. Carnegie Institution of Washington publication. 12 NEW METHOD FOR DETERMINING COMPRESSIBILITY \ FT. 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 ne
. Carnegie Institution of Washington publication. 12 NEW METHOD FOR DETERMINING COMPRESSIBILITY \ FT. 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 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 mm., the sensibility of the instrument is reduced; if it is much less than 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
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