Measurements of small Strains in Testing Materials, <r. 123 



May 16, 1895. 

 The LORD KELVIN", D.C.L., LL.D., President, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



The following Papers were read : 



I. "On Measurements of small Strains in the Testing of" 

 Materials and Structures." By J. A. EWING, M.A., F.R.S.,. 

 Professor of Mechanism and Applied Mechanics in the 

 University of Cambridge. Received April 24, 1895. 



Many forms of " extensometer " have been devised for measuring 

 the small strains of extension or compression which precede yielding 

 in materials subjected to stress by direct pull or push. Such instru- 

 ments are employed, in the testing of metals, for determining 

 Young's modulus and for observing the behaviour of the material as 

 the limit of elasticity is passed. Apparatus of the same general 

 kind has also been applied to examine the strains produced in the 

 members of bridges and other structures by the application of load 

 to the structure as a whole ; the amount of stress in any member 

 being in that case inferred from observation of the strain. Professor 

 Un win, who has himself designed more than one form of extenso- 

 meter, has described in his work on the ' Testing of Materials of 

 Construction ' a number of instruments of this class, and has pointed 

 out that a first condition of accuracy is that the measurements of 

 extension be made on both of two sides of the piece, in such a way 

 that a mean is obtained representing the extension of a central line. 

 In practice, a rod always bends more or less on being pulled. Even 

 when the rod is initially straight the pull is rarely, if ever, so sym- 

 metrically applied or the elasticity so uniform as to make the exten- 

 sion equal at all parts of the section. Hence, to avoid errors which 

 would be great relative to the quantity under examination, the 

 extensometer must be arranged in such a fashion that its indications 

 depend only on the change of distance between two points on the- 

 axis of the rod, and are independent of those inequalities which are 

 found to exist in the strains as measured on the surface. The 

 condition is met either by taking two separate observations of the 

 strain on two sides of the rod and averaging the two, or by making 



