120 



EWING EXTENSOMETER. 



balance the weight of the microscope. In this instrument 

 the point 13 rests in a conical hole instead of a slot. 



The object at Q, which is sighted through the micro- 

 scope, is a fine wire stretched across a hole in a plate, and 

 illuminated by a mirror behind. Readings are taken to 

 one edge of the wire, and a micrometer scale engraved 

 on glass is fixed in the eye-piece of the microscope. J n 

 the instrument in question the distance O P is made 

 equal to O Q, so that the movement of Q, as seen through 

 the microscope, is just double the extension of the speci- 

 men. The divisions on the scale, and the length of the 



FIG. 50. 



microscope, are so chosen that readings can be taken which 

 correspond to 5^^ of an inch of extension of the speci- 

 men. In order to check the scale readings the screw P is 

 turned through one revolution. The pitch of this screw 

 is /o of an inch, therefore this one revolution of the screw 

 should displace the edge of the wire at Q through 500 



