21*4 COKER'S INSTRUMENTS. 



that the calibration test gave 36 divisions of the scale as 

 corresponding to one minute of arc. Angular displacements 

 of one second of arc can be measured with this instrument. 

 On Fig. 10-4 is shown a most important attachment to 

 this instrument. This is the clamp used when putting the 

 instrument on the test bar. Its object is to fix the apparatus 

 on the bar with its plane accurately perpendicular to the 

 axis of the bar, and its centre coinciding with the axis. 

 This clamp consists of a pair of divided collars, the halves 

 of which are pivoted together and secured by nuts. The 







FIG. 104. - Fixing C'amp 



collars are wedge-shaped in radial section to engage with 

 corresponding wedge-angled grooves upon the chuck plate 

 and cylinder. The lower halves of the divided collars are 

 connected by one or more distance pieces. When using 

 the clamp, the two parts of the instrument are clamped 

 together and so held rigidly in the proper position, and 

 then the test bar is inserted and the gripping screws 

 tightened, when the clamp may be removed, leaving the 

 two main pieces accurately spaced on the bar, while the 

 graduated circle remains perpendicular thereto, and very 

 approximately centred.* 



119. Example of a Complete Test of a Mild Steel Bar in 

 Torsion. The following is an example of a test under 

 torsional stress carried out on a turned bar of mild steel. 

 This bar was made with enlarged ends, and fitted with 

 keys, by means of which it was held in a torsion machine of 

 the kind illustrated on Fig. 94. After carefully measuring 



* Instruments for Measuring Small Torsional Strains." British 

 Association, 1898. 



