The paper describes a study of methods of measuring dielectric losses which has been carried out with the object of determining the nature and magnitude of the various errors to which different methods are liable. The most important errors are those due to stray capacitance currents, which affect especially the phase angle of the currents in resistors commonly used in wattmeter methods. A method is developed of shielding a simple resistor of flowing water, so that the current is in phase with the voltage within about 0.0001 radian. A resistor has been developed on these lines for 150 kV.The standard of reference has been a specially designed air condenser of the parallel-plate type. The separation of the plates is variable; and at 10 cm separation, suitable for 100 kV, the capacitance is about 500 cm. The curvature of the edges and of the guard plates was decided as a result of studying the electric stress distribution by models in an electrolyte. The use of a condenser with the Schering bridge form of circuit has been found to be almost free from errors of importance which are difficult to allow for. The work has been carried out for the British Electrical and Allied Industries Research Association, more particularly with reference to the measurement of the power loss in cable dielectrics.