CHAPTER III. 

 AUXILIARY MEASURING APPLIANCES. 



57. In addition to the testing machines already described, 

 other appliances are needed for the purpose of taking 

 linear measurements of the test specimens. Such 

 measurements are required both before, during, and after a 

 test. Previous to a test, all the dimensions of the 

 specimen must be carefully determined. Measurements, 

 often of a delicate character, are frequently taken while a 

 test is proceeding, to determine the strains produced by 

 the stresses to which the specimen is being subjected ; and 

 it is necessary to carefully measure the specimen after 

 the completion of the test. The instruments used for these 

 purposes may be divided into two classes, namely, those 

 which are used for taking the dimensions of specimens 

 before and after the test, as well as the rougher the 

 measurements taken during the test ; and, secondly, those 

 used for taking the minute measurements required during 

 the test, while the material is in the elastic state. These will 

 now be considered separately and in the order mentioned. 

 58. Instruments for Measuring Test Specimens. 

 The first, and one of the most useful of measuring instru- 

 ments, is the ordinary steel rule. One or two of these are 

 absolutely necessary in a testing laboratory. The most 

 useful length is about 16 in., and, in addition, one 24 in. 

 and one 4 in. long will be found useful. The best way to 

 have the rules divided is into inches on both edges, and to 

 have these inches subdivided into tenths, fiftieths, and 

 hundredths on one of the edges, and into eighths, six- 

 teenths, and thirty-seconds on the other. For all measure- 

 ments of test specimens, both in and out of the machine, 

 inches and decimals of an inch will be found the most 

 useful units, just as tons and pounds divided into decimal 

 parts are the most convenient units of force in testing 

 work. The ordinary steel rule is most often used in con- 

 junction with a pair of spring dividers, or screw-clamp 

 dividers. A useful size for these is from six to eight inches 

 length of leg. When using the dividers to take measure- 

 ments on a test specimen in the machine, the two points 

 are applied to two centre-punch marks which have pre- 

 viously been made on the test bar, and then the length so 



