BRIDGMAN. — A SECONDARY MERCURY RESISTANCE 'GAUGE. 225 



De Forest Palmer, 2 working with the high pressure apparatus of 

 Barus, made measurements of the electrical resistance of mercury up 

 to 2000 kgm., and suggested it as a suitable secondary standard. He 

 gives data from which the pressure can be calculated if the change of 

 resistance is known. It appears from his work that the pressure co- 

 efficient is large enough to make accurate measurements of the change 

 of resistance easy. The additional advantage of presumable repro- 

 ducibility made it seem worth while to examine with some care its 

 suitability as a secondary standard. The conclusion reached is that 

 with ordinary care the mercury resistance gauge is good to about 

 T \>- per cent. 



In order to attain this probable degree of accuracy, however, it was 

 necessary to examine several minor points with somewhat greater 

 detail than de Forest Palmer found necessary for the purpose of his 

 work. The probable error in de Forest Palmer's work was -fa per cent 

 on the total resistance, which means an error of 1.5 per cent on the 

 pressure at 2000 kgm. The percentage error at lower pressures is of 

 course proportionally greater. Within these limits of error he found 

 the pressure coefficient to be constant. Furthermore, the mercury 

 was placed in a capillary of some glass not specified, so that the data 

 given will not apply to other mercury gauges with a greater degree of 

 accuracy than the possible error introduced by variations in the com- 

 pressibility of the glass. It is known that different grades of glass may 

 differ in compressibility by as much as 100 per cent. 



In fact, this matter of the glass containing vessel proved to be the 

 chief source of possible error. Pure mercury may with confidence be 

 assumed to be perfectly reproducible, and since internal strains can- 

 not be set up in it, to be also perfectly free from hysteresis. The glass, 

 however, is a solid in which it is particularly difficult to get rid of in- 

 ternal strains. It cannot be assumed, therefore, that a pure hydrostatic 

 pressure will not produce hysteresis, or even set analogous to the 

 volume set shown in thermometers after exposure to changes of tem- 

 perature. It is an advantage, however, that the total effect of the 

 glass envelope is unusually small, both because of the comparative 

 largeness of the pressure effect on the resistance of the mercury, and 

 because the correction factor is only ^ instead of the whole of the 

 compressibility. This latter fact is due to the simultaneous shortening 

 of the capillary which contains the mercury, and the decrease of the 

 bore, the one resulting in an increase of resistance and the other in a 

 decrease. The total correction on the observed change of resistance 



2 de Forest Palmer, Amer. Jour. Sci., 4, 1-9 (1897), and 6, 451 (1898). 

 vol. xliv. — 15 



