Ductile Materials under Combined Stress. 83 



load, but the travelling poise (500 lbs.) was removed and 

 found to be correct, and the distance between the knife-edges 

 was measured, with the same result. In a few of the tests 

 there was evidence that the axes of the specimen and of the 

 pull did not exactly coincide, causing a small bending moment 

 on the specimen. This tends to produce an effect similar to 

 that of the elastic limit. 



27. The Application of the Torque. — In order to apply 

 torque to the specimen, it was placed in the testing machine 

 and a small load (250 lbs.) placed upon it, to line it up and 

 steady it. The cross-bars were then placed in the holders 

 and nipped in position by the set screws E. Steel points F 

 were then placed in the upper cross-bar, and a pair of screws 

 in a bracket, bolted to the casting of the testing machine, 

 were adjusted to just touch the lower cross-bar. Finally, 

 equal and opposite forces were applied to the steel points 

 by means of bent pieces of iron, as shown in the left-hand top 

 corner of fig. 2, attached to belts. These belts were belt 

 laces, which being thin, flexible, and sufficiently strong, 

 appeared to be the best means of transmitting round pulleys 

 such forces as were required. The belts led off horizontally 

 from the ends of the cross-bar, which was adjusted vertically 

 by the screw A. of the tube-holders, to pullejs, from which 

 they went vertically downwards to knife-edges fixed in the 

 ends of a horizontal cross lever, from a knife-edge at the 

 centre of which hung the carrier for the weights which pro- 

 duced the torque. The effect of the cross lever and weight- 

 carrier was neutralized by balance- weights hung over the 

 pulleys. These pulleys were made by filling the rims of 

 bicycle wheels with plaster-of-paris, and, before the plaster 

 had set hard, rotating the wheel in contact with a former, 

 thus producing a suitable groove true with the axis of rotation. 

 The torque thus applied to the specimen was practically pure, 

 and produced no bending moment on the specimen. It was 

 resisted by the forces produced by the set screws on the lower 

 cross-bar, there being thus little or no sideways force at the 

 lower grip. 



28. The Friction of the Torsion Rigging. — A fourth pulley 

 was used in testing the friction of the torsion apparatus. 

 The greater part of this friction, when any considerable load 

 was on the specimen, occurred in the ball-bearings in the 

 shackles. To ascertain the friction two equal and opposite 

 torques were applied to the top cross-bar by means of belts 

 leading over each of four pulleys and terminating in a weight- 

 carrier. The lower cross-bar was removed. The desired 

 tension load P was then placed upon the specimen, and 



G2 



