THERMOMETER AND PYROMETER. 

 Fig. 12. 



19 



of a mean expansibility among the me- 

 tals, ef is the bar to be measured, 

 resting on two notches, one attached to 

 the fixed upright a b, and the other to 

 the principal lever h i. A is a strong 

 arbor fixed to the basis, and intended 

 to receive the ends of two screws h, I, 

 upon which the principal lever h, i 

 turns ; o is a slender steel spring in- 

 tended to press the lever against the 

 extremity of the bar; and p is a check- 

 rod to support the lever, when the bar 

 is removed, t is called the feeler ; it is 

 in the form of the letter T, and is sus- 

 pended freely, but without shake, be- 

 tween the points of the screws m, n. q 

 is the handle of the feeler, which is 

 movable on a loose joint, so that the 

 feeler may be moved by the handle 

 without being irregularly affected by 

 the pressure of the hand. The princi- 

 pal part of the instrument is s, the 

 micrometer screw, and w the graduated 

 circle or index-plate fixed on the screw, 

 which indicates the revolutions of the 

 screw on the index v. The micrometer 

 screw passes through two solid heads 

 perforated by a corresponding screw ; 

 the piece y z is made somewhat springy, 

 and tends to draw the micrometer screw 

 backward from d; by which its threads 

 press uniformly against the correspond- 

 ing threads in the holes, and keep the 

 motion equable and easy. 



When the instrument is used, its 

 basis and the bar are immersed in a tin 

 vessel containing water, as marked by 

 the dotted line, which is heated by se- 

 ven lamps applied below. The vessel 

 is provided with a cover ; and a deli- 

 cate mercurial thermometer is sus- 

 pended in the water, for regulating and 

 ascertaining the temperature employed, 



which is not intended to exceed that of 

 boiling water. 



The expansion of the bar presses the 

 lever and feeler towards the end of the 

 micrometer screw, which, as well as the 

 extremity of the feeler, is tint with har- 

 dened steel. The handle q is laid hold 

 of, and by it the feeler is moved up and 

 down, while the screw is turned, until 

 its steel point comes in contact with the 

 end of the screw. Mr. Smeaton found 

 that he could judge of that contact more 

 accurately by the ear, than by the eye 

 or the touch. 



The turns of the index-plate counted 

 by its edge and the divisions of the 

 index, show the expansion of the bar ; 

 and its length when cool may be found 

 in the same manner, either before or 

 after the experiment above described. 

 In this instrument the bar acts against 

 the centre of a lever of the second order t 

 the fulcrum of which is in the basis ; 

 and when both are expanded, the free 

 extremity of the lever moves through 

 a space double of the difference between 

 the expansion of the bar and of the 

 basis : hence, when we know the length 

 of the lever from its axis to the point of 

 suspension of the feeler, the distance 

 from that axis to the point of contact of 

 the bar, the number of threads of the 

 micrometer screw in an inch, and the 

 number of degrees on the circumference 

 of the index-plate, we can compute the 

 value of these degrees in fractions of an 

 inch. In the original pyrometer the 

 following were the proportions : 

 From axis of lever to point of inches. 



suspension 5.875 



fulcrum to point of contact 2.895 



Length of 70 threads of the screw 2.455 

 Division of index-plate 100. 

 c 2 



