Abstract :
The consideration of the extreme care necessary in preparing samples of a dielectric for test for electrical properties led to the investigation of the effect of air spaces, purposely formed of definite thickness and location, upon the power factor. This work in a way is an extension of some work done by Clark and Shanklin and Shanklin and Matson several years ago on air spaces in high voltage cables and wrapped armature coils. In their investigation the effect of assumed air spaces of indefinite thickness, extent, pressure and location was shown by plotting effective resistance from the formula RI = E2/W against potential gradient. A characteristic curve was obtained, a sharp inflection point in the curve being interpreted as indicating the starting point of corona. In the work by the writer, various materials were investigated both with air spaces excluded as much as possible, and with air spaces of definite thickness, extent, and location at atmospheric pressure. The results were plotted showing variation of power factor with potential gradient. A definite increase in power factor with potential gradient indicated the starting of corona. The thicker air space with a given thickness of dielectric showed the more abrupt change in power factor, and this took place at a lower potential gradient. By plotting power factor against voltage, a maximum was shown indicating that a saturation of ionization was approached which resulted in a decrease in power factor.
