BRIDGMAN. — A SIMPLE PRIMARY GAUGE. 215 



gauge. With the former gauge the leak was almost imperceptible 

 after pressure had been kept at 7000 kgm. for an hour. It is a curious 

 fact that the leak around the more loosely fitting piston was distinctly 

 most rapid at 2000 kgm. The decreased leak at higher pressures may 

 probably be taken as proof of the efficiency of the application of 

 pressure to the outside of the cylinder in decreasing the size of the 

 crack, although there is a slight possibility that the effect is due to 

 increased viscosity of the molasses under pressure. 



With this calibration, the critical examination of the behavior of 

 the gauges might have been terminated, because the simplicity of the 

 construction is such as to make improbable any error in their use. 

 As a matter of fact, the indications of the various types of gauge de- 

 scribed above have usually been accepted at their face value, without 

 comparing with any other absolute gauge. There were means at hand 

 in the present case, however, of so easily comparing the one gauge with 

 the other that it seemed worth while doing. The method adopted was 

 an indirect one, depending on the secondary mercury gauge described 

 in the second part of this paper. It had been found from a great many 

 preliminary comparisons of different mercury gauges that the indica- 

 tions of the mercury gauge were constant, giving a trustworthy meas- 

 urement of pressure, if once the calibration with a primary gauge 

 could be effected. More detailed proof of this statement will be found 

 in the second part. The two absolute gauges described above were, 

 therefore, compared at different times against the same mercury gauge, 

 and the two sets of readings compared. 



The results of the comparisons are shown in Table I. Gauge I was 

 compared twice with the mercury resistance, and Gauge II once. 

 Each number entered in the table is the mean of two or four readings 

 made at increasing or decreasing pressures. The agreement of the 

 two readings under increasing or decreasing pressure, as also of the 

 readings of Guage I on two separate occasions, was as close as it was 

 possible to make the measurements of change of resistance, and, 

 therefore, only averages have been tabulated. The change of resist- 

 ance could be read to one part in 3000, at the maximum pressure. 

 The average divergence of the readings of either gauge from the mean 

 is well under T V per cent. The readings of Gauge II are consistently 

 higher than those of Gauge I, a discrepancy which would point to a 

 slight error in determining the effective area of the pistons. The dis- 

 crepancies also show a tendency to become larger at the higher pres- 

 sures. This is probably no fault of the gauges themselves, but may be 

 due to the increased difficulty of making fine adjustments of pressure 

 at the higher values. The method of procedure was to apply a known 



