10-t VEHNIKH CAL1PER. 



measured is transferred to the steel rule for a determination 

 of the length in question. For the reason that it is al\\ ;i \ ! 

 easier to measure a distance on a rule when beginning from 

 some definite point or division rather than from the -end, it 

 is advisable to have the inches numbered from a mark 

 which is not quite at the end of the rule, so that one leg of 

 the dividers can be placed on this mark instead of at the 

 actual end. 



When the extensions of very long tension specimens 

 have to be measured, it will be found necessary to have a 

 pair of trammels. These are not so convenient to use as 

 dividers, but they are necessary in some cases. 



The dimensions of most metal test pieces should be 

 measured to thousandths of an inch, and for this purpose 

 something more accurate than an ordinary steel rule is 

 required. The two instruments used for this work are the 

 vernier caliper and the micrometer screw caliper. A 

 general view of the vernier or bar caliper is shown on Fig. '- } >- '. 

 It consists of a graduated steel bar having a projecting 

 jaw at one end. This jaw projects at right angles to the 

 main bar, and the object to be measured is placed between 

 this jaw and a second one, which is capable of being 

 moved up and down the bar into any required position. 

 It will be seen that the moving jaw can be fixed in any 

 position by means of a small set-screw at the top. In 

 addition to the moving jaw itself there is a second piece of 

 steel sliding upon the bar, and attached to the sliding jaw 

 by means of a small adjusting screw. This sliding piece 

 can be fixed in any position on the bar, and the sliding 

 jaw moved with respect to it by means of the adjusting screw. 

 By this means the sliding jaw can be roughly moved into 

 position and then finally adjusted into the exact position 

 required by the screw. On Fig. 40 an enlarged view 

 of the fixed and sliding jaws is shown. It will be seen 

 that the bar itself is graduated, and also that the sliding jaw 

 carries a small scale or vernier. In the most convenient 

 arrangement the main scale on the bar is divided, in the 

 first place, into inches ; each inch is subdivided into tenths, 

 and each tenth is again subdivided into four equal parts, 

 these being fortieths of an inch. By means of the vernier 

 each of the fortieths can be further subdivided into 

 twenty-five equal parts, these smallest division? being 

 thousandths of an inch. The twenty-five parts on the 

 vernier correspond in their total length to twentv-four of 

 the smallest divisions on the parent scale. Therefore, each 



