Thermometer 639

A Thermometer of Contact has lately been invented by M. Fourrier "It is well known," says M. Fourrier, "that on touching different substances maintained at the same temperature, the same calorific impression is not received in consequence of the different conductibility of those bodies. It is even sufficient to cover those bodies with a thin sheet of paper, sensibly to change the effect of the contact. If, then, on a support kept at a constant temperature, for example, at that of melting ice, thin sheets of different substances are successively applied, the simple contact of the naked hand will suffice to class a great number of them according to their order of conductibility; but this method is by no means accurate, and is liable to other inconveniences. M. Fourrier's instrument may be considered as an improved hand, and minutely establishes the facts to which the application of the hand only makes an approximation. It is extremely simple; it consists of a cone of very thin iron, filled with mercury, and terminated at its circular base by a skin of moderate thickness. A thermometer is placed in the mercury; it is this skin which is put on the thin sheet applied to the support.

The contact is very intimate, in consequence of its flexibility; and the thermometer indicates the variations of temperature. By this instrument many curious facts have already been ascertained. For instance, it has been shown, that the order in which thin sheets of different substances are placed one upon another, influences the quantity of heat which passes through them under the same external circumstances. Thus, the interposition of a sheet of leather facilitates the transmission of heat from skin to cloth, and it obstructs it from cloth to marble.

The thermometers hitherto described are very limited in their extent; they indeed point out to us the lowest degrees of heat which are commonly observed even in cold climates, but they by no means reach to those degrees of heat which are very familiar to us. The mercurial thermometer extends no farther than to 600 of Fahrenheit's scale, the heat of boiling mercury; but we are sure that the heat of solid bodies, when heated to ignition, or till they emit light, far exceeds the heat of boiling mercury. In order to remedy this defect, Sir Isaac Newton attempted, by an ingenious experiment, to extend the scale to any degree required. His plan, however, was not found convenient for practical purposes. Hut following the idea suggested by Newton, the late Mr. Wedgwood invented a very simple thermometer, which marks with much precision the different degrees of ignition from a dull red heat visible in the dark, to the heat of an air-furnace.

a b c d, in the annexed figure, is a smooth flat plate; on which are fixed two rulers, or flat pieces, a quarter of an inch thick, lying flat upon the plate. with the sides that are towards one another made perfectly true, a little further asunder at one end, than at the other end; thus they include between them a long converging canal, which is divided on one side into a number of small equal parts, and which may be considered as performing the offices both of the tube and scale of the common thermometer. It is obvious, that if a body, so adjusted as to fit exactly at the wider end of this canal, be afterwards diminished in its bulk by fire, as the thermometer pieces are, it will then pass further into the canal, and more and more so according as the diminution is greater; and conversely, that if a body so adjusted as to pass on to the narrow end, be afterwards expanded by fire, as is the case with metals, and applied in that expanded state to the scale, it will not pass so far; and that the divisions on the side will be the measures of the expansions of the one, as of the contractions of the other, reckoning in both cases from that point to which the body was adjusted at first. i is the body whose alteration of bulk is thus to be measured.

This is to be gently pushed or slid along towards the narrow end till it is stopped by the converging sides of the canal. Mr. Walker, to whom we have already alluded, suggests the idea of a metallic thermometer which shall embrace the medium between the highest point of the mercurial thermometer, which terminates at 600 degrees, and the lowest of Wedgwood's, just described, which commences at 1077 degrees of Fahrenheit A metallic composition is formed, not liable to alteration in its quality or quantityby epeated exposure to heat, the melting point of which .sat a little below 600 degreeJof Fahrenheit, and its boiling point at 1200 degrees. A case resembling inform the glass case for the ordinary thermometer, but somewhat larger, contains the metallic composition, and the scale consists in a slender graduated rod, equal in height at the commencement of the scale; that is, when the metallic composition is just liquid to the top of the tube, the graduated rod terminating at the bottom in a thin, circular, flat plate, which rests or floats as it were upon the liquid metal; and in proportion as the latter expands and rises in the tube by heat, the graduated rod is buoyed up, or raised above the top of the tube, passing through a perforated cover to the maximum, or boiling point.

The thermometer case and graduated rod are formed of pipe-makers' clay, previously prepared by having been exposed to a sufficient degree of heat. The scale of this new thermometer is an exact continuation of the scale in the mercurial thermometers; the lower degrees of the former corresponding with, or indicating like, temperatures with the upper degrees of the mercurial thermometer.

Thermometer 640

"The same principle," says Mr. Walker, "I might observe, admits of being extended, for the purpose of ascertaining the variation in temperature, up to the most intense heat, perhaps, that can be required. It is unnecessary to state here, that the influence of the incumbent atmosphere upon the surface of the liquid metal within the open tube is too inconsiderable, even at the commencement of the scale, to deserve notice, and at a higher temperature diminishes to nothing; especially if the whole of the liquid contained in the thermometer, as ought to be the case in every thermometer, be completely immersed, or subjected to the temperature, the degree of which it is intended to indicate. A method similar to the above, I should think, might be applicable to the purpose of showing, in a ready way, the degree of expansion in metals by heat; but the elongation of a cylinder of any metal, by increase of temperature, is much too small to admit of its being a convenient measure of temperature. I should not despair, however, availing myself of every advantage, viz., increasing the length of a metallic wire, by giving it a spiral form, in order to comprise a considerable length in small compass, with the application of the lever-index, and a good magnifier, upon constructing a thermometer upon this principle, so as to render the scale apparent even to single degrees; using silver for the lower temperatures, and platina for the higher, or employing iron wire, only up to its ultimate point of expansion in a solid state.