TESTING OF WIRE. 257. 



may be done to the wires in some cases by the heating 

 during the process of pouring in the molten metal. Where 

 this is used the metal itself should be of such a nature 

 as to allow of no yielding of the wires, and for this 

 reason it should be comparatively hard on setting. If 

 lead is used a small admixture of antimony hardens it and 

 a little bismuth causes it to expand on setting. It is some- 

 times asserted that the rope itself ought to yield a result 

 better than the total strength of the wires because the 

 wires as they lie in the ropes are inclined to the axis, and 

 therefore present an area to the axial pull greater than 

 the area on a section at right angles to the axis of the wire. 

 The rope, however, must not be regarded as a rigid bar of 

 the metal, and though the wires are to some extent 

 supported in position, the pull is necessarily axial to the 

 wires and not to the rope. If the wires were not supported 

 laterally the opposite effect would be produced of the 

 tension in the wires being greater than the axial tension 

 on the rope. The conditions would appear to be analagous 

 to the case of a wire passing over a pulley, when the 

 tension is sensibly constant at all points of the wire. 



135. The Testing of Wire. The difference between a wire 

 and a larger bar of the same material is not in size alone, but 

 in the condition of the metal forming the wire, a condition 

 caused by the process of manufacture. The effect of wire- 

 drawing, as opposed to rolling, is to produce a condition 

 of greater hardness than the original metal from which the 

 wire has been produced, and also greater tensile strength. 

 This artificially produced hardness may be made to dis- 

 appear by annealing after wire-drawing, but the tensile 

 strength may or may not be reduced to the strength of the 

 original metal ; in some cases annealing lowers the strength 

 below that of the original metal. 



The tests to which wire is usually subjected, in order to 

 obtain definite information as to its strength properties 

 are three in number, and are as follow : 



(a) Repeated bendings through a definite angle. 

 (6) Torsion or twisting tests, 

 (c) Ordinary tensile tests. 



136. Bending Test. The bending test for wire may 



be carried out in a special piece of apparatus made for 



the purpose, or in an ordinary vice. The jaws of the 



holding vice should be rounded, and made either to 



s2 



