but the 

 integral twist of the mirror or pointer may be made of any 

 magnitude we please, without exceeding the elastic limit of 

 the metal, by simply increasing the length of the gauge-tube. 

 As, by using a tube of suitable thickness, it is easy to keep 

 the stresses in the steel, produced by fluid pressures up to 

 several thousand pounds per square inch, below the yield- 

 point, and since there is no mechanism nor friction, the gauge 

 is at once simple and accurate. By comparing the readings 

 of two gauges, one having thicker walls than the other, the 

 fluid pressure corresponding to the yield-point of the lighter 

 gauge can easily be found, as it occurs when the readings 

 cease to be proportional. The constant of the heavy pressure- 

 gauge may be found by comparing it with the lower gauges. 

 Having first ascertained the permissible working pressures, 

 a thin-walled gauge may be compared with a mercury 

 column, and its constant thus found ; this gauge may then 

 be compared with a heavier gauge, and since the constant of 

 the first gauge cannot alter, that of the second may be found. 

 Thus the constants of all the gauges can be found. The axis 

 of such a gauge-tube can evidently be coiled (as a line) and 

 produce other desirable movements. 



The gauge used in the calibration of the commercial gauge 

 was made of tubing originally \ inch in diameter ; it was 

 about 8 inches long, and from my knowledge of the yield- 

 points of thinner-walled gauges I think that its yield-point 

 w r ould be reached under a fluid pressure of from 4000 to 

 5000 lbs. per square inch. 



In Table II. will be found the comparison of the com- 

 mercial gauge by the two methods. They give practically 

 the same result. 



