498 Prof. E. G. Goker on a Maeliine for applying 



the inner sides, and light rods V screwed into these rings 

 carry the weights. This arrangement of knife-edge is very 

 easy to adjust accurately, and when bending and twisting 

 stresses are applied simultaneously the rolling line contact 

 adjusts itself to the bending and twisting of the specimen. 

 The bending of the specimen causes a change in the effective 

 arm of the bending levers, which is generally negligible, but 

 a correction may be necessary with a very long specimen. 

 For if a is the length of the lever-arm and b is the radius of 

 the circular knife-edge, an angular deviation of amount 6 

 will cause a change of a— (a cos + b sin 6) in the lever- 

 arm, and this is zero when = and also when a =a cos 6 

 + b sin 6. 



In the machine described a is 10 inches and b is 0*5 inches,, 

 and the angles = and = 5 O, 75 both correspond to an 

 effective length of 10 inches. The maximum correction 

 between these values is easily shown to be at an angle 6 

 given by the equation b cos 6 = a sin 6 3 in the present case 

 2°*9 approximately, for which value the correction is 0'12 

 per cent. For values of greater than 6° the correction 

 increases more rapidly, and its amount may be obtained from 

 the diagram, fig. 7, which shows the percentage error for all 

 angles up to 10°. In the majority of tests the angular change 

 at the ends rarely exceeds 5°, and the correction is therefore 

 so very small as to be practically negligible. 



The worm-wheel W and the casting V for the weigh- 

 levers are bored out to receive the ends of the specimen, and 

 are provided with fixed keys which slide in corresponding 

 key-ways cut in the specimen. When tubes are subjected to 

 stress they are provided with solid ends secured by transverse 

 pins, thereby avoiding brazed joints since these latter are 

 troublesome owing to the state of the metal being altered, 

 by the brazing. The end of the specimen projecting through 

 the worm-wheel is fitted with a lever X for applying 

 bending moment, and both levers for bending may be loaded 

 independently or by a cross-bar suspended from stirrups as. 

 shown in fig. 2. 



A photograph of the machine is shown by fig. 8 (PL XII.) 

 with a specimen inserted which has failed under the combined, 

 effect of bending moment and twisting moments. 



Measurement of the Strams. 



The worm-wheel is graduated in degrees and a vernier 

 circle enables 0*1 of a degree to be read with ease, while in 

 order to start with a zero reading this vernier is carried on a 

 ring sliding in a groove in the casing so that it can be- 



