The Measurement of the Dielectric Constant of Standard Liquids

Abstract

New determinations have been made of the dielectric constants of benzene, cyclohexane, dichlorethane and nitrobenzene with the object of providing values of a higher accuracy than has hitherto been available for four standard liquids covering the range of values commonly required. Liquids of specially high purity were obtained with the co-operation of Professor J. Timmermans of Brussels and Dr. Herington of the Chemical Research Laboratory, Teddington, and the measurements were made by the a.c. bridge technique developed at the National Physical Laboratory for precision capacitance measurement and also by a cavity resonator method. Part I of the paper discusses the general problem and the special features of the capacitor designed to receive the liquids and surveys the results obtained. Part II gives details of the measurements made by capacitance methods at frequencies from 50 c/s to 10 kc/s, and Part III describes the measurements made at microwave frequencies. The values obtained confirm estimates of the most reliable values and their probable accuracy published in 1951 by A. A. Maryott and E. R. Smith as a result of a survey of existing data made at the National Bureau of Standards, Washington. It is considered that the new values narrow the range of uncertainty by a factor varying from 3 to 10 for the different liquids. The accuracy obtained for benzene at audio frequencies was within about 0.01%; at 9000 Mc/s the accuracy was a trifle worse but the results show clearly that although the dielectric constant exceeds the square of the optical refractive index by about 1%, no measurable diminution in dielectric constant occurs in passing from 50 c/s to 9000 Mc/s. A considerable increase in power absorption is however noticeable at 9000 Mc/s and an even greater increase at 25 000 Mc/s, and it is shown that this absorption is not entirely dependent on the water that most samples of benzene contain and that can only be removed by weeks of drying treatment. The necessity of applying such treatments is a serious disadvantage to the use of benzene as a standard liquid for measurements of dielectric constant, and cyclohexane, which is free from this disadvantage, is recommended instead.