If a physical conductor with constant electromotive forces in two specific points on its surface is connected to any linear conductor, then in its place one can always substitute a linear conductor with a certain electromotive force and a certain resistance, which in all applied linear conductors would excite exactly the same currents as the physical one.... The resistance of the linear conductor to be substituted is equal to that of the body when a current is passed through it from the two entry points of the linear conductor.