Semi-automatic High-Pressure Installation. 155 



In any case small, and under working conditions is reduced 

 to a negligible amount by keeping the piston in rotation. 



The pressure-ranges for which the three gauges are 

 designed are — 



to 200 atmospheres for Gi, 



200 atmos to 800 „ G 2 , 



800 „ to 1500 „ G 3 . 



The corresponding piston-areas being approximately — 



0*10 sq. in. for G 1} 0*02 sq. in. for G 2 , 



and 0*01 sq. in. for G 3 . 



The needle-valves V 1? V 2 (fig. 1) are used for shutting off 

 the gauges G l9 G 2 when these are not in use. No such 

 valve is required for the high-pressure gauge G 3 , which 

 carries a sufficient load to keep it from lifting when the 

 pressure is low enough to be measured by G x or G 2 . 



Details of gauges. — A piston intended for high-pressure 

 measurements must have a small sectional area if the weights 

 are to be kept within manageable limits, and it must be very 

 closely fitted. The oil which supports the rotating piston 

 must consequently be under a considerable pressure before 

 the lubrication is free enough to render the arrangement 

 sufficiently frictionless. It is for this reason that any 

 single gauge is only serviceable over a restricted range 

 of pressure. 



The gauge G 2 , used for intermediate pressures, is shown in 

 more detail in figs. 2 and 3. The upper end of the piston* 

 has the form of a spherical cap, upon which rests the 

 hardened steel pin P spherically hollowed to a somewhat 

 greater radius. This pin is inserted in, and thus supports, 

 the tubular weight-carrier C. To the lower end of C an 

 annular plate Q (fig. 3) is pivottally connected, with freedom 

 to tilt about a horizontal axis A A, while two pins B project 

 downwards from Q and engage in radial slots in the 

 worm-wheel V. The worm-wheel is kept in rotation, and the 

 weight-carrier and weights must necessarily partake of this 

 rotation, the transmission of the motion being such that the 

 forces applied to the carrier are sensibly equivalent to a 



* 

 * The pistons and cylinders were made for us by Messrs. Elliott, and 

 are copies of those used at the National Physical Laboratory. The 

 designs were given us by Mr. Jakeman of that laboratory, to whom our 

 best thanks are due. 



