As soon as any one of the rings
begins to soften under the heat, it is squeezed together by the pressure,
and, as it melts, it is completely squeezed out and disappears. The rod is
then made to rise by the thickness of the melted ring, and a simple
apparatus shows at any moment the number of rings which have melted, and
therefore the temperature which has been attained. This instrument cannot
be used to follow variations of temperature, but indicates clearly the
moment when a particular temperature is attained. It is of course entirely
dependent on the accuracy with which the melting-points of the various
alloys have been fixed.
Yet another principle is involved in the instrument called the
thalpotasimeter, which may be used either with ether, water, or mercury.
It is based on the principle that the pressure of any saturated vapor
corresponds to its temperature. The instrument consists of a tube of metal
partly filled with liquid, which is exposed to the medium which is to be
measured. A metallic pressure gauge is connected with the tube, and
indicates the pressure existing within it at any moment. By graduating the
face of the gauge when the instrument is at known temperatures, the
temperature can be read off directly from the position of the needle.
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