uniformly charged with electricity throughout its volume. Divide the sphere into two parts, namely a concentric nucleus of radius
and a shell of thickness
. Then the electromagnetic mass of the whole sphere is not the sum of the electromagnetic masses of these two parts, but is to be calculated by a quadratic law from the charges.
A material object expresses the spatial distribution of a quantity of 'material,' when the quantity is absolutely extensive.
59.3 The volume-density, at a time
in the