is always outside of the arc comprised between the brushes, R. In
order to secure a continuity of the circuit in the effective arc, W S_ o_,
it is only necessary to arrange a rubber, M, in such a way as to
establish a communication between the two parts of the divided touch
as soon as this latter enters the arc under consideration.

In order to produce a current in the direction of the arrows shown in
Fig. 4, the spiral and axle must revolve from right to left. In this
case the rubber, M, occupies the position shown in the same figure,
the brushes embracing an arc of a little less than 180°. As soon as
the lower touch comes in contact with the brush, R, when the
revolution is being effected from left to right, the rubber, M,
establishes a communication between the two halves that have until now
been isolated, and the current is no longer interrupted. The second
touch during this time is at any point whatever of the arc, W N _o_,
and the spirals corresponding to the latter arc outside of the
circuit. In short, thanks to the rubber, M, we have an ordinary Gramme
collector in that portion of the circuit comprised between the
brushes, and a collector with a breakage of the circuit in the portion
to the right.

[Illustration: FIG. 5.]

This type of machine is entirely theoretical. In the apparatus used
for Prof. Pfaundler's experiments in 1870, the armature revolved with
the solenoid. The core and armature were of soft iron, and the core
was arranged in a manner analogous to the preceding, and remained in
place under the action of its weight, and the shell, forming a
complete circle, revolved with poles fixed in space.