Helmholtz's starting point was a result published by Gustav Kirchhoff in 1848. Like Helmholtz, Kirchhoff was concerned with three-dimensional, electrically conducting systems. Kirchhoff considered a system consisting of two parts, which he labelled parts A and B. Part A (which played the part of the "active network" in Fig. 2) consisted of a collection of conducting bodies connected end to end, each body characterized by an electromotive force and a resistance. Part B was assumed to be connected to the endpoints of A via two wires. Kirchhoff then showed (p. 195) that "without changing the flow at any point in B, one can substitute for A a conductor in which an electromotive force is located which is equal to the sum of the voltage differences in A, and which has a resistance equal to the summed resistances of the elements of A".