UNWIN EXTENSOMETER. 



121 



units on the scale. It must here be remembered that Q 

 moves through twice the distance moved by 0. 



This screw at P may also be used where the extension 

 of the bar exceeds the full range of the scale, when Q may 

 be brought back to zero after the end of the scale has been 

 reached ; its use is also necessary for adjusting Q to the 

 zero of the scale. A special form of this instrument is also 

 made for use on the members of actual structures, such as 



fej 



] B 



FIG. 51. 



FIG. 53. 



the members of roofs and bridges. On Fig. 50 is illustrated 

 the latest form of the Ewing extensometer. This instrument, 

 while making use of the principles adopted in the older 

 forms, differs considerably in detail, and possesses the 

 important advantage that it can be applied equally well to 

 horizontal, vertical, or sloping bars. 



68. Unwin Screw Extensometer with Levels* This 

 instrument, which was designed and has been used by Prof. 



* Unwin's "'Jesting of Materials." 



