The constant A varies, depending on the HD sphere and its SD
surrounding dielectric constants, between 1 and 0.5 for εSD/
εHD/ 0 and εHD/εSD / 0 respectively.
Considering two spheres weakly interacting spaced by a distance
2D, i.e. one placed at z ¼ D and the second one placed at
z ¼ þD, the electric field is expressed as the superposition of the
fields created by each of the sphere: