Strains in the Testing of Material* and Structures. 125 



tively to C, so long as the two are not otherwise connected. But the 

 piece B has fixed to it an arm B', which ends in a rounded point P, and 

 this point gears in a \/-slot cut transversely across the end of the piece 

 C. The contact of P with the two sides of this slot removes the two 

 degrees of freedom which B would otherwise have relatively to C, 

 and makes the position of the pieces definite. Then, when the rod 

 extends, the point P acts as a fulcrum, and the opposite end of C, 

 namely, Q, moves down through a distance proportional to the mean 

 displacement of the two pairs of set screws. The displacement of Q 

 Is a multiple of the strain, in the ratio of PQ to OQ. This dis- 

 placement is measured by means of a microscope, which hangs from 

 B and sights an object at Q. The strains to be observed are so 

 small that it is easy, without making the piece C inconveniently 

 long, to prevent its angular movement from becoming sufficient to 

 prove troublesome or to affect the accuracy of the indications. The 

 arm B', which carries the point P, must be so attached to B that, as 

 regards oscillation about the set- screws of B, the two pieces B and 

 B' move as a rigid whole. But B' may be flexible in the direction 

 which would give P motion at right angles to the plane of the paper, 

 and by giving it flexibility of this kind (while preserving its rigidity 

 as regards motion in the plane of the paper) the point P may be 

 made to gear in a hole instead of a slot in C. There is a practical 

 advantage in doing this, for if P gears in a slot any unequal extension 

 of the front and back of the specimen makes P work along the slot, 

 and it is difficult to secure that no error will result in consequence of 

 the slot not being perfectly parallel to the axes of the set-screws. 

 This consideration has led the author to adopt the plan of putting a 

 transverse joint between the pieces B and B', with the object of 

 giving P freedom to adjust itself to a hole (instead of a slot) in C, by 

 movement about this joint in a direction perpendicular to the plane 

 of the sketch. Incidently, such a joint has the further advantage 

 that it allows the whole apparatus to be more conveniently and 

 quickly applied to the rod under examination. 



The displacement of Q due to the strain is measured by means of 

 a micrometer in the eye-piece of the microscope. A micrometer 

 scale engraved on glass re convenient for the purpose, and by esti- 

 mating tenths of a division on such a scale readings are readily taken 

 which correspond to -5^0^ f an i ncn f extension on the part of 

 the specimen, with an objective which allows the whole of the 

 elastic extension to occur without displacing Q beyond the field of 

 view. 



There is a fine screw with a divided head between B' and the point 

 P. This serves to bring Q into a convenient position for sighting, 

 and also to determine what is the absolute amount of extension corre- 

 sponding to a division of the eye-piece scale. Should a strain occur 



