

258 TESTING OF WIRE. 



some pre-arranged radius or to a radius equal to the 

 diameter of the wire which is being tested. The test 

 consists in holding the wire in the vice and then bending 

 the portion outside the vice backwards and forwards 

 through a constant angle until fracture takes place, the 

 number of bendings before fracture, the radius of curvature 

 of the jaws, and the an^le through which the bending has 

 taken place being notea. To give the reader an idea of 

 what occurs, it may be stated that a piece, of iron wire 

 about one-tenth of an inch in diameter, when bent at 

 right angles backwards and forwards over jaws having 

 radii of 0'4 in., fractures after about twenty bendings. 



FIG. 125. 



137- Torsion Tests for Wire. The torsion test consists in 

 holding a certain definite length of the wire in such a way 

 that one end is firmly fixed and the other end is tAvisted 

 with respect to it until fracture takes place. Special 

 machines are used for this purpose. A. typical example is 

 shown on Fig 125. This machine is made by Messrs. 

 Denison, of Leeds. The grippers for holding the ends of the 

 wire are automatically self-tightening. The end of the wire 

 on the left is prevented from rotating while the right-hand 

 end is twisted by means of the hand-wheel until fracture 

 has taken place, a counter being provided for the purpose 

 of ascertaining the total number of revolutions through 

 which the wire has been twisted. The piece of wire is of 

 a certain definite length, and it is often specified by buyers 

 of wire that it must be able to withstand a certain number 

 of twists on a certain length before fracture takes place. 

 For instance, a piece of good crucible steel wire, 8 in. 

 in length, and Xo. 20 B.W.G., requires over 100 revolutions 



