BRIDGMAN. — MEASUREMENT OF HYDROSTATIC PRESSURES. 335 



This change of gauge constant with time demands that frequent 

 calibration be made. In all the work of the following papers in which 

 considerable accuracy was desired, such calibrations were made every 

 few days. It is not much trouble to make this calibration. The 

 twenty-two readings with increasing and decreasing pressures can be 

 made in the course of half an hour. When the gauge is so calibrated, 

 and the procedure for avoiding hysteresis is adopted, the readings are 

 consistent to the limit of sensitiveness. This has already been stated 

 to be about 8 kgm./cm.l 



The gauge in actual use has shown itself very convenient. The 

 readings may be made rapidly and immediately after the application 

 of pressure, since there are no thermal effects such as dissipation of 

 heat of compression. The temperature effect is so small that it may 

 be used directly in the room without a thermostat, and the leak is so 

 slight that it "may be used in entire comfort in measuring many high- 

 pressure effects. Where applicable, the gauge is more convenient than 

 the electrical resistance gauge and it has been used whenever possible. 



The Manganin Resistance Gauge. 



In the earlier paper a method was described for measuring high 

 pressures by measuring the electrical resistance of pure mercury under 

 pressure. The method had the advantage of being perfectly repro- 

 ducible, so that any one could at any time measure pressure without 

 recourse to the then inconvenient fundamental standard of pressure. 

 The advantage has been in large part offset by the devising of the 

 convenient form of absolute gauge described in the first part of this 

 paper. Moreover, the method becomes inapplicable at somewhat 

 higher pressures than those reached formerly because of the freez- 

 ing of the mercury. Thus at 0°, the freezing pressure is about 7500 

 kgm./cm.^. Aside from this difficulty, which might be avoided by 

 placing the mercury in a separate vessel, maintained at higher tem- 

 perature, there are numerous inconveniences of manipulation, as, for 

 instance, that the mercury must always be kept in an upright position. 

 But the greatest inconvenience of all is that the glass capillary con- 

 taining the mercury is always shattered by an explosion, and explosions 

 become more and more frequent at high pressures. 



There are frequently situations, however, where the absolute gauge 

 becomes unavailable, and where a gauge with some of the properties 

 of the mercury gauge becomes desirable. For instance, it is often 

 necessary to secure absolute freedom from leak, and this is obviously 

 impossible with a freely moving piston as in the absolute gauge. 



