used in Mechanical Testing, 285 



of rotation is observed by noting the image of a scale in the 

 mirror through a reading-telescope. In this way measure- 

 ments to x t h of an inch can be taken. To eliminate 

 errors due to curvature, two rollers are placed, one on each 

 side of the bar, and two sets of readings are taken. This 

 involves the adjustment of two instruments and the taking of 

 two sets of readings. But the principle is perfect; and no 

 more accurate measurements than Bauschinger's have pro- 

 bably been made. 



Touch-Micrometer Extensometer. — The first instrument used 

 by the author was a kind of callipers. Two bars, one sliding 

 in the other, could be set by touch to the distance between 

 two fixed clips on the test-bar. A scale was engraved on 

 silver on one bar ; and the distance of the nearest division from 

 a fixed zero-mark on the other was taken by a microscope- 

 micrometer. Readings could be taken to - — I th of an inch. 



, . te . 10000 



The instrument is easy and rapid to use. Readings can be 

 taken on both sides of the test-bar; and the readings are direct 

 on to a carefully graduated scale, so that no calibration of the 

 instrument is necessary. 



Screw- Micrometer Extensometer. — This aims at obtaining 

 the extension along the axis of the bar by a single reading. 

 Two clips are fixed on the bar, each by a pair of steel points, 

 one on each side, gripping the bar in a plane through its axis. 

 If, then, these clips can be made to preserve the same relative 

 position to the bar, the middle points of the clips will move in 

 the same way as points on the axis of the bar. Fig. 2 is a 

 diagrammatic sketch of the apparatus, a a and b b are the 

 clips on the test-bar, fixed to it by points in its middle plane. 

 c c are projections on the clips, to which are fixed delicate 

 spirit-levels ; d is a small screw which just touches the test-bar; 

 e is a micrometer-screw with graduated head, which supports 

 the upper clip on the lower clip. In use the lower clip is 

 first levelled by the screw d ; then the upper clip is levelled 

 by the micrometer-screw, and a reading taken. The clips being 

 always accurately levelled, in a plane perpendicular to that in 

 which the four points attaching the clips to the test-bar lie, 

 the micrometer-readings are the distances between the middle 

 points of the two parallel clips ; and their differences are the 

 mean of the elongations on the two sides of the test-bar, or 

 virtually are readings at the axis of the test-bar. Headings 

 to 10 Q 00 th of an inch can be taken. 



Roller- and-Mirr or Extensometer.— Fig 3 is a diagrammatic 

 sketch of another instrument on the same principle, a and b 

 are two clips similar to those in fig. 2 ; the lower clip is sup- 



