Chapter 7 239 where a and b are again the major and minor semi-diameters. The resistance of the hair to bending is proportional to ab3 rather than a2b2 because an elliptical hair will always bend over its minor axis. Scott and Robbins49 described a balanced fiber method for measuring the bending stiffness of hair. A long hair is draped over a fine wire hook with small weights attached to each end of the hair. The bending stiffness can be calculated by measuring the distance, D, between the two weighted ends. The apparatus is illustrated in Figure 10. The bending stiffness determined by this method is well correlated with the elastic modulus, as seen by the plot of stiffness index versus the slope in the Hookean or linear region as seen in Figure 11. Wortmann and Kure53 made innovative use of this method to investigate effects of changes in bending properties on the strength Figure 8. The torsion pendulum used by the Textile Research Institute (image courtesy of Trefor Evans) Figure 9. Illustration of the three zones in the bending of a cylindrical object