154 



COMPRESSION TESTS. 



extensometer, care being taken that the diameter of the bar 

 is sufficient to render unlikely any possibility of buckling. 

 The elastic deformations of short compression specimens 

 have not often been measured. An instrument for this 

 purpose, designed and used by Professor Unwin, is shown 



on Fig. 73. This was primarily designed for measuring 

 the elastic compressions of small blocks of stone, concrete, 

 or similar substance, but there is no doubt that it could be 

 used for similar purposes on metal compression pieces, or 

 the same principle might be made use of with small 

 differences in detail. The working of this instrument will 

 be appreciated by reference to the figure. The lower clip 

 B is held to the specimen by means of four set screws, and 

 the upper one clips the block by means of one pair of 

 screws. These two clips have prolongations, D and E 

 respectively. There is a vertical pillar C, which carries 

 the knife edge about which the upper clip rotates. As the 

 block is compressed, and the clip A descends with respect 

 to B, the point of this upper clip F rises with respect to 

 the end of the lower clip G, and this movement is two and 

 a half times the compression of the block. On these 

 ends are two fine lines drawn upon silver, and their 

 relative distance is measured by means of a micrometer 

 microscope H. 



Readings to from o^^ to 5 ^<7u of an inch can be taken.* 



In most cases, however, it is only the semi-plastic 

 strains that are measured. In the case of a tension tosr, 

 the extent to which the loads can be increased is limited 

 by the point of fracture occurring, when the test is at an 



* Unwin's "Testing," p. 225. 



