473 



In fact this "normal volume" is derived bj' Kohnstamm and Walstra ') 

 from the volume which corresponds to the pressui'e given by the 

 pressure-balance according to the isotiiermal of hydrogen as deter- 

 mined by ScHATKWijK ^) at Leiden by means of the open manometer 

 of Kamerijngh Onnks '). In order to reduce the observed pressures 

 and volumes in the investigations by Kohnstamm and Walstra to 

 real pressures and volumes, which are requii'ed for tiie deduction of 

 the equation of state, an investigation as to the real pressure, 

 corresponding to a definite indication of tiie pressure-balance, is thus 

 indispensable. 



As the open manometer in question allows absolute pressure- 

 measurements up to 120 atmospheres of great accuracy, a calibration 

 of the small pressure-balance, used in the experiments of Kohnstamm 

 and Walstra, would at any rate yield the norma! volume belonging 

 to the measurements at lower pressures. 



Independently of the absolute calibration itself of the pressure- 

 balance in tiie region explored, the comparison of this balance with 

 tiie open gauge was also of great value for forming an estimate of 

 the accuracy of the determination of the very high pressures. The 

 desirability of such comparison was insisted upon by Kohnstamm 

 and Walstra not long ago. 



Of the tiieory of the pressure-balance only little is known and 

 even that has not been at all adequately tested by experiment. 

 Worst of ail the experiments made so far do not confirm the theory. 

 We are chiefly referring to E. Waoner's^) investigation, whose calcu- 

 lations about an AMAGAT-gauge are also mutatis mutandis applicable 

 to a pressure-balance. Waonkr calculates the force wliicii the cylinder 

 of an AMAGAT-gauge experiences owing to tlie viscosity of the oil 

 which flows through the narrow interspace between piston and 

 cylinder and finds liiat this force cannot always be neglected in the 

 practice of accurate measurements. In order to calculate the true 

 pressure from the indications of the gauge a correction has to be 

 applied to the latter, but since in the expression for the force, 

 besides constants of the instrument, only the pressure occurs as a 



1) Ph. Kohnstamm and K. VV Walstra. These Proceedings 16. p. 754, 822. 

 1913 and 17 p. 203. 1914 and K. W. Walstra, Dissertation Amsterdam 1914, 

 where also a description of the pressure-balance will be found. 



2) J. G. Schalkwijk. These Proceedings 3. p. 421, 481 1901. Comm. 67 and 

 Dissertation, Amsterdam, 1902. 



S) H. Kamerlingh Onnes. These Proceedings 1. p. 213. I89S. Comm. 44. 

 ^■) E. Wagner, Dissertation. München, 1904 and Ann. d. Pliys. (4) 15 p. 9Ü6, 

 1901. Comp. also G. Klein, Disscitation Techn. Hochsch. Berlin, 1909. 



