BRIDGMAN. — MERCURY UNDER PRESSURE. 



355 



by tightly forcing the narrow edge C on the upper part B into the flat 

 seat at D. It is not desirable to attempt to use any packing material. 

 At the lower part of B there is a very fine channel, E, opening into A. 

 This channel is made by carefully drilling in B a 1/16 inch hole and 

 then plugging the hole with a pin along which a fine scratch has been 

 made. This fine scratch takes the 

 place of the glass capillary when the 

 piezometer is made of glass. It is 

 thus possible to regulate the size of the 

 channel at pleasure. It is a matter 

 of the greatest ease to make such a 

 channel that a pressure of only a few 

 pounds to the square inch will force 

 mercury through it in drops just 

 barely perceptible to the eye, and 

 much too small to be measured by 

 any ordinary balance. In all the work 

 with these piezometers no effect was 

 ever found suggesting in any way the 

 possibility of error introduced by cling- 

 ing of mercury to the mouth of the 

 channel. 



The piezometer may be used in either 

 of two positions, upright or inverted. 

 When used upright, the body A is 

 filled with the liquid to be investigated, 

 water for example, and the cup at B 

 with mercury. The entire piezometer 

 is then placed in a pressure chamber 

 surrounded on all sides by a liquid, to 

 which pressure is applied. This takes 

 the place of lowering into the sea in 



Aim^'s experiment. Mercury flows in through the narrow channel to 

 equalize the pressure within and without, and drops to the bottom. 

 On release of pressure, the water bubbles out through the mercury in B. 

 The piezometer is then unscrewed, the mercury in A weighed, and the 

 total change of volume at the maximum pressure calculated. Into this 

 calculation enter the compressibility of the steel of the piezometer and 

 the compressibility of the mercury. The latter can then be found in 

 terms of the compressibility of water by nearly filling the piezometer 

 with mercury on which floats a little water to fill the piezometer 

 completely. On application of pressure mercury drops through the 

 layer of water, and on release water comes out. 



Figure 1. The steel piezom- 

 eter. This is filled with the Hquid 

 under investigation and exposed 

 to hydrostatic pressure all over. 

 The pressure forces a measurable 

 quantity of mercury into the 

 chamber A through the fine chan- 

 nel at E, and from this quantity 

 of mercury the change of volume 

 of the liquid originally filling A 

 may be found. 



