BRIDGMAN. — A SIMPLE PRIMARY GAUGE. 203 



sq. cm. The first part is occupied with a description of the gauge, 

 calculation of the corrections to be applied, and a comparison of two 

 gauges to determine the accuracy and sensitiveness. 



Description of the Gauge. 



Besides Amagat's 5 manometer, other forms of direct pressure gauge 

 have been used, examples of which are the pressure balance at Stuck- 

 rath, 6 and the differential manometer at the National Physical Labora- 

 tory at London. 7 Lisell, 8 in his measurement of the pressure coeffi- 

 cient of resistance of wires, used a gauge much like that at Stuckrath. 

 These gauges differ in the manner in which the pressure exerted on 

 the piston is measured. Amagat measures it by measuring with a 

 mercury column the hydrostatic pressure acting on a larger piston 

 which balances the total thrust exerted by the high unknown pres- 

 sure on a much smaller piston. The thrust is measured at Stuckrath 

 or by Lisell by hanging weights on the piston either directly or with 

 the aid of a lever. At London the action of weights is used to equili- 

 brate the differential effect of the pressure on two pistons of nearly the 

 size. A common feature of all these gauges is the piston fitting ac- 

 curately in the cylinder, which is subjected to pressure on the inside. 

 The distortion produced by the pressure is, therefore, a compression of 

 the'piston, accompanied by a stretching of the cylinder, the resultant 

 effect being to increase the breadth of the crack between piston and 

 cylinder. The leak, therefore, at higher pressures increases because 

 of the increased pressure expelling the liquid and the increased breadth 

 of the crack. 



This effect is avoided in the gauge used in this work by subjecting 

 the cylinder in which the piston plays to pressure on the outside as 

 well as on the inside. It is well known that a cylinder subjected to 

 the same pressure externally and internally shrinks to the same 

 extent as a solid cylinder subjected to the same external pressure. 

 By properly decreasing the external pressure on the hollow cylinder, 

 the shrinkage at the inner surface may be made as small as we please, 

 or may be made an expansion. Practically the same result may be 

 obtained by subjecting only a portion of the external surface of the 



6 Amagat, Ann. de Chim. et Phys., (6), 29, 544 (1893). 



6 Zeit. f. Instrk., 14, 307 (1894). Manometer fur hohe Dmck. 



7 Engineering, 75, 31 (1903). The Estimation of High Pressures. 



8 Lisell. Om Tryckets Imflytande pa det Elektriska Ledningmotstandet 

 hos Metaller, samt en ny Metod att Mata Hoga Tryck. Upsala, 1903 (C. J. 

 Lundstrom). 



