In using this pyrometer, the bar of metal is laid on the standard bar. The heat of a lamp is first applied to the .standard bar E, and its expansion, as indicated on the moveable plate, is marked. The expansion of the bar E, by the heat communicated to it from the standard bar, is also measured, as marked by the brass circle. The in strument is then allowed to stand till the whole is tho roughly cold; then removing the bar E, and laying another bar of any other metal in its place, heat the standard bar to the same degree of heat as before, which is seen by the moveable plate's marking the same degree of expansion. The index will then show the degree of expansion of the second metal, as it did of the first, and in the same way the expansions of different metals, by the same degree of heat may be exactly estimated. The results obtained by Mr. Ellicott were as follows, the heat being the same : Steel. Iron. Gold Copper. Brass. Silver. Lead.
56 60 73 89 95 103 149 See Phil. Trans. 1736, vol. xxxix. p. 297, for the de scription of this pyrometer, and the same work, 1751, vol. xlvii. p. 485, for an account of the experiments.
of Graham's Pyrometer.
That celebrated artist Mr. Graham, constructed also a pyrometer for measuring minute alterations in the length of metallic bars, but unfortunately he has left no descrip tion of it behind him. Mr. Smeaton saw the instrument itself at the house of Mr. Short, and he mentions that those alterations were determined by advancing the point of a micrometer screw till it sensibly stopped against the end of the bar to be measured. This screw being small, and very lightly hung, was capable of agreement with the 3500th part of an inch. See Phil. Trans. 1754, vol. xlviii. D. 598.
5. Description of Smeaton's Pyrometer.
This instrument, founded on the same general prin ciple as Graham's pyrometer, is represented in Plate CCCCLXXI. No. II. Fig. 6, which exhibits it without the cistern in which it is used. The bar ABCD is the basis of the instrument. It is one, by one and a half inch thick, and is made of brass. It stands edgewise upwards, and one end is continued of the same piece at right angles, to the height of three and a half inches, to be a firm support for the end of the bar under examination, while the other end acts on the middle of a lever of the second kind, whose fulcrum is on the basis ABCD. The motion at the ex tremity of the lever is therefore double the difference be tween the expansion of the bar and the basis. The bar EF, to be examined, lies in two notches, one of which is fixed to the upright standard AB, and the other to the principal lever HI. The end E of the bar EF bears against the point G, a screw of use in examining the micrometer screw. The other end of the bar EF bears against a small spherically protuberant bit of hard metal, fixed at the same height as G in the principal lever HI. An arbor K is fixed
in the basis, which receives at each end the points of the screws H, L, on which the lever HL turns as upon a ful crum. A slender spring 0 keeps the lever in a bearing state against the bar, and P is a check to prevent the lever from falling forward when the bar is taken out. The top of the lever is furnished with an appendage N, called a feeler, in the shape of a T, suspended and moveable up and down on the points of the screws I, NI, which, as well as L, H, are so adjusted as to leave the motion free, but without shake. The handle QR of the feeler is moveable in a loose joint at R, so that, laying hold of it at Q, the feeler is moved up and down without being affected by the irregular pressure of the hand. The extremity S of the feeler is also furnished with a bit of protuberant hard me: tal, to render more perfect its contact with the point of the micrometer screw. The divided index plate is shown at V, and W, a knob for the handle. The micrometer screw passes through two solid screwed holes at D and Y. The piece is made a little springy, and endeavours to pull the screw backwards from the hole at D ; consequently the micrometer screw is constantly bearing against its threads the same way, and thus renders its motion perfectly steady and gentle. The index X has divisions upon it answering to the turns of the screw. This piece points out the divi sions of the plate, as the face of the plate points out the divisions of the index. In using the instrument, lay hold of the knob at Q with one hand, and moving the feeler up and down, with the other move forward the screw 1 till its point comes in contact with the feeler, then will the plate and index V and X show the number of turns and parts of a turn. In Fig. 7. is represented the instrument when ready for use, and immerged in its cistern of water AB. The cover C of the cistern goes on between the bar EF and the basis BC when the instrument is raised on blocks. The handle D is for taking off the cover when hot ; E is the mercurial thermometer; F the cock to let off the water ; Gil a hollow piece of tin, which supports seven spirit lamps, which are raised higher or lower by the screws I and K in order to give the water in the cis tern a proper degree of heat. The following are the mea sures in Mr. Smeaton's instrument ; From these data it will be found that the value of one division will be the 5876th part of an inch. When the screw is altered one-fourth of one of these divisions, the difference of contact will be'Very perceptible to the slightest observer ; and consequently the 23145th part of an inch will be per ceptible in the instrument.