Bruce developed a series of electrode shapes that approximated an ideal uniform field. The Bruce profile is a figure of revolution, starting with a flat plane in the center, with a sine curve used as a transition to a circular section at the edge. The idea is to have a large area of uniform field (2 flat plates) with a gradually increasing radius of curvature to the edge. The Bruce profile apparently wasn't originally developed with the intention of finding an analytically nice E field, but was an empirically derived method to reduce the edge effects.
( Drawing after Naidu and Kamaraju , p184)
Bruce (referenced in Craggs and Meek, and in Kuffel, et.al.) claimed that electrodes with diameters of 4.5, 9.0, and 15 inches can be used for maximum voltages of 140, 280, and 420 kV, respectively. The reference I have doesn't say if the diameter is of the flat part (AB in the above drawing) or of the outer curved rim.
A comment made by Naidu, et.al, in connection with their discussion of uniform field gaps is that there is no significant difference in breakdown voltage between sphere gaps and the various uniform field gaps, given the fairly large uncertainty in all the measurements. In practice, breakdown measurements are made with spherical electrodes, because small changes in the surface of a uniform field electrode can cause field irregularities which in turn dramatically affects the breakdown voltages. Also, a deviation from parallelism of the opposing electrode faces can cause significant deviations from a large uniform field area. Spheres are easier to make and keep smooth, even though there isn't a "nice" analytical solution to the field between the electrodes. Furthermore, if there is a misalignment between the spheres, the region between the spheres is still geometrically the same.
Copyright 1998, Jim Lux / bruce.htm / 14 Dec 2000 / Back to HV Home / Back to home page / Mail to Jim