ON STRESS DISTRIBUTION IN ENGINEERING MATERIALS. 479 



(fe) The auto-collimator, which measures the slope of the film at any- 

 desired point by an optical reflection method. 



The most important practical problem which may be solved by this 

 method is that of the torsion of a cylinder, or beam of uniform cross-section. 

 In this case, the dilation is zero, and the appropriate soap film may be 

 formed on a hole, in a flat plate, of the same shape as the cross-section 

 of the beam. The film is slightly displaced from the plane of the plate 

 by maintaining a small excess of pressure on one side of it. The slope 

 of the film at any point is proportional to the stress at that point, the 

 direction of the contour lines is the direction of the stress, and the volume 

 between the film and the plane of the plate is proportional to the torsional 

 stiffness of the beam. The constants of proportionality are obtained 

 by comparison with a circular film under the same pressure difEerence. 



Sections of any shape whatever may be dealt with, but in the case of 

 hollow beams the inner boundary or boundaries must be parallel to, but 

 not coincident with, the outer one. The proper relative positions may 

 be found by measuring auxiliary soap films. 



A further problem of considerable practical importance, which may be 

 solved by the soap film method, is the determination of the shearing 

 stresses in a bent beam. In this case, the hole representing the boundary 

 must be cut in a curved plate, the shape of the elevation of the edge being 

 calculated from the dimensions of the section. This film is not blown up. 

 The components of shearing stress are calculated from the components 

 of the slope of the film, in the directions ox and oy respectively. The twist 

 of the beam, under a load applied at any given point, may also be found. 



The following papers deal with the subject : 



(1) ' The Use of Soap Films in Solving Torsion Problems.' Vroc. 



Inst.Mech.E., December 14, 1917. 



(2) Reports of the Advisory Committee for Aeronautics : — 



R. & M. 332, R. & M. 334, T. 1040, T. 1076, T. 1275. 



Eccentric Loading in Tension and Compression Tests. 

 By Walter A. Scoble. 



The earlier tests were made to determine the degree of variation of 

 stress across the section of a specimen subjected to tension or compression. 

 Different arrangements for the application of the load Were tried, and the 

 specimens were prepared from various materials. 



The efiect of the eccentric loading was surprisingly pronounced, and 

 a further series of experiments was made on different metals to estimate 

 the effect on the test results of the unequal stress distribution across the 

 section of the specimen. 



A distinction must be drawn between the two series of tests. The 

 first set represents conditions which are realised in practical testing, but 

 for the second series the load was occasionally purposely made eccentric 

 to yield evidence of the effect of bending on the behaviour of the metal 

 under test. 



The extensometer used (fig. 13) consisted of two independent clamps 

 attached to the specimen by pointed screws. The lower clamp carries 



