This paper is an account of work which was undertaken in the first place to study the performance under various conditions of paper-dielectric condensers such as are commonly used for power-factor correction.It shows that breakdown of dielectrics in service is due to ionization of entrapped air; and that, removal of the air being impossible, the design of cables and condensers should be such as to prevent ionization. The principle of this design is to subdivide the dielectric.The effect of surges in causing breakdown is indicated, and a new test for finished products is suggested.