475 



become more an more difficnlt as the pressure rises, until the 

 above investigation should liave been carried out. 



2. Experimental method. A simultaneous reading of the open 

 gauge and the pressure-balance turned out to be practically impos- 

 sible. In fact in the pressure-balance the pressure in the oil-passages 

 IS only constant, while the piston with its weights is turning freely, 

 and this motion does not continue longer than a few minutes at 

 the utmost; when the rotation has come to a stop and the piston 

 is again set in motion by hand, Ihei'e are always, however carefully 

 the operation is conducted, small vertical forces exerted on the piston 

 which are propagated in the tubes as pressure-impulses and disturb 

 tlie pressure-equilibrium. On the other hand the various readings on 

 the open gauge require much more time than the two or three 

 minutes which the pressure-balance, while left to itself, allows; in 

 fact, when all the tubes are at the proper pressure, a coin|)lete 

 reading carried out by two coopei'ating observers requires al)out 

 three quarters of an hour. 



A simultaneous reading of pressure-balance and open gauge being 

 therefore attented with practically unsiirmountable difticulties, we 

 resolved to carry out the comparison through the intermediary of 

 the two closed hydrogen-manometers of the Leiden- Laboratory 

 M„ and il/j^o, the former of which has a range from 20 to 60 

 atmospheres, the latter from 60 to J 20. We already will mention 

 here, that this procedure did not impair the accuracy aimed at in 

 any respect, as will moreover appear from the discussion in the 

 next section. 



The accompanying plate shows the open manometer (^>. J/., the two 

 closed manometers M^„ and il/,„ and the pressure-balance D. B. 

 with its oil-forcing pump (>. P. besides the connections and stop- 

 cocks by which the various apparatus are joined up together. The 

 construction and method of working of the various gauges having 

 been repeatedly described and represented need not be gone into 

 on this occasion '). 



A small complication arose in connection with the transmission 



1) Open manometer: H. Kamerlingh Onnes. These Proceedings I. p. 213. 1898. 

 Comm. 44, and J. G Schalkwijk Dissertation, Amsterdam 190->. H. Kamerlingh 

 Onnes, C. Dorsman and G. Holst These Proc. Supra Comm. 146a. Manometer 

 ifj,: H. Kamerlingh Onnes and H. H. F. Hyndman, These Proceedings 4 p. 7f)l. 

 1902, Comm. 78. § IT, H. Khmerlingh Onnes and G. Bkaak. These Proceedings 

 9 p. 754. 1!'06. Gomm. 97a, § 3. Manometer ilfi-;,, : These Proceedings Supra. 

 Comm. l4Ga, 146b. Pressure-balance: Ph. Kohnstamm and K. VV. Walstra j.i. c.c. 



31 



Proceedings Royal Acad. Amsterdam. Vol. XVIII. 



