GENERAL CONSIDERATIONS. 



123 



length between the two clips A and B, there is one, 

 E, of tixed length whose ends serve as fulcra, about 

 which the clip B rotates as the bar varies in length ; 

 and, in consequence of this movement, the outer end of the 

 clip A moves with respect to the clip B. To B is attached a 

 finger-piece F, which presses against a roller R carried by 

 A. As the bar varies in length this finger-piece, pressing 

 against the roller, causes it to rotate through a small angle. 

 To the roller is attached a small mirror M, and the amount 

 of rotation of the mirror is observed by means of a reading- 

 telescope and scale. The graduated scale is placed some 



FIG. 54. 



distance away from the mirror, and its reflection is observed 

 by means of the telescope. The whole arrangement thus 

 constitutes what may be called a system of optical leverage. 

 It is stated that readings can be taken to T<7TyVo7>th of an 

 inch. A perspective view is shown in Fig. 54. 



70. General Considerations. Many of the extenso- 

 meters in use are somewhat costly instruments, and it there- 

 fore behoves experimenters to consider well the qualities 

 necessary in a good extensometer before proceeding to select 

 one for their own use. 



The first quality necessary is accuracy. By this is 

 meant that the readings given by the instrument shall 

 indicate really and truly the measurements they represent. 



