The electrical properties of the earth play an important part in many modern applications of physics and electrical engineering. In the early portion of this paper a summary is given of the published information on the electrical properties of soil for alternating currents at frequencies from 50 to nearly 200 million cycles per sec. Particular interest is attached to the properties at radio frequencies, and it is shown that, while a certain amount of knowledge has been acquired by the use of field methods depending upon some characteristic of electric wave propagation, the results available apply to certain rather scattered ranges of frequencies and to some fairly localized portions of the earth's surface.The later portions of the paper are concerned with the investigation of the conductivity and dielectric constant of samples of soil by a laboratory method of measuring resistance and reactance at radio frequencies. This method was used for the majority of the measurements which were carried out at frequencies between 100 and 10 000 kilocycles per sec., but, by the aid of a standard type of capacitance bridge, the work was extended through the audio-frequency range to a power frequency of 50 cycles per sec.A series of preliminary measurements showed that reliable results could be obtained which were independent of the size and shape of the fixed condenser, in which the soil was packed as the dielectric. The conductivity and dielectric constant were both found to be very dependent upon the moisture content of the soil, except when this was within the normal range for the site under consideration. In a typical case the conductivity for dry soil, at a frequency of 1 200 kilocycles per sec., was of the order of 105 electrostatic units (corresponding to a resistivity of 9 megohms per cm cube), while at a moisture content of between 12 and 26 per cent a limiting value of between 108 and 2 × 108 electrostatic units (resistivity 9 000 to 4 500 ohms per cm cube) was obtained. The corresponding values of dielectric constant ranged from 3 or 4 for dry soil up to 30 or 40 for very moist soil. The effect of frequency on conductivity was more marked for dry than for moist soil, and in the latter case there was a small although definite rise in the conductivity at the highest frequencies. The dielectric constant decreased with increase of frequency towards a limiting value for moist soil, whereas for dry soil the variation of dielectric constant with frequency was very small. From measurements made on samples of different types of soil it was found that the temperature coefficient of the conductivity at 20°C. was about 2.3 per cent per deg. C., whereas the effect of temperature on the dielectric constant was negligible. In both cases, however, a sudden decrease in the value took place in passing through O° C., which is attributable to the partial or total freezing of the water content.When the validity of the laboratory method had been established, and it had been found that the results obtained thereby agreed with those previously available from field measurements, the investigation was extended to a study of the properties of soil from 12 different sites in England and Scotland. In the majority of these cases, samples of soil were obtained for measurement at various depths down to 10 ft., while in one case the depth was extended to 300 ft. The results are given in detail in tabular form in the paper. It is shown that, at a typical frequency of 1 200 kilocycles per sec., the conductivity of surface soil varies between 0.16 × 108 and 4.0 × 108 electrostatic units for different sites, while at various depths the values range from 1.6 × 105 to about 1010 electrostatic units. In general it is shown that the clay soils have a high conductivity (i.e. above 108 electrostatic units) accompanied by high dielectric constant, and the loam and chalk soils an average value of about 108 electrostatic units for conductivity and 20 for dielectric constant, while soil of a sandy or gritty nature gives a much lower conductivity value. The lowest values (of the order of 105 electrostatic units) mentioned above were obtained on the solid granite or slate subsoils found at some of the sites.The concluding section of the paper illustrates in graphical form the effect of the electrical properties of the soil on the penetration of alternating currents below the surface. Curves are plotted for certain of the sites from which samples of soil were obtained for measurement. These curves illustrate the fact that while at a frequency of 100 kilocycles per sec. a few per cent of the current at the surface may penetrate to a depth of 20 metres (over 60 ft.), at a frequency of 10 000 kilocycles per sec. the current is practically confined to the first 3 or 4 metres of depth. The effect of such current penetration on the field at the surface due to an electric wave being propagated along this surface is a subject that must remain for further investigation.