Potential gradient and flux density: Their measurement by an improved method in irregular electrostatic and magnetic fields

The safety of electric power cables is dependent in part upon the maximum electrostatic flux density in the dielectric. The core loss in revolving machinery is, in part, a pole face loss, dependent on variations in the magnetic flux density. Alternators work best in parallel, and give less interference with communication circuits, if of a good wave form, of flux density. These fields of electrostatic and magnetic flux are often too irregular to lend themselves to an accurate computation, and hence experimental methods of study are needed. The writers have improved a method suggested by F. W. Carter, and the results obtained give promise of a field for useful study. The method itself may be briefly described as measuring an electric model of the field to be studied cut out of sheet metal. By the use of a contact-making device, with two contacts, held at a fixed distance apart, but movable to any part of the edge of the metal sheet, voltage gradients may be measured upon a galvanometer. From these measurements, the flux densities may be calculated. The paper describes the method in detail and also gives some results of study on the three problems mentioned. It is shown how this method of test can be used to improve pole-shoe design. The importance of pole-shoe design is pointed out. An appendix is included showing that correct pole-shoe design is needed to eliminate the so-called “tooth ripples.”