SEVEI<~L of my former students have asked me where they might look up the electrostatic theory that is necessary to fully understand the important paper which was read before the American Institute of Electrical Engineers, on March I4, by Mr. C. Fortescue, on " Insulation Problems," and, in view of these requests, it seems worth while to publish the following discussion of Mr. Fortescue's paper which was given before the instructing staff and students of the Departments of Physics and of Electrical Engineering at Lehigh University. The discussion is made as brief as possible, and it is assumed that the diagrams will make the meaning clear.
The following propositions are extensively used in theoretical discussions of electrostatics:
(a) Any equipotential surface may be wholly or in part replaced by a thin nzetal sheet witho,t distnrbiny an electric field distribution. Thus Fig. I shows the lines of force between two metal cylinders A and B. The dotted line represents an equipotential surface, and this equipotential surface may be replaced wholly or in part by a thin metal sheet without disturbing the field between A and B.
(b ) A closed metal shell screens its interior from all outside electrostatic effects. This action of a metal shell may be understood in terms of a mechanical analogue as follows: Fig. 2 represents a mass of steel B completely separated from a surrounding mass of steel A by an empty space. The steel B remains unstressed however the steel A may be stressed, because stress cannot be transmitted across an empty space. Fig. 3 represents a mass of dielectric B completely separated from the surrounding dielectric A by a metal shell. The dielectric B remains unstressed however the dielectric A may be stressed, * Communicated by the Author.