[ 400 ] 



XL VII. A Comparison of the Bourdon, the Tait, and the Amagat 

 High-Pressure Gauges. By C. Bakus*. 



1. PRELIMINARY. — In my earlier work f I was obliged 

 •*- to content myself with empiric pressure measure- 

 ment. I found, by comparing different Tait gauges of my 

 own with a large Bourdon gauge, that so long as pressure 

 continually increased from zero, the relation between the indi- 

 cations of each under like conditions of pressure was linear ; 

 but that when pressure again decreased from a high value (1000 

 atmospheres or more), this relation changed to a markedly 

 curvilinear locus, terminating, however, in the original zero J. 

 I inferred that so long as increasing pressures alone were 

 applied, both gauges could be used with safety. The present 

 work, in which the two gauges are directly compared with 

 Amagat's " manometre a pistons libres," corroborated this in- 

 ference, and shows that the bow-shaped cycles represent the 

 actual motion of the free end of the Bourdon tube. From 

 this point of view the data are important, for they supply an 

 example of purely mechanical hysteresis, and possibly of 

 metallic volume-lag. The data also show that cj^clic change 

 is absent in the Tait gauge, and indicate a way in which this 

 instrument may be adapted for the precise measurement of 

 pressures of any value whatever. 



2. Amagat 's Manometre. — The instrument § in possession 

 of the Physical Laboratory, U.S.G.S., is constructed for 

 measuring pressures as high as 3000 atmospheres. At this 

 limit the height of the compensating column of mercury (the 

 diameters of the two pistons being '549 centim. and 12*171 

 centim. respectively) will be about 464 centim. To read so 

 long a column with reasonable facility, I fixed a painter's 

 ladder in a vertical position, and permanently attached to one 

 side of it both the manometer-tube, and a millimetre-scale 

 which I ruled with care on a sufficiently long strip of brass. 

 A round leather belt, passing from top to bottom of the 

 ladder, and everywhere within easy reach, was suitably con- 

 nected with the mechanism for rotating the two pistons. I 

 was thus able to give the two pistons the rotational movement 

 immediately before taking the gauge-reading, no matter what 

 my position on the ladder might be. 



I found by trial, that by using the thick mineral machine- 



* Communicated by the Author. 



t This Magazine, [5] xxx. p. 338 (1890). 



% Ibid, plate x. The zero is not quite regained. 



§ Marked " E. H. Amagat, Lyon, 1890." 



