DIELECTRIC PROPERTIES OF INSULATING MATERIALS 505 



Dielectric studies of this kind have also shown, for example, that 

 H2O is not a symmetrical linear molecule, H — O — H, but rather a 



triangular structure 0<( . CO2 on the other hand, being non-polar, 



is determined to be a linear molecule O = C = O. Thus, dielectric 

 measurements interpreted by the Debye theory have become estab- 

 lished as one of the standard means of studying molecular structure. 



Since dipole polarizations depend upon the relative orientations of 

 molecules, rather than upon the displacement of charges within the 

 atom or molecule, the time required for a polarization of this type to 

 form depends upon the internal friction of the material. Debye 

 expressed the time of relaxation of dipole polarizations in terms of the 

 internal frictional force by the equation: 



where ^ is the internal friction coefficient, 7/ is the coefficient of viscosity, 

 a the radius of the molecule and T the absolute temperature.^ This 

 latter expression, because it depends on Stokes' law for a freely falling 

 body, is rigidly applicable only to gases or possibly to dilute solutions 

 of polar molecules in non-polar solvents in which the polar molecules 

 are far enough apart that they exert no appreciable influence on each 

 other. 



Applying this equation to the calculation of the relaxation-time of 

 the orientational polarizations in water at room temperature we obtain 

 T = lO""^** seconds, assuming a molecular radius of 2 X 10~^ cm. and 

 taking the viscosity as 0.01 poises.* The relaxation-frequency corre- 

 sponding to this relaxation-time is about 1.6 X 10^ cycles/sec, agreeing 

 with the results of experimental studies on water which show that 

 in the range of frequencies extending from 10^ to 10^^ cycles the 

 dielectric constant decreases from its high value to a value approxi- 

 mately equal to the square of the refractive index. Thus the drop in 

 dielectric constant occurs in the frequency range which corresponds to 

 the calculated value of the relaxation-time. 



Similar experiments on dilute solutions of alcohols ^ in non-polar 

 solvents yield values of r of about 10~^ seconds. The shortest relax- 

 ation-times which dipole polarizations can have are probably not 



^ P. Debye, "Polar Molecules," Chem. Cat. Co., 1929, p. 85. 



* The viscosity of a liquid in poises is given by the force in dynes required to 

 maintain a relative tangential velocity of 1 cm. /sec. between two parallel planes in the 

 liquid each 1 cm.^ in area and 1 cm. apart, the distance being measured normal to their 

 surfaces. 



6 R. Goldammer, Phys. Zeit., 33, 361 (1932). 



