Changing the Shape of Hair 160 is used to keep the hair straight as is it wound on the rod. Robbins used 1-gram alligator clips to weight the hair.5 This author has found that small “split-shot” fishing weights weighing about 0.4 grams are very convenient for this purpose. They can be crimped to the free end of the hair and then snipped off before the hair is removed from the coiling rod.7 It is instructive to look at the equation for the spring constant, k, of a helical spring at low extensions: Eq. 2 Where: G = the shear modulus of the material (in this case hair) D = the diameter of the spring d = the diameter of the hair. (For an elliptical hair d4 should probably be replaced by a2b2 where a and b are the major and minor diameters of the ellipse.) n = the number of turns of the coil. We see that the strength of the hair “spring” is related to the fourth power of fiber diameter and inversely related to the third power of the coil diameter. Thus, fiber diameter is a critical factor in determining the strength of the spring produced when hair is curled. In addition to producing a strong spring for a given curl, thicker Figure 3 A: Method of winding a helical coil of hair at diameter D, with coil angle B: Hanging coil of length l, diameter D, pitch P and coil angle.