TENSILE TESTING MACHINE. 49 



end firmly attached to a rigid support, and applying the 

 loads in the form of deadweights to the lower end. This 

 is shown in the accompanying Fig. 17, where a wire, S, is 

 being tested by placing weights in a scale pan attached 

 to its lower end, while its upper end is fastened to a hook 

 in a massive beam, resting at its ends upon supports. 

 This is at once a simple and inexpensive form of testing 

 machine, but it is an arrangement which can only be 

 used to a very limited extent. 



So long as the cross section of the specimen is small, or 

 the material of such a nature as is unable to resist great 

 tensile stress, a " deadweight " arrangement of this kind 

 may be used. But when larger specimens of stronger 

 materials have to be tested, other means must be sought 

 for, both for applying the loads and for measuring them. 

 When" the loads are movable weights of a few pounds they 

 are easily lifted on and off the scale pan, but when hundred- 

 weights and tons come to take the place of pounds, it is no 

 longer possible to deal with these in the same way. 



21. Tensile Testing Machines. In the deadweight 

 machine the application and measurement of the load are 

 effected by one and the same operation; in the larger 

 testing machines, which we are now about to consider, 

 these two are not quite the same thing. The principle in all 

 tensile testing machines is the same. The specimen to be 

 tested is firmly gripped or held at its two extremities. 

 The shackles in which these ends are held are attached 

 respectively to a measuring appliance at the one end, and 

 at the other to a part of the machine which is so arranged 

 as to be capable of exerting the required pull upon the 

 specimen, and not only to exert a statical pull, but to 

 exert this pull steadily through such a distance as shall 

 be determined by the stretching of the bar, in order that 

 the end at which the magnitude of the load is measured 

 shall not alter its position relatively to the measuring 

 apparatus. 



22. Methods of Applying the Load. It is perhaps neces- 

 sary to state a little more definitely Avhat is meant by 

 " applying the load." The application of the load is in 

 reality the controlling or regulation of the load at the will 

 of the operator. When a bar is being subjected to a tensile 

 load there must necessarily be at the respective ends two 

 equal and opposite forces, and provision must be made for 

 the application of these two. If one end is firmly held, 

 and the load applied at the other, an equal and opposite 



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