High tension insulators are generally built of several porcelain discs in series, the discs being connected by galvanized steel hardware. When alternating voltage is impressed across such a string the voltage is not distributed symmetrically because the charging current is not the same through the string. There is capacitance from each connector to the next, from each connector to the ground and to the line wire, as well as capacitance from one connector to one which is not adjacent. If the voltage impressed is direct and all the units or discs are exactly alike, then such an air insulator string would have across each unit the same value of voltage. A certain leakage current flows through each insulator, and the d.c. voltage across each unit will be the leakage current multiplied by the resistance of each unit. In the transmission of high-voltage direct current, such as the Thury system found in Europe, high-tension porcelain insulators can be used to insulate the line from the ground. Such insulators are exposed to changing weather conditions of humidity and temperature. Also, these insulators on account of being subjected to mechanical stresses, electrical stresses, and repeated temperature cycles, depreciate in resistance, become faulty, and unless detected may interfere with the operation of the transmission system. If the resistance of the insultors forming the string, and the effect produced on the resistance by temperature, humidity, etcetera, are known then it is possible to predict to a certain degree of approximation what service the insulators can give. Besides determining the condition of such insulators, a knowledge of the resistance would give a basis for comparing different types of insulators, and likewise would furnish a means for arriving at the voltage distribution of different makes and types and the uniformity with which a given type could be expected to run. In the investigation carried out, the results of which are tabulated below, particular attention was given to the determination of the surface and interior resistance, and the effect of change of temperature and humidity on the electrical resistance of various types and makes of high-tension porcelain insulators. The humidity effect is not an internal one, but manifests itself by changing the surface of the insulator so that a greater freedom is given to the flow of current over the surface between the cap and the pin of the insulator.