12 



NEW METHOD FOR DETERMINING COMPRESSIBILITY 



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jacket. It was filled with the liquid metal, and the change in volume 

 for different pressures was measured very simply by placing the whole 

 jacket under the liquid in the Cailletet barrel, adding successive weighed 

 portions of mercury, and noting each time the pressure 

 needed just to break and then again make the electrical 

 connection between the meniscus and the platinum 

 point. The electrical method of indication has often 

 been used for similar purposes, especially by Barus and 

 Amagat ; but never in exactly this way. If the plati- 

 num wire is very finely pointed, the fine tube around it 

 about 1.5 mm. in diameter and the meniscus perfectly 

 clean, the indications of this instrument are surpris- 

 ingly constant and trustworthy. Even with a substance 

 no more compressible than mercury it is easy to be 

 certain of the necessary pressure within one atmos- 

 phere a very small fractional error in many hundred 

 atmospheres. The pressure at which the connection 

 was made was taken as the true point, rather than that 

 at which the connection was broken, since there is 

 sometimes a slight adhesion between the point and the 

 mercury under the last named circumstances. Often, 

 however, the making and breaking occurred within an 

 atmosphere's pressure of one another. 



If the fine tube is larger than 1.5 mm., the sensibility 

 of the instrument is reduced ; if it is much less than 

 1.5 mm., drops of mercury are likely to be caught and 

 held by the wire. 



The most serious possible cause of error arises, how- 

 ever, from the faulty fitting of the ground stopper of 

 the glass jacket. If a poorly ground stopper be used, 

 the mercury during the process of compression is forced 

 into the tiny interstices between stopper and tube a 

 complication which makes the compressibility of the 

 liquid seem slightly greater than it is. This difficulty 

 may be obviated wholly by always wetting the ground 

 surfaces with a minute drop of water or some other 

 liquid, thus displacing all the air, and preventing the 

 ingress of mercury. The infinitesimal variations in the compression 

 of this practically constant drop of lubricating liquid are quite too 

 small to produce any perceptible effect, and successive trials always 

 yield the same result. 



Fig. 2. 



