TORSIONAL PROPERTIES OF HAIR 579 place under the loading conditions described. However most fibers do exhibit creep when they are subjected to stress, the deformation observed continues to increase with time. The treatment by the simple spring theory is limited to the extent that hair fibers exhibit viscous mechanical properties and depart from ideal elastic behavior. While it is intended to consider the properties of "springs" made of hair, it was thought useful first to examine the characteristics of the spring material--the hair fiber. It was, therefore, decided to study the mechanical properties of hair in torsion to obtain some measurements of the usual elastic constants (torsion modulus) and some information as to the creep behavior. Since moisture and permanent waving are im- portant in cosmetic practice the effects due to these were also investi- gated. Work was undertaken to measure the physical properties of hair in torsion using two different methods. The first technique will be referred to as the torsion pendulum method. It will not be described in detail since the method has been used by a number of workers in studies of textile fibers (3-5). In brief, the test hair is utilized to suspend a small bob which can be set into free rotational oscillation. By measuring the period of oscillation (T), the fiber length (l), and diameter (d), and by calculating the moment of inertia (I) of the bob (from its weight and shape), it is easy to compute the fiber torsion modulus: 128 ,r I 1 G - T2d4 (2) The measurements were made in a constant temperature room at 21 ø m 1 øC, the fibers being suspended in glass jars over saturated solu- tions of different salts. Equilibration of the fiber with the constant humidity atmosphere in the jars was allowed to take place for at least two days. It was also possible to make observations useful in assessing the tendency of the fiber to flow or creep under the influence of torsional strain. Lochner (6), using textile fibers, showed that the damping or the decrease in amplitude of successive oscillations of the torsion pendulum is related to the internal viscosity of the fiber. He used a parameter called the logarithmic decrement (S) which is the natural logarithm of the ratio of the amplitude of successive swings (a•, a•, aa... a,) of the torsion bob' S = 2.3 log•0 a• (3)

580 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS - HYPODERMIC NEEDL:E MOUNT •---•f•'--'"•---- ,':-OR FIBER • TEST FIBER TORSION BOB •. SATU R/• TED SALT • •'" :--.-, ,,, ...-. . ::..: SCHEMATIC DIAGRAM OF APPARATUS USED FOR TORSION MEASUREMENTS BY PENDULU•,• METHOD. A - PLAN VIE'N OF ARRANGEMENT OF JAR CONTAINING TEST FIBER, 'FORSIOII BOB, OP TICAL LENSES AND SCALE. B - ENLARGED VIE'It OF FIBER AND TORSION BOB. Figure A The sense of the meaning of S can be seen by asserting that for fibers which are highly elastic by nature any rotational oscillation induced will continue for a long time with only very small decay in amplitude, any change being produced largely by the frictional resistance of the air on the bob. Conversely, in the case of fibers of low elasticity, the me- chanical work stored in the fiber with a swing of the bob in one direction will not be entirely recoverable on the reverse oscillation. Thus, fibers which show appreciable damping are said to creep or to have high internal viscosity and will exhibit large decreases in the amplitude of