BRIDGMAN. — A SIMPLE PRIMARY GAUGE. 217 



the sensitiveness of the gauge. It was found that throughout the 

 entire pressure range the pistons would respond to differences of 

 pressure that could not be detected by the change of electrical resist- 

 ance. At 7000 kgm., therefore, the gauges remain sensitive to at least 

 2 kgm. per sq. cm. The continued sensitiveness of the piston with the 

 crack only 0.0001 in. furnishes an argument against the maximum 

 value set, in the discussion above, on the distortion of the cylinder. 

 For, if we accept the above maximum, we shall find that at 7000 the 

 crack must decrease 0.00018 in., or in this case completely close up. 

 There cannot well be an error of this magnitude in the micrometer 

 measurement of the diameter, and the probable correctness of the 

 average value of the distortion used above is thus increased. 



Conclusion. 



In this first part of the present paper a description has been given of 

 an absolute gauge, so designed that leak does not become trouble- 

 some, at least to G800 kgm. per sq. cm. The various corrections to 

 be applied have been discussed, and the method by which the dimen- 

 sions were determined has been described. From a comparison of 

 two gauges of this type with one of another type, the probable accu- 

 racy of the gauge is estimated to be at least -^ per cent, and the sensi- 

 tiveness, 2 kgm. per sq. cm., at 7000 kgm. per sq. cm. 



