276 The Use of Soap Films in Engineering [April 19, 



WEEKLY EVENING MEETING, 



Friday, April 19, 1918. 



General E. H. Hills, C.M.G. R.E. D.Sc. F.R.S., Secretary and 

 Yice-Presideiit, in the Chair. 



Major G. I. Taylor. 

 The Use of Soap Films in Engineering. 



[Abstract.] 



The similarity, first noticed by Prof. Prandtl, between the equations 

 representing the strains in a twisted bar of any section and the 

 deformation of a membrane stretched uniformly over a plane 

 boundary of the same shape as the cross-section of the bar, and 

 deformed by the action of a uniform pressure, has been used to find 

 the strains in twisted bars in cases where they cannot be calculated. 

 The uniformly stretched membrane used in the experiments was a 

 soap film, and films were projected on the screen in such a way as 

 to illustrate, by means of Prandtl's analogy, the principle features 

 of the strains in twisted bars. 



In this way it was shown that the strain at an internal corner 

 becomes infinite, while that at an external corner vanishes. 



As an example of the usefulness of the method in solving prac- 

 tical problems in engineering, the amount by which it is necessary 

 to round off the internal corner of an L-shaped beam in order that 

 the strain may not become too great when the beam is twisted, was 

 worked out by means of soap films. 



Apparatus designed for measuring the soap films was shown. 



Some of the results obtained from the measurement of soap films 

 are not easy to understand when applied to twisted bars. Some 

 gelatine models were exhibited and projected on the screen which 

 showed how the strains arise in these cases. 



[G. I. T.] 



