Measurement of the Dielectric Constant and Index of Refraction of Water and Aqueous Solutions of KCl at High Frequencies

Dielectric constant of distilled water.-The dielectric constant of distilled water was measured with a vacuum tube source of voltage throughout a range of periods extending from about T=1.31×10^-8 second to T=8.49×10^-8 second corresponding to vacuum wave-lengths of 3.918 meters to 25.47 meters. The value obtained for the dielectric constant is ɛ=78.57 at 25°C and is independent of the period within the above range of periods. Temperature coefficient of dielectric constant for distilled water.-The values of the dielectric constant ɛ obtained for various temperatures t between 10°C and 60°C satisfy the empirical equation ɛ_t=78.57[1-0.00461(t-25)+0.0000155(t- 25)²...]. Between 4°C and 10°C the applicability of this equation is somewhat uncertain. Dielectric constant of conducting solutions.-With distilled water, tap water, and aqueous KCl solutions of conductivity ranging from 0.97×10⁷ e.s.u. to 178×10⁷ e.s.u., and separating the conduction effects from the dielectric effects, we found the dielectric constant only slightly dependent on the ionic concentration of the solution and nearly the same as that of pure water. The errors of measurement are of the order of 0.1 percent for low conductivities but, because of attenuation, become of the order of 1 percent at the high concentrations of the salt. Method.-The method employed consists in the measurement of standing electric waves between a pipe and a wire concentrically located within the pipe. The liquid is the dielectric between the wire and the pipe. The frequencies employed were accurately measured by a piezo crystal standardizing a zero beat method, and the half wave-length within the dielectric were obtained by moving a piston in the liquid and observing reactions on the source. A theoretical discussion of the method is given.