PYROMETER, an instrument for mea. suring the expansion of bodies by heat. The whole art in forming an instrument, adapted to this purpose, is so as to ren der it capable of showing very small ex pansions of solid bodies. Different instill- ments have. been invented for this pur pose ; of the greater number of which it is scarcely necessary to give a detailed account. The difficulty of contriving an unexceptionable instrument of this kind has arisen partly from the difficulty of -• finding a substance not liable to be alter ed by a high temperature, and which shall suffer a change of volume sufficiently perceptible to be accurately measured ; and partly from that of finding a measure, which shall not itself be affected by the high temperature, and be, at the same time, sufficiently delicate.
The pyrometer, in which, perhaps, these difficulties have been most effectu ally surmounted, and which has come in to most general use, is that invented by • the late Mr. Wedgwood. The pure earth, named alumina, and the different earths, (the clays) in which it predomi nates, have the singular property of not expanding, but of contracting by heat. This contraction begins to become evi dent, when the clay is raised to a red heat, it continues to proceed until it vi- • trifles, and the total contraction, in pure clays, exceeds considerably one-fourth part ofthe volume in every direction. It ' occurred to Mr. Wedgwood, that from this property it might be employed in the construction of a pyrometer. The contraction that the clay suffers is per-, manent, or it does not return to its for mer dimensions when cold. The degree of contraction it has suffered, therefore, can be ascertained without any source of fallacy, and will indicate the extreme of temperature to which it has been expos ed.
This pyrometer consists of a gauge, composed of two straight pieces of brass, twenty-four inches long, divided into inches and tenths, and fixed in a brass plate, so as to converge ; the space be tween them, at the one extremity, being five-tenths of an inch, and at the other three-tenths. The pyrometrical pieces
of clay are small cylinders, flattened on one side, made in a mould, so as to he adapted exactly to the wider end. It is evident that, in exposing one of these pieces to a high temperature, the can traction it has suffered may be measured, by the length to which it can be slid into the converging groove or gauge.
The utility of this instrument, it was obvious, would be much increased by connecting it with the mercurial ther mometer, and by ascertaining the pro portion between the degrees of each ; and this was done by Mr. Wedgwood. The scale of his pyrometer commences at red-heat fully visible in day-light. The mercurial thermometer cannot easily measure any temperature above 500° or 55u°; and hence, between the termina tion of the scale in the one, and its com mencement in the other, there is a range of temperature requiring to be measur ed. This Mr Wedgwood did, by the ex pansions of a square piece of silver, mea sured in a gauge of earthen-ware, con structed in the same way as his pyrome ter ; and by the same method, he found out the proportion between each degree of his scale, and that of any of the usual thermometrical scales. Each degree of his pyrometer lie found to be equal to 130° of Fahrenheit. The commencement of his scale, or the point marked 0, cor responds with 1077P of Fahrenheit's scale. From these data, it is easy to re duce either to the other, through their whole range. The scale of Wedgwood includes an extent of temperature equal to about 32,000° of Fahrenheit, or 54 times as much as that between the freezing and boiling points of mercury. Its commencement, as has been stated, is at of Fahrenheit, or red-heat fully visible in daylight; its extremity is 240°; but the highest heat that he measured with it is 160°, or 21,877° of Fahrenheit ; being the temperature of a small air furnace, and 30° degrees of his scale above the point at which cast-iron melts.