[coker] APPAEATUS for APPLYING BENDING MOMENTS 63 



figure 3, from a ring pivoted on a circular knife edge, screwed on to the 

 weigh lever arm, and secured by clamping nuts on each side. 



The general arrangement of the apparatus, with a test bar in posi- 

 tion, is shown in perspective in Fig. 3a, where it is seen that the 

 specimen is stressed beyond its yield point by the oombined stresses 

 and is bent considerably, the angular distortion due to the twisting 

 moment is not apparent since the lever R has been brought 

 back to the horizsontal by the worm-wheel gear. In determining 

 the angle of twist of a specimen, use is made of an angle 

 measurer designed by the author,^ which can be arranged to read 

 accurately on a bent bar. For reaidings before the yield point is 

 reached some such device is necessary, but afterwards the worm-shaft 

 can be used, as the errors due to the twdsting of the enlarged ends can 

 then be neglected. In the present arrangement the worm-wheel has 

 forty-eight teeth, so that one complete turn of the worm shaft corre- 

 sponds to a twist of 7^°. A graduated disc upon the shaft is divided 

 so that each division corresponds to five minutes of arc, which is suflS- 

 ciently fine for these relatively rough measurements. To prevent mis- 

 takes in reading, the worm-wheel is also divided to check the number of 

 revolutions of the worm shaft, since in testing most metals to destruc- 

 tion, one end must be twisted round several complete turns, and with- 

 out some counting device, mistakes in measurement are likely to arise. 

 As an example of the results obtained in a case of combined stress, 

 reference may be made to a few experiments on some samples of 

 wrought iron, all of which were cut from the same bar and turned up to 

 a standard size of 0.375 inches. The length of the specimen under test 

 was in all cases, four inches, and the calibration value of the measuring 

 instrument gave one minute of arc as corresponding to 8.68 divisions 

 of the scale. A specimen was first tested by applying a gradually 

 increasing torque without bending, and the bar failed with a torque of 

 about 285 inch pounds, the readings taken are shown in Table I., and 

 the stress-strain curve from these is plotted in Figure 4. Similar spe- 

 cimens to which bending moments of various amounts were applied 

 were then tested, and it was found that an increase in the bending 

 moment lowered the yield point of the material, but as long as the bar 

 remained perfectly elastic, no difference in the angular deflection 

 occurred, beyond what might be reasonably accounted for by unavoid- 

 able experimental error and also by the fact that the specimens differed 

 very slightly in diameter. 



^ Philosophical Mag-azine, December, 1899. 



