Retarded dispersion forces between conducting spheres

The retarded dispersion forces between conducting spheres are calculated. It is shown that for perfect conductors, the contribution of magnetic dipole terms and higher-multipole terms are important, amounting to about 50% of the force asymptotically, and an even greater percentage at shorter distances. It is argued that ordinary conducting and superconducting spheres will behave as perfect conductors insofar as retarded dispersion forces are concerned, provided that the radius of the spheres is greater than about 10^-4 cm for superconductors, and provided that the separation is less then 10²a for ordinary conductors. Some comments on the measurability of these forces are presented.