This paper deals with investigations of the electric birefringence of colloidal solutions in alternating fields; the most detailed measurements were carried out with monodisperse bentonite sols. In general the double refraction consists of a direct and an alternating part, both of which depend on field strength and frequency of the applied field, particle size, temperature, and concentration of the solution. The direct birefringence, which can be positive or negative, can reverse its sign with a change of any of the five parameters mentioned. The measurements are interpreted by means of a relaxation theory which can explain all phenomena connected with the vibrating component in a satisfactory way. Likewise this theory is capable of explaining the variations and the reversal of the direct birefringence with changes of the frequency, field strength, particle size, and temperature. The character of the dependence of the steady birefringence on concentration which has not hitherto been observed with other colloids was first discussed by H. Mueller. The changes and reversal of sign of the direct component with concentration are accounted for by the assumption that the micelles are surrounded by a compressed water hull; the pressure of the water hull depends on particle size and concentration. Direct and alternating birefringence have different relaxation times.
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