PYROMETER (Gr. pyr, fire, and matron, a measure) is a term originally applied by Muschenbroek in 1731, to an instrument which he invented for measuring the changes produced in the dimensions of solid bodies by the application of heat. It is, however, now applied to any instrument the object of which is to measure all gradations of tem perature above those that can be indicated by the mercurial tliermonleter (q.v.). Desa guliers gives a description of Muselienbroek's instrument, as improved by himself, in 1119. Etperimental Philosophy. Numerous pyrometers have since been invented, among which may be noticed those of Ellicott (described in The Philosophical Transactions for 1730 and 1751), Graham (in do. for 1754), Wedgwood (in do. for 1782. 1784, and 1780), and Guyton (in the Annales de Mink, tome 40). None of these instruments, however, gave accurate results for very high temperatures; and it was not till the year 1821 that prof. Daniell announced the invention of his pyrometer, which has supplanted all others, and for which, in an approved form, lie received the Rumford medal from the royal society. It consists of two distinct parts, the register (1) and the scale (2). The register is a solid bar of black-lead earthenware, A, 8 in. long, cut out of a common black-lead crucible. In. the axis of this, a hole is drilled, reaching from one end of the bar to within half an inch of the other extremity; and in this cylindrical cavity a bar aa, of metal (as platinum or iron, for example) is placed. A cylindrical piece of porcelain, cc, sufficiently long to project a short distance beyond the extremity of the black-lead bar, is placed on the top. of the metallic bar. This is termed the index, and it is kept firmly in its position by a. ring or strap of platinum, d, which is.tightened by a wedge of porcelain, e. When the. register is exposed to a high temperature, the expansion of the metallic rod, aa, forces. the index forward; and when the register has afterward cooled, the tension of the strap will retain the index at the furthest point to which it has been protruded. The scale (2)• consists of a frame composed of two rectangular plates of brass, f, g, joined together by their edges at a right angle, and fitting square upon two sides of the register. .Near the
end of this frame is a small brass plate, Is, which projects at a right angle. To the extremity of the frame nearest the brass plate is attached a movable arm, D, round a fixed center, i, and at its free end carrying the arc of a circle, E, the radius of which is 5 in., and, which is accurately graduated into degrees and thirds of a degree. Upon this arm, at the center, k, another lighter arm, C, is made to turn. carrying at its. longer part a vernier (q.v.), II, which moves on the face of the arc, and divides it into minutes, together with au eye-glass, 1, to assist the reading; while the shorter part termi nates in a knife-edge in, turned inwards at a right angle.
To use the instrument, the scale is carefully applied, the brass plate, h, being pressed upon the shoulder of the register, and the lighter arm being so placed that the steel point, m, may rest on the top of the index in a notch cut for it which coincides with the axis of the rod. The position of the index being then read off on the scale, the register is detached and exposed to the heat to be measured; after it is removed and cooled, it is again placed in the scale, and the new position of the index read off; the difference of the two readings determining the expansion of the metallic bar above that of the black lead. In order to employ the instrument as a measure of temperature as well as of expan sion, prof. Daniell adopted the doubtful assumption that equal increments of length are the effects of equal increments of temperature. For further information on this instru ment and its uses, we must refer to the original memoir in the Philosophical Transactions for 1830-31.
In the great exhibition of 1851, Mr. Ericsson exhibited in the United States' depart ment a pyrometer in which temperatures were indicated by the tension of a permanent volume of air or of nitrogen gas, which was measured by the reading of a column of mercury under a vacuum. For a description of the instrument, we must refer to the jury report. M. Edmond Becquerel published in 1864 a very complete essay on pyrom etry in the Annales du Conservatoire.