condenser expressed in microfarads is given by the formula
where is the surface of the dielectric in square metres, is the thickness of the dielectric in millimetres, and has the value given in the above table.
In developing the theory of the potential we started with the experiment of bringing unit electricity from the wall of the room to a point outside the charged sphere; and, as a limiting condition, to its surface. Beyond that we did not go. But what happens if we pass the surface and carry our unit through to the inside? A mathematical investigation shows that in this case no force at all is acting on the unit charge, and in fact on any body carrying a charge of any magnitude. Since in moving such a body about within the hollow sphere we experience no resisting force whatever, no energy is required to perform the motion, and consequently all points of the interior space must have the same potential. Any point of the inner surface of the sphere is a point in the interior, but since the surface has the potential it follows
