1895.] Strains in the Testing of Materials and Structures. 127 



parallel steel rods united by a cross-bar at top and bottom, and carry- 

 ing, besides the screw P, the microscope, which is hinged to B' about 

 the point E vertically above Q, and is provided with a focussing 

 screw at F. The counterpoise D, which is also attached to the piece 

 B', serves to balance the weight of the microscope and make the 

 pressure vertical between P and the hole into which it gears. There 

 is a supplementary counterpoise D' for adjusting the balance about 

 the axis of the joint between B and B'. These counterpoises are 

 adjusted until when the heavy end (Q) of C is raised, so that P 

 ceases to be in gear with C, P has no tendency to move in any 

 direction. The excess of weight on the right hand side of C suffices 

 to produce the requisite pressure at the point P. The frame BB' 

 with the microscope may be lifted off, leaving only the two clips 

 attached to the rod. 



The object sighted is one side of a wire stretched horizontally 

 across a hole in a plate at Q, and illuminated by a small mirror 

 behind. The distances OP and pQ are in this instance equal, with 

 the effect that the movement of Q is double the extension of the rod. 

 The eye-piece scale and the length of the microscope are chosen so 

 that the numbers read on the scale correspond to TOOTTO' of an inch of 

 extension. This adjustment is tested by turning the screw P, which 

 has a pitch of -^ inch, through one revolution, and observing that 

 the displacement of Q is 500 units of the eye-piece scale. In the 

 instrument illustrated in fig. 2 the whole scale comprises 1,400 units, 

 and calibration tests show that throughout the middle 1,200 of them 

 the proportionality of the scale readings with the real movements of 

 Q is practically perfect. 



To facilitate the application of the apparatus to any rod a clamp or 

 distance piece, H (fig. 3), is added by which the two clips, B and C, 

 may be held at the right distance apart, and with the axes of their 

 set screws parallel. This makes it in many cases unnecessary to 

 prepare the rod beforehand by punching or drilling holes for points 

 of the set screws ; the clamp is readily held so that the clips stand 

 fairly round the rod ; the set screws are advanced to grip the rod, and 

 the clamp is then removed by releasing the screws, GG. The con- 

 nection of the clamp to each of the clips is by means of three points 

 gearing with a hole, slot, and plane. The clamp is specially con- 

 venient when the strain of the specimen has been carried beyond 

 the elastic limit, and it is desired immediately to reset the clips to the 

 standard distance apart after the length between them has been 

 materially changed by the permanent extension of the specimen. 



Another form of the apparatus designed for use on the members of 

 actual structures, is shown in fig. 4. There are essentially the same 

 parts, but the arrangement is altered in order to make all the appa- 

 ratus lie on one side of the piece under examination a desirable and 



VOL. LVIIl. L 



