PYROMETER. A machine contrived to measure the expansion of metals, and other bodies, occasioned by heat. Muschenbroeck was the original inven tor of the pyrometer; the nature and construction of his instrument may be understood from the following account.—If' we suppose a small bar of metal, twelve or fifteen inches in length, made fast at one of its extremities, it is obvious that if it be dilated by heat it will become lengthened, and its other extremity will be pushed forwards. If this extremity then be fixed to the end of a lever, the other end of which is furnished with a pinion adapted to a wheel, and if this wheel move a second pinion, the latter a third, and so on, it will be evident that ,by multiplying wheels and pinions in this manner, the last will have a very sensible motion ; so that the moveable extremity of the small bar cannot pass over the hundredth or thousandth part of a line, without a point of the circumference of the last wheel passing over several inches. If this circumference then have teeth fitted into a pinion, to which an index is attached, this index will make several revolutions, when the dilation of the bar amounts only to a quantity altogether insensible. The portions of this revolution may be measured on a dial plate, divided into equal parts ; and by means of the ratios which the wheels bear to the pinions, the absolute quantity which a cer tain degree of heat may have expanded the small bar can be ascertained ; or conversely, by the dilatation of the small bar, the degree of heat which has been applied to it may be determined. Such is the construction of Muschenbroeck's pyrometer. It is necessary to observe, that a small cup is adapted to the machine, in order to receive the liquid or fused matters subjected to experiment, and in which the bar to be tried is immersed. When it is required to measure by this instrument a considerable degree of heat, such as that of boiling oil or fused metal, fill the cup with the matter to be tried, and immerse the bar of iron into it. The dilatation of the bar, indicated by the index, will point out the degree of heat it has assumed, and which must necessarily be equal to that of the matter into which it is immersed. This machine evidently serves to determine the ratio of the dilatation of metals, &c.; for by substituting in the room of the pyrometric bar other metallic bars of the same length, and then exposing them to an equal degree of heat, the ratios of their dilatation will be shown by the motion of the index.
The most celebrated instrument for measuring very high temperatures, is that invented by the late Mr. Wedgwood, founded on the principle, that clay pro gressively contracts in its dimensions, as it is progressively exposed to higher degrees of heat. He formed white porcelain clay into small cylindrical pieces, in a mould, which, when they were baked in a dull red heat, just fitted into the opening of two brass bars, fixed to a brass plate, so as to form a tapering space between them. This space is graduated, and the farther the pyrometric guage
can enter the greater heat does it indicate. The limits of the converging scale are five-tenths of an inch at the beginning towards the opening, and three-tenths at the end, or towards the line to which the bars converge. The next thing to be done, was to ascertain and establish a connexion between the indications of his instrument and those of the mercurial thermometer; to accomplish this, he employed a heated rod of silver, of which he measured the expansion. The clay piece and the silver rod were heated in a muffle, and as soon as the muffle indicated a low red heat it was drawn forward towards the door of the oven ; and its own door being then nimbly opened by an assistant, Mr. Wedgwood pushed the silver rod as far as it would go. But as the division to which it reached could not be distinguished in that ignited state, the muffle was steadily and cautiously lifted out, and left to cool. When the muffle was sufficiently cold to be examined, the degree of expansion at which the silver stood was carefully noted, and the degree of heat shown by the clay pieces was measured by their own guage ; after which, the whole was returned into the oven, and exposed to a more intense heat, in order to obtain another point of correspondence between the two scales , the graduated silver rod serving as an intermediate scale, with which Wedgwood's and Fahrenheit's might be readily compared. The first of these points of correspondence was 2*0 of Wedgwood's, to 1370° of Fahren heit's; the second was 6i° of Wedgwood's, to 1890° of Fahrenheit's. Hence, because 6-1 — 2i = 4, and 1890 — 1370 = 520, it appears, that an interval of 4 degrees on Mr. Wedgwood's instrument is equivalent to 520 degrees on that of Mr. Fahrenheit's, and consequently, that I degree of the former equals 130 of the latter, and the zero (or o) on Wedgwood's scale corresponds with 1077 and a fraction on Fahrenheit's. Hence, we have the means of reducing the degrees at any point of one scale, to the corresponding degrees on the other, through the entire range. Mr. Wedgwood's instrument includes an extent of about 31,200 of Fahrenheit's, or about 50 times that between the freezing and boiling points of by which points the performances of mercurial thermometers are necessarily limited. Also, if we conceive Mr. Wedgwood's scale to be extended downwards below his zero, as Fahrenheit's is supposed to extend upwards above the boiling point of mercury, the freezing point of water will fall on 8°.421, or somewhat above 81° below the zero of Wedgwood's scale, and that of mercury on 8°.596, or a little below 810; so that the distance between the freezing points of mercury and water is an inter val of .175 of a degree on Wedgwood's scale; 8° and a decimal from the freezing point of water to complete ignition ; and 160° is the highest point or degree of heat to which our ingenious philosopher was able to extend his observations.