PYROMETER

(Yunanca köken — orijinale bakınız.) An instrument for measuring the degrees of heat. According to this definition, the pyrometer is synonymous with thermometer;but though the two terms are sometimes applied Indifferently to the same instrument, the term pyrometer is generally understood to denote either an instrument intended to measure higher temperatures than can be me as ured by the ordinary thermometer, or an instrument for comparing the expansions of different metals. Various contrivances have been employed for the above purposes. Mu chenbroek, the original inventor of the pyrometer, adopted the following method: A prismatic rod (about six inches long) of the metal under trial being attached at one extremity to an immoveable obstacle, and heated by lamps, the other end is nece arily pushed forward;and this being fastened to the end of a rack playing into a pinion, communicates a revolving motion to an axle to which a train of wheel work is attached, whereby the minutest expansion of the heated bar is rendered sensible, and measured by an index on a dial. The principle of this apparatus is sufficiently simple; but the uncertainty attending the motion of so many loosely connected wheels and pinions must have rendered its indications of little value; and the method is liable to a still more serious objection, namely, that the temperature communicated to the bar by the lamps is entirely unknown. Desaguliers, and afterwards Ellicott, made several improvements in the construction of the instrument, tending to give it a more equable motion and to increase its delicacy. Graham substituted a micrometer screw for the wheels and levers that bad formerly been employed; and on this principle Mr. Smeaton contrived an ingenious vol xlviii. apparatus, which is described in the Phil. Trans, [s. 1021]