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PYROMETER (literally,' fire-measurer,' from TrSp,' fire,' and pirpov,' a measure'). No fluid has hitherto been found applicable to the construction of thermometers capable of indicating higher temperatures than that of boiling mercury (about 650" tahr.). The term pyrometer was first employed by Musschenbrock to designate an instrument invented by him for measuring the effects produced in the dimensions of solid bodies by tho application of heat; but the signification of the term has since been extended so as to include those instruments the object of which is to measure all gradations of temperature above those which can be indicated by the mercurial thermometer.

Musschenbroek's pyrometer consisted of a metallic bar, about six inches in length, one extremity of which was fixed, while the other was left free to advance as the metal elongated from the effect of several spirit-lamps placed beneath, which, at each experiment, were charged with the same quantity of highly rectified spirit of wine. The advance of the moveable extremity gave motion to a pinion and wheel, the latter of which drovo an index over a graduated circle, each degree of which corresponded to a linear cxpausion of 12,500th of an inch. The instrument, as it was originally constructed, is described in the second part of Musschenbroek's translation of the ' Saggi di Naturali Esperienze fatte nell' Academia del Cimento,' Leyden, 1731; and as improved by Dcsaguliers (who substituted fine cords and friction-rollers for the wheel and pinion), in the first volume of his ' Experimental Philosophy,' p. 444.

To Musschenbroek's pyrometer succeeded those of Ellicott (described in the ' Philosophical Transactions' for 1736, p. 297, and 1 751, p. 485), Graham (' Phil. Trans.,' 1754, p. 598), SmeatoB, Ferguson ('Lectures,'vol. i., pp. 14 and 301), Sec, which, like those that have since been constructed, with few exceptions, down to the preseut time, evince but little originality in the principle upon which they rest. A bar of metal is in most cases subjected to the direct action of tlame, or immersed in a tluid of convenient temperature. The minute resulting expansion is multiplied, and thereby rendered appreciable by the intervention of a succession of levers or a system of wheels and pulleys. Supposing this intervening machinery to perform with theoretical accuracy, and that the same quantity of heat is successively communicated to different substances, the indications of such an instrument would give the relative expansions of those substances under the same circumstances. But where wheels, funions, levers, &c. are employed, there must be considerable iability to error, arising from flexure, obliquity of action, and •ther causes, the magnitude of which it would bo difficult (o estimate, and wbivh, even if it be supposed small in tho first instance, will be magnified almost in the same proportion as the delicacy of the instrument is increased. Moreover the substance itself, if its nature be such as to bo softened by heat, is very liable at high temperature* to undergo compression in giving motion to the machinery. Even therefore as measures of cxpausion they cannot be considered as deserving of much confidence, A similar remark is applicable, though in a less degree, to the contrivance employed by Lavoisier and Laplace, in which the expansion of the metal deflected a telescope from the poaition that it had at the commencement of the experiment, and the absolute expansion was deduced from the extent of tho deflexion, which was read off upon a graduated scale placed at a considerable distance in front of the telescope. See a description of lhe apparatus employed in Biol's ' Physique Experimentale,' tome i., pp. 207-9, where also is given a table of the expansions of the several substances ex pen mented on between the temperatures of 32" and 212* Fabr. Troughton, in 1794, constructed an instrument which bort some resemblance to the preceding, the principal differeuot consisting in the employment of a spirit-level, the deviations of which from the horizontal determined the expansion of the metal.

Tho 'Phil. Trans.' for 1777 contain a description of the method employed by De Luc in the construction of bi» compensating pendulums, in order to determine the length of one metal whose expansion is equal to a given length of another metal. For this purpose he suspended the bar of known length from an arm, projecting horizontally from an upright deal plank. To the lower extremity of this bar was adjusted a small horizontal platform, upon which a bar of the other metal rested in a vertical position. Upon raisins the temperatures of both bars, every point on the surface of the second bar would obviously become subjected to t»o motions tending to move it in opposite directions; it would be depressed by the expansion of the first bar, and elevated by the expansion of the second. One point would therefor* remain stationary, and this point, being ascertained by raising or lowering a microscope adjusted to the edge of tho plank, determined the portion of the second bar, measured from its lower extremity, whose expansion was equal to tho whole of the first bar.

The rods employed by Borda in measuring the bnse-Iinr of the great French Survey consisted of a rule of brass laid upon a somewhat longer rule of platinum and attached at one extremity. The portion of the platinum rule not covered by I lit one of brass was divided into millionth* of the entire length of the rule, and further subdivided b> means of a leinior and microscope adjusted to the extremity of lhe brass rule. The value of each of these divisions having been previously ascertained by first surrounding the compound lule with melting ice, and then immersing it in boiling water, it was only necessary to observe the indications of the vernier in order to apply the requisite correction for reducing the length of the rod to the standard temperature.

For liw temperatures, the contrivance of Ramsden, described in the 'Philosophical Transactions' for 1785. and employed l.y General Roy in determining the expansion of the rods used in measuring the base on Hounslow Heath fur thu Trigonometrical Survey, was perhaps unexceptionable. Tho rod was immersed in a trough of water, ami uvor each extremity was placed a microscope, to which a slow ntobua could be given in the direction of the length of the rod by means uf a fine micrometer screw. The lines of collimaticn of the microscopes being thereby adjusted at the cumniencrmont of the experiment so as to accurately coincide with two points near the extremities of the rod, the temperature of tho water was gradually raised, till a thermometer placed in tho tiuugli indicated an advance of 10°, 20°, 30 , or any requiiud number of degrees. The consequent elongation of the rod destroyed the coincidence of its extremities with the lines of collimation of the microscopes, which was reestablished by turning the micrometer screws, and carefully noting the number of turns and fractiun of a turn nccc*sarV for that purpose; when, the value in parts of an inch of each turn being previously known, a direct measure of the expansion was obtained, free from the errors of a system of levers or of a train of wheels and pinions.

The property of alumina whereby it undergoes a diminution of bulk when heated, was employed by Wedgwood asameasure of high temperatures. His pyrometer ctiiuitten' of cylinders of fine white clay, and an apparatus for actaralely measuring their length. This apparatus consisted of a metallic plate, upon which were fixed two brass rules slightly inclined to each other. The rules used by Wedgwood were 24 inches long, and divided into 240 equal parts. The distance between the rules at one extremity was three tenths and at the other five-tenths of an inch; consequently the difference between their distances at any two consecutive divisions was the 1200th part of an inch. But it is obvious that these numbers aro quite arbitrary; and that by increasing the length of the rules and diminishing their inclination, the difference between their distances at any two consecutive divisions may be made as small as we please. The clay cylinders were first baked at a red heat, estimated at 947° Fahr., and then reduced to exactly five-tenths of an inch in length, so as to fit the first division of the scale. When afterwards exposed to a greater heat, they underwent contraction, and the amount of this contraction was determined by observing the division of the scale corresponding to their diminished length. If we then assume, with Wedgwood, that the contraction is proportional to the temperature at which it took place, the latter will likewise be determined; but independently of the difficulty of procuring pieces of clay of uniform composition, from which it resulted that two cylinders of equal length when exposed to the same heat seldom underwent the same degree of contraction, it has been found that the duration of the experiment has considerable influence upon the contraction, the longer continuance of a low temperature producing the same contraction as a higher degree of heat continued for a shorter time. As a measure of temperature therefore this method cannot be relied on, though as a direct measure of expansion we doubt if it has been surpassed either in the simplicity of its principle or in the minuteness of the indications of which it is susceptible. A description of the instrument and of the experiments made with it will be found in the Philosophical Transactions of 1782, 1784, and 1786.

A pyrometer was constructed by Achard, similar in form and principle to the common thermometer, but intended to indicate much higher degrees of heat. It consisted of a bulb and graduated tube of semi-transparent porcelain highly baked, and containing a very fusible alloy, composed of bismuth, lead, and tin, which became liquid at about '2X2°, and indicated higher temperatures by its expansion, which was visible through the semi-transparent tube.

Dulong and Petit employed a very direct mode of measuring the absolute, not linear, expansions of various substances. By observing the difference of altitude at which mercury of different temperatures stood in the two arras of an inverted glass siphon, they determined the absolute expansion of the mercury, and by comparing this with the apparent expansion of mercury in a glass tube, they deduced the absolute expansion of the glass. A cylinder of the metal whose expansion was sought was then placed within a filass tube, closed at one extremity and terminating at the other in a capillary opening, and the rest of the tube occupied with mercury. Upon the whole being heated, a portion of the mercury was expelled equal to the excess of the absolute expansions of the mercury and metal above that of the glass; and as the expansions of the mercury and glass were-previously known, the weight of the expelled mercury determined the expansion of the metal.

Dr. Brewster has proposed to measure expansions by the number and intensity of the polarized tints produced by the inflexion of a plate of glass against which the expanding substance is made to press. The reader will find some account of this in Brewster's ' Cyclopredia' under the articles 'Pyrometer' and 'Optics.' Guyton's pyrometer, which was exhibited before the National Institute in 1803, and described in the ' Annales de Cbimie,' xlvi., p. 276, and in Nicholson's 'Philosophical Journal,' vi., p. 89, consisted of a bar of platinum nearly two inches iu length, placed in a groove of porcelain. One extremity of the bar rested against the solid end of the groove, while the other pressed upon the short arm of a lever, the longer arm of which carried a vernier over a graduated circular arc. The whole was constructed of platinum, and a spring was made to press upon the vernier to prevent its displacement while in the act of withdrawing the instrument from the furnace. The indications of the vernier at the commencement and termination of the experiment were the data from which the expansion was subsequently computed. 'The defect of this instrument,' observes Mr. Daniel),' arose from the nature of platinum, which at a red heat becomes soft and ductile, so that the lever would be liable to bend, and thereby frustrate the experiment; and this is supposed to have been the reason why the inventor never extended his experiments to temperatures higher than that of the melting point of antimony. As early as 1821, the last-named gentleman, Mr. Danicll, the present Professor of Chemistry-at King's College, London, had invented an instrument which, he states,'afforded correct determinations connected in an unexceptionable manner with the scale of the mercurial thermometer;' but it was only suited to the experimental furnace of the chemist, so that, he continues,' the great desideratum still remained of a pyrometer which might be universally applied to the higher degrees of heat, as the thermometer had long been to the lower, and which, in addition to its use in delicate researches, might effect for the potter, the smelter, the enameller, and others, in the routine of their business, what the latter daily performs for the brewer, the distiller, the sugar-refiner, and the chemist.' The annexed'diagram represents the second pyrometer invented by Mr. Daniell, for which the Rumfurd medal was awarded to him by the Royal Society. A description of it is given in the ' Philo sophical Transactions' for 1830, and an account of the experiments made with it is inserted in the * Transactions' of that and the following years.

It consists of two distinct parts, the register and the scale. The register is a solid bar of black-lead earthenware, DDDD, eight inches long and seven-tenths of an inch wide and thick, cut out of a common black-lead crucible. In this a hole is drilled three-tenths of an inch in diameter, and seven inches and a half in depth. At pp the upper end of this bar, and on one of its sides about six-tenths of an inch in length of its substance, are cut away to the depth of half the diameter of the bore. When a bar of any metal six inches and a half long is dropped into this cavity, it rests againsts its solid end; and a cylindrical piece of porcelain, <7, about one inch and a half long, called the index, is placed on the top of it, which, projecting into and beyond the open part, is firmly confined to its place by a stiap of platinum, r, which passing round the black-lead bar and over the piece of porcelain, is made to press upon the latter with any required degree of tension by means of a small wedge s of porcelain inserted between the bar and the strap. When the register is exposed to the heat of a furnace, it is evident that, the expansion of the metallic bar exceeding thai of the black-lead, the porcelain index will be forced forward; and when the register is afterwards cooled. the tension of tho strap will retain the index at the point of greatest elongation.

The object of the scale is the accurate measurement of the distance through which the index has advanced. It consists of a frame aaaa composed of two rectangular plates of brass joined at right angles by their edges, and fitting square upon two sides of the register. At one extremity of this frame is a small plate of brass a', which, when the two former plates are applied to the register, is brought down upon the shoulder formed by cutting away the blacklead at B, and the whole may be thus firmly adjusted, when required, to the black-lead bar by three planes of contact. To the outside of this frame is firmly attached, by means of the screws bb, a brass plate AA, the extremity of which d projects so that a point c near to it may be immediately opposite to the cavity in the black-lead bar when the latter is adjusted to the frame. About c as a centre, turns an arm dnB slightly bent at n, carrying at its extremity a graduated circular arc ee. The radius of this arc is five inches, and its moveable centre n is distant from the fixed centre c exactly half an inch. About n turns a straight and lighter arm hg, five inches and a half in length, the distance from A to n being half an inch. The extremity g of this arm carries a vernier, by which the divisions of the graduated arc are subdivided into minutes, and also an eye-glass i to assist the reading. The other extremity terminates in a steel point A, or, as the instrument is now constructed, a knifeedge, which, when the register is adjusted to the frame, is inserted in a small cavity t, formed for its reception at the extremity of the porcelain index. A small steel spring let into the larger arm at m is made to press upon the lighter arm, whereby the latter has a constant tendency to move towards the commencement of the graduation.

When the instrument is used, the metallic bar to be experimented on is placed in the cavity of the register, and the index pressed down upon it and firmly fixed in its place by thi! platinum strap and porcelain wedge. The scale is then applied by carefully adjusting the frame to the register and fixing it by pressing a' upon the shoulder. Holding the whole together steadily in the left hand, the lighter arm is so placed that the steel point A may rest upon the edge of the index, against which it will be pressed by the spring: then by slightly turning the larger arm, the point will move along the surface of the index till it drops into the cavity /. The indications of the vernier being then read off, the register is detached from the scale, placed in the furnace, and after it is removed and cooled, it is again applied to the scale in the same manner as before, and the second indication of the vernier noted. From the two readings of the vernier may be deduced the excess of the expansion of the metallic bar above that of the black-lead, though a correct formula for this purpose has not, to the writer's knowledge, been hitherto given.

The one employed by Mr. Daniell, tbouyh probably sufficiently correct for all practical purposes, gives the expansions one per cent, too great without exception, and in many cases much more, so that more than the first significant figure can seldom be depended upon in those published by him in the 'Philosophical Transactions'of 1830-31. The error thus introduced is perhaps within the limits of the error to which the instrument itself is liable; but should this not be the case, it might bo desirable to employ the correct formula, for which reason we subjoin its investigation.

Let cnB, hng, represent the positions of the two arms of the scale relative to the register, before the expansion has taken place, and cn'W, h'n'g', their positions after the expansion; A and A' the two positions of the steel point, the line joining which passes through the fixed centre c; e and e' the two positions of the zero of the graduated arc. Put the angle cnB = cn'B' = a; enB = e'n'B' = /3; eng (the first reading of the vernier) = 0; c'n'g' (the second reading) = 0'; also en = en' = nh = n'h' = r; and AA' = f, tho excess of the expansion of the metal above that of the black lead: then sin hnn' — sin k'n'n The formula used by Mr. Daniell is i = sin J (0' - 0), or its equivalent, since 0' — 0 is generally a smal angle, i = 2 sin i (0' - 0); from which it appears that all the expansions given by him should be dimini&hid in the ratio of 1 : cos {7° 30' + J (0' + 0)} ; but ashcliu recorded only the difference 0'— 0 of the readings of the vernier, and not the readings themselves, this correction can only be made by a repetition of the whole of the experiments. The error is inconsiderable so long as 0 and 0' are both small, but it increases with the increase of either of those angles.

The excess of the expansion of the metal above that of the black-lead being thus obtained, and increased by the expansion of the latter (the determination of which is lew direct and conclusive), the expansion of tho metal become* known. In order that the instrument may then be employed as a measure of temperature as well as of expansion, the doubtful assumption is introduced that equal increments of length are the effects of equal increments of temperature, and thence, having determined the expansion between any two known points on the thcrmometric scale say the temperatures of melting ice and boiling mercury, a mere proportion will of course give the temperatuio'at which any other observed expansion took place.

It remains to notice a paper communicated to the Royal Society by the late Mr. Prinsep. the assay-master of the Mint at Kenares,' On the Measurement of high Temperatures ' and published in their'Transactions' for 1828. 'The fusing-rornu of pure metals,'observes that gentleman, 'are determinate and unchangeable; they also comprehend nearly the whole range of temperature; the unoxidable or noble metals alone embrace a range from the low melting-point of silver to the high ignition of platina. There are it is true only three fixed points in this scale, but as many intermediate links may be made as are required, by alloying the three metals toeetbc in different proportions. When such a series has been once prepared, the heat of any furnace may be expressed br the alloy of least fusibility which it is capable of meluoi' As the melting-points of silver and gold are comparatively near to each other, Mr. Prinsep assumed only ten intermediate gradations of heat, the lowest of which correspondcu to the fusing-pomt of pure silver, and the others to the fu» ing-points of silver alloyed with 10, 20. 30, &c. per cent, of gold. From the melting-point of gold to that of platina. he assumed ono hundred gradations of heat, which were the melting-points of pure gold and of gold alloyed with 1 •> ^ &c. per cent, of platina. Among the advantages o'rTlm mode of identifying temperatures are:—the smallness of the requisite apparatus, nothing more being needed than o «m»l cupel, containing in separate cells eight or ten pyrvmctriP alloys, each of the size of a pin's head; the indestructibility of the specimens, since those melted in one experiment need only to be flattened under a hammer, when they will be again ready for use; and the facility of notation, since two letters and the decimal of alloy will express the maximum heat: thus S -3 G expresses the temperature of the fusing-point of silver when alloyed with gold in the proportion of 7 to 3; and G -23 P expresses the fusing-point of gold when alloyed with platina in the proportion of 77 to 23. For a more particular account of this mode of determining temperatures we refer the reader to the memoir cited.

Several suggestions have been made for employing the expansion of air, on the principle of the differential thermometer, as a measure of high temperatures. It is proposed that one-half of the instrument be composed of platinum, so as to fit it for exposure to a great heat, and the other part of glass. The suggestion, we believe, is originally due to Mr. Schmidt (Nicholson's Journal, xi., p. 141); but was brought forward under another form by Mr. Nicholas Mill, in the ' Monthly Medico-Chirurgical Review and ChemicoPhilosophical Magazine,' vol. i., Lond., 1824; again by Dr. Ure, in his 'Dictionary of Chemistry ;* and lastly by Mr. Pnnsep. The instrument, wo believe, has been constructed upon each of the plans proposed. That of Mr. Prinsep appears tho most complete (see a drawing of the apparatus in full operation at page 87 of his Memoir above referred to), and was employed by him to connect the fusing-points of his alloys with the thermometric scale; but the principle upon which they all rest involves the assumption that the increase of temperature is proportional to the expansion of the air.

A valuable tableof the expansionsof different substances, collected from various sources by Mr. Francis Baily, is given in the first volume of the 'Transactions of the Astronomical Societv," p. 416.

(BioYs Physique Experimentale; Philosophical Transactions; Thomson's Chemistry; Brewster's Cyclopaedia; Encyclopaedia Britannica; and the works cited.)

Note - this article incorporates content from The Penny Cyclopaedia of the Society for the Diffusion of Useful Knowledge (1840)

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