122 IN WIN I:\TI:NSOMI;TER. 



Unwin, is represented by the diagrammatic sketch on Fig. 

 .")!. Here the test piece, which is vertical, is gripped at the 

 ends of the portion to be measured by two screw clips A, B. 

 These are maintained horizontal by ordinary spirit levels. 

 placed upon the arms C and D. The arm 1) is k<;pt in 

 a horizontal position by the small set-screw E, which serves 

 to rotate the clip about its two centre points The bar C 

 is likewise maintained horizontal by means of the s< -n .>- 

 distance-piece F, whose lower end rests upon D, and whose 

 upper end presses upon the under part of C. The length 



Fig. 52. 



of this distance-piece is variable, and can be adjusted by 

 the graduated wheel G fixed upon the fine pitch screw H. 

 As the bar stretches the level on C tends to fall ; this is 

 restored to a horizontal position by turning the screw G, 

 and when this has been done the amount of extension is 

 read on the graduated circumference of G. The pitch of 

 the screw and the graduation of the wheel are such that 

 the instrument reads to one ten-thousandth part of an 

 inch. A perspective view of this instrument, as made by 

 Messrs. Nalder Bros., is shown on Fig. 52. 



69. Unwin Mirror Extensometer. - - The general 

 arrangement of this instrument, Fig. 53, is not unlike 

 that of the one which has just been described. 

 Instead, however, of having a distance-piece of variable 



