the coils or circuits are coincident. In cylindrical coils in which the
current is uniformly distributed through all the parts of the conductor
section, what I here term the electrical middle, or the center of
gravity of the ampere turns of the coils, will be the plane at right
angles to its axis at its middle, that of B and C, in Fig. 4, being
indicated by a dotted line. To repeat, then, when the centers or center
planes of the conductors, Fig. 4, coincide, no indication of
electro-inductive repulsion is given, because it is mutually balanced in
all directions; but when the coils are displaced, a repulsion is
manifested, which reaches a maximum at a position depending on the
peculiarities of proportion and distribution of current at any time in
the two circuits or conductors.

[Illustration: FIG. 9.]

It is not my purpose now to discuss the ways of determining the
distribution of currents and mechanical effects, as that would extend
the present paper much beyond its intended limit. The forms and relative
arrangement of the two conductors may be greatly varied. In Fig. 5 the
parts are of equal diameter, one, B, being a closed ring, and the other,
C, being an annular coil placed parallel thereto; and an iron core or
wire bundle placed in the common axis of the two coils increases the
repulsive action. B may be simply a disk or plate of any form, without
greatly affecting the nature of the action produced. It may also be
composed of a pile of copper washers or a coil of wire, as before
indicated.

[Illustration: FIG. 10.]

An arrangement of parts somewhat analogous to that of a horseshoe