522 Mr. E. G. Coker on Instruments for Measuring 



To calibrate the instrument it is therefore only necessary 

 to measure the number of divisions corresponding to a small 

 angular displacement of, say, 10'; and this is easily accom- 

 plished by setting the instrument in position with the circuit 

 complete, and afterwards following up a known angular dis- 

 placement of the vernier-plate by the micrometer-screw. 



A simple form of detector-galvanometer, in circuit with a 

 single dry cell, has been found to be a convenient arrangement 

 for indicating when contact takes place, and the feeble current 

 required does not injure the contacting surfaces. 



It is essential that the graduated circle be set accurately 

 upon the bar, with its plane perpendicular thereto and its 

 centre coinciding with the longitudinal axis of the bar. An 

 arrangement has been devised to effect this, consisting of two 

 similar and equal clamp-bars, the eyed ends of which take 

 over outwardly projecting cones arranged diametrally upon 

 the chuck-plate and ring. Each main piece has one degree 

 of freedom with respect to the clamp-bars, and therefore two 

 degrees of freedom with respect to the other ; these degrees of 

 freedom are suppressed by projecting plates fitting against 

 corresponding projections on the main pieces ; and this con- 

 nexion makes the apparatus a rigid whole. The bar is now 

 inserted and the screws adjusted by hand as accurately as 

 possible. 



The clamp-bars are"'afterw r ards removed, leaving the two 

 main pieces accurately spaced on the bar, while the graduated 

 circle remains perpendicular thereto and very approximately 

 centred . The light contacting arm is then clamped in position, 

 and the bar may now be set in the testing machine. 



An improved clamp described with reference to the second 

 form of apparatus may be used instead of the arrangement 

 described above, and the hand-operated chuck-plate may be 

 replaced by a form of self-centering chuck described below. 

 An example of tests made with this apparatus is given 

 below. 



The test-bar was adjustably secured at one end and a 

 balanced lever of fixed length secured upon the free end, and 

 hung from the arm of a scale-beam. The load was applied 

 by placing equal weights in the suspended pans of the balanced 

 lever and scale-beam, so that bending movement was as far as 

 possible eliminated. Before making a reading the torsion- 

 arm was brought to a horizontal position by aid of a spirit- 

 level. 



The mean of the calibration tests gave 18*6 divisions of the 

 micrometer-screw as corresponding to an angular displace- 

 ment of 1 minute of arc. 



