124 Prof. J. A. Ewing. On Measurements of small [May 16, 



the extensometer itself indicate a mean strain. Bauschinger's appa- 

 ratus, in which two mirrors are used to indicate separately the 

 strains on opposite sides of the rod, is an example of the first 

 method. Professor Unwin's extensometer, in which the rod is 

 grasped by a pair of clips at the extremities of two parallel 

 diameters, the change of distance between the clips being measured, 

 in an example of the other. 



The object of the present paper is to describe an instrument of the 

 same class, but embodying some novel features, and to mention a 

 few results that have been obtained by its means. 



The instrument is self-contained, is entirely supported by the rod 

 under test, and touches the rod only at the points where the clips are 

 attached a feature which is of advantage in view of the tendency 

 the rod has to bend. The principle of the construction will be seen 

 on referring to the diagrammatic sketch, fig. 1. To the rod A, which is 



FIG. 1. 



B 



the piece under test, the two pieces B and C are attached each by a 

 pair of set screws. Each of these pieces has, separately, one degree of 

 freedom with respect to the rod, namely, freedom to rotate about the 

 axis of its set screws. Hence, B has two degrees of freedom rela- 



