This paper presents some of the results of a series of high-voltage tests on cables and insulators, extending over a period of eight years, to determine the electrical characteristics of the insulation. Leakage current, insulation resistance, and watt input tests were made with direct and alternating current. Paper-insulated and rubber-insulated cables and a 27,000-volt porcelain insulator were tested. From these tests several conclusions were drawn, the principal ones being as follows: 1. Insulation resistance of paper and rubber-insulated power cables increases to a maximum with increasing applied d-c. voltage, the characteristic depending upon the temperature. 2. It is necessary to use shields as well as guards in making tests to determine the electrical characteristics of cable insulation under d-c. voltage stresses above the ionization point. 3. The watt input to the insulation of a paper cable under d-c. stress, at a given temperature, depends upon the character of the voltage wave; the greater the ripple, the greater the watt input. 4. It is necessary to use shields as well as guards in making tests on short cable samples to determine the a-c. electrical characteristics, such as dielectric loss of cable insulation under voltage stresses above the ionization point. 5. The ionization point is very variable depending upon the physical circuit, together with the atmospheric conditions, and represents, in reality, local air breakdown. 6. The better (i. e.