IRVING EXTENSOMETER. 



119 



These have each only one degree of freedom with respect 

 to the test bar, this being freedom to rotate about the axes 

 of its set screws. To the piece B is attached at right 

 angles the bar B'. The end of this bar is a rounded point 

 P, resting in a transverse V-slot, cut in the other piece C. 

 As the bar extends the lower point O descends with regard 

 to B B 1 , the bar C turns about the point P as a fulcrum, 

 and the opposite end of C, marked Q in the figure, is 

 moved in a downward direction also. The extent of this 

 movement is ascertained by observing a mark on Q, by 

 means of a microscope M. An adjusting screw is fixed to 



49. 



the lower end of B 1 for purposes of adjustment of the 

 point Q. The microscope, M, is slung by means of the rod 

 N from the end of the piece B. The movement of Q, as 

 read by the microscope, is not the actual extension of 0, 

 but a multiple of it in the ratio of P Q to P 0. On Fig. 

 49 is a general view of the instrument as applied to a flat 

 test bar. The different parts will be recognised from the 

 above description. In addition to what has already been 

 mentioned there is a counterpoise D, which serves to 



