from The Century Dictionary.
- noun That position from which as an origin the mean value of all the rectangular or oblique coördinates of the particles of a body is zero.
Sorry, no etymologies found.
The shapes and angles of the plates have nothing to do with it, except that the perpendicular distance of the plate edges from the mass-center line determines thickness of the beam of parallel current-rays.
This line is the theoretical straight line passing through the mass-center of each plate.