Tremeschini’s pyrometer is based on a different principle, viz., on the expansion of a thin plate of platinum, which is heated by a mass of metal previously raised to the temperature of the medium.  The exact arrangements are difficult to describe without the aid of drawings, but the result is to measure the difference of temperature between the medium to be tested and the atmosphere at the position of the instrument.  The whole apparatus is simple, compact, and easy to manage, and its indications appear to be correct at least up to 800 deg.C.

The Trampler pyrometer is based upon the difference in the coefficients of dilatation for iron and graphite, that of the latter being about two-thirds that of the former.  There is an iron tube containing a stick of hard graphite.  This is placed in the medium to be examined, and both lengthen under the heat, but the iron the most of the two.  At the top of the stick of graphite is a metal cap carrying a knife-edge, on which rests a bent lever pressed down upon it by a light spring.  A fine chain attached to the long arm of this lever is wound upon a small pulley; a larger pulley on the same axis has wound upon it a second chain, which actuates a third pulley on the axis of the indicating needle.  In this way the relative dilatation of the graphite is sufficiently magnified to be easily visible.

A somewhat similar instrument is the Gauntlett pyrometer, which is largely used in the north of England.  Here the instrument is partly of iron, partly of fireclay, and the difference in the expansion of the two materials is caused to act by a system of springs upon a needle revolving upon a dial.

The Ducomet pyrometer is on a very different principle, and only applicable to rough determinations.  It consists of a series of rings made of alloys which have slightly different melting-points.  These are strung upon a rod, which is pushed into the medium to be measured, and are pressed together by a spiral spring.  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.  From 100 deg. to 220 deg.F. ether is the liquid used, from thence to 680 deg. it is water, and above the latter temperature mercury is employed.