1'YHOMETER. 



rVIMMlTEH. 



S'.J 



of the term has since been extended so as to include those 

 instrument* the abject of which U to measure all gradations of 

 temperature above thow which can be indicated by the mercurial 

 thermometer. 



Mtuchcobrock's pyrometer consisted of a metallic bar, about nix 

 inches in length, one extremity of which wan fixed, while the other 

 was left free to advance at the metal elongated from the effect of tin- 

 Hum of aeveral ipirit-Umpi placed beneath, which at each experiment 

 were charged with the name quantity of highly rectified spirit of wine. 

 The advance of the moveable extremity gave motion to a pinion and 

 wheel, the latter of which drove an index over a graduated circle, each 

 degree of which corresponded to a linear expansion of 12,500th of an 

 inch. The instrument, as it was originally constructed, is described in 

 the second part of Muschenbroek's translation of the ' Saggi di Natural! 

 Esperirnze tatte nell' Academia del Cimento,' Leyden, 1731 ; and as 

 improved by Desaguliers (who substituted fine cords and friction- 

 rollers for the wheel and pinion), in lib ' Experimental Philosophy,' 

 vol. i. 



To Muschenbroek's pyrometer succeeded those of Ellicott (described 

 in the I'hil. Trans.' for 1786 and 1751), Graham (' PhiL Trans.,' 1784), 

 Smeaton, Ferguson (' Lectures'), Ac., which, like those that have since 

 been constructed, with few exceptions, down to the present 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 flame, or 

 immersed in a fluid of convenient temperature. The minute resulting 

 expansion U multiplied, and thereby rendered appreciable by the inter- 

 vention of a succession of levers or a system of wheels and pulleys. 

 Supposing this intervening machinery to perform with theoretical 

 accuracy, and tUat the same quantity of heat is successively communi- 

 cated to different substances, the indications of such an instrument 

 would give the relative expansions of those substances under the same 

 circumstanced. But where wheels, pinions, levers, Ac., are employed, 

 there must be considerable liability to error, arising from flexure, 

 obliquity of action, and other causes, the magnitude of which it would 

 be difficult to estimate, and which, even if it be supposed email in the 

 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 be softened by heat, is very liable at high 

 temperatures to undergo compression in giving motion to the machinery. 

 Even, therefore, as measures of expansion 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 

 position that it had at the commencement of the experiment, and the 

 absolute expansion was deduced from the extent of this deflexion, 

 which was read off upon a graduated scale placed at a considerable 

 distance in front of the telescope. A description of the apparatus 

 employed is given in Biot's ' Physique Experiment-lit.',' tome L, and a 

 table of the expansions of the several substances experimented on 

 between the temperatures of 32 and 212 Fahr. Troughton, in 1794, 

 constructed an instrument which bore some resemblance to the pre- 

 ceding, the principal difference consisting in the employment of a 

 .-pint-level, the deviations of which from the horizontal determined 

 the expansion of the metal. 



The 'Phil. Trans.' for 1777 contain a description of the method 

 employed by De Luc in the construction of his compensating pendu- 

 lums, 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 horizon- 

 tally 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 raising the tempe- 

 ratures of both bars, every point on the surface of the second bar 

 would obviously become subjected to two 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 therefore remain stationary, and this point, being ascertained 

 by raising or lowering a microscope adjusted to the edge of the 

 plank, determined the portion of the second bar, measured from its 

 lower extremity, whose expansion was equal to the whole of the 

 first bar. 



The rods employed by Borda in measuring the base-line of the great 

 French Survey consisted of a rule of bras* laid upon a somewhat longer 

 rule of platinum and attached at one extremity. The portion of the 

 platinum rule not covered by the one of bran was divided into 

 millionth* of the entire length of the rule, and further mibdivided by 

 means of a vernier and microscope adjusted to the extremity of the 

 brass rule. The value of each of these divisions having been previously 

 ascertained by first surrounding the compound rule 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 tempe- 

 rature. 



!<iw temperatures, the contrivance of Tlanuden, described in the 

 I'hil. Trans.' for 1785, and employed by General Hoy in determining 

 the expansion of the rods used In measuring the base on H 

 Heath for the Trigonometrical Survey, was perhaps unexceptionable. 

 The rod was immersed in a trough of water, and over each extremity 



was placed a microscope, t> which a slow motion could be pi-. - n m the 

 direction of the length of the rod by means of a 



The lines of cullimation of the microscopes be: . Ijuated at 



the commencement of the experiment so as to coincide acci: 

 two points near the extremities of the i 

 water was gradually raised till a thermometer placed in 

 indicated an advance of 10*, 20*, 30*, or any required number of 

 degrees. The consequent elongation of the 1 the coin- 



cidence of its extremities with the lines of oollimation of the 

 scopes, which was re-established by turning the micrometer .- 

 and carefully noting the number of turns ami fnu-timi of a turn 

 necessary for that purpose; when, the r.ilue in part* of an inch of each 

 turn beinK previously known, a direct measure of the expansion was 

 obtained, free from the errors of a system of levers or of a train of 

 wheel* and pinions. 



The property of alumina whereby it undergoes a diminution of bulk 

 when heated, was employed by Wedgwood as a measure of high temjK-. 

 raturea. His pyrometer consisted of cylinders of fine white cla; 

 an apparatus for accurately 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 \V< .l^u-oo.l were 

 24 inches long, and divided into 240 equal parts. The distance 

 between the rules at one extremity was 3-10ths an.l at the other 

 5-lOths of an inch; consequently the difference between their distances 

 at any two consecutive divisions was the 12ui>th part of .-in inch. But 

 it is obvious that these number* ore quite arbitrary, and that by 

 increasing the length of the rules and diminishing their inclin 

 the difference between their distances at any two consecutive di>. 

 may be made as small as we please. The clay cylind r- were first 

 baked at a red heat, estimated at 947 Fahr., and tin: 

 exactly 5-lOths of an inch in length, so as to fit the fir 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 pro- 

 curing 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 In-m 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 lias 

 been surpassed either in the simplicity of its principle or in the minute- 

 ness of the indications of which it is susceptible. A description of the 

 instrument, and of the experiments mode with U, will be found in the 

 1 Phil. Trans.' of 1782, 1784, and 1788. 



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 anil 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 '21 '2, and indicated higher temperatures 

 by its expansion, which was visible through the scmi-transpareut 

 tube. 



Dulong and Petit employed a very direct mode of measuring the 

 absolute, not linear, expansions of various substances. By ob.- 

 the difference of altitude at which mercury of different tempei 

 stood in the two arms 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 

 expulsion was sought was then placed within a glass tube, closed 

 at one extremity and terminating at the other in a capillary open- 

 ing, and the rest of the tube occupied with mercury. I'pon tin- 

 whole being heated, a portion of the mercury was expelled equal to 

 the excess of the absolute expansions of the mercui \ 

 that of the glass ; and as the expansion* of the mercury and glass 

 were previously known, the weight of the expelled mercury detoi i 

 the expansion of the metal. 



Sir David Brewster has proposed to measure expansions by the 

 number and intensity of the polarized tint* produced liy the inflexion 

 of a plate of glass against which the expanding substance is made to 

 press. (Brewster's ' Cyclopaedia/ articles ' Pyrometer' ami ' < >]>' 

 (iuyton's pyrometer, which was exhibited before the National 

 tute in 1803, and described in the 'Annalesde ('himie,' xlvi., i 

 and in Nicholson's ' Philosophical Journal,' vi., p. Ml, consisted of a 

 bar of platinum nearly 2 inches in length, placed in a groove of por- 

 celain. One extremity of the bar rested against the solid cud of the 

 groove, while the other pressed upon the short arm of a lever, the 

 longer arm of which carried a vcrni. > 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 com- 



