Calculation of mechanical forces in electric circuits

V. Karapetoff: Professor Dwight´s equation (5) may be deduced directly from the general formula (2) for the mechanical force. Let the return conductor be a concentric cylinder of radius b. Let the virtual deformation of the given conductor consist of an axial lengthening of the portion of radius a2 by an infinitesimal amount d s and of a corresponding shortening of the portion of radius a1 by the same amount ds. The tapered portion is supposed to move bodily without a change in shape.