The paper discusses the conditions governing the design of suitable electrodes for the measurement of the permittivity and power factor of solid dielectrics at radio frequencies. Two-terminal mercury electrodes intended for the testing of sheet and tubular materials by resonance methods are first described. Typical conditions in bridge circuits used for dielectric measurements are then investigated, and the effects of the resistance and inductance of the condenser leads and those of defective insulation between the electrodes and earth are deduced from the more general solution. Two types of 3-terminal mercury clamps are subsequently described which have been designed to render the preceding effects as small as possible.