Until quite recently it was believed that the force c -2∂(E×M)/∂t e.m.u. or e.s.u. known to be applied to a Coulombian magnetic dipole of moment M by an electric field E did not exist in the Ampère picture of magnetism. In 1967 independent research groups have stressed that the above « magnetodynamic force » is necessary for ensuring the equality of action and reaction applied (in the case of a slowly varying fields) to the sources of the fields. Among them, Penfield and Haus have discovered that the forgotten momentum in the Ampère picture was precisely the one associated with the relativistic variation of the mass of the charge carriers, while we gave the (gauge invariant) formula e -2∮(∂(Vi)/∂t)d l with ∂ t A=0 and E=∂V, for the magnetodynamic force exerted by an electric field on a current loop of intensity i. This paper is intended first to discuss, by using an ad hoc Gedankenexperiment, the detailed mechanism of the Penfield-Haus effect and the generation of our integral formula. The distinct roles of the external and internal forces applied to the current loop is emphasized. We then examine how it happens that there exists no angular momentum analogue of the Penfield-Haus effect, whence, as already pointed out by Feynman in his Lectures on Physics, the possibility of « electromagnetic bootstrap merry-gorounds », of which we give a more thorough discussion. © 1969 Società Italiana di Fisica.
