e
same apparatus.

The commutator consists of a small boxwood cylinder, carrying around
its cylindrical surface two rows of eight holes, one above the other,
in which are fitted sixteen contact pieces of brass which slightly
project above the surface of the wood, the positions of those in the
upper circle alternating or "breaking joint" with those in the lower,
and each contact piece is in metallic connection with its
corresponding conducting wire, and, therefore, with the junction of
two of the helices on the armature. Against the edge of the commutator
are pressed by means of adjustable levers two small brass contact
rollers, k k, which are respectively connected with the positive and
negative poles of the voltaic battery (either through or independent
of the coils of a fixed electro-magnet, to which we shall presently
refer), and the magnetic axis of the ring will lie in the same plane
as the line joining the points of contact of the battery and rotating
helix, this axis remaining nearly fixed notwithstanding the rotation
of the iron ring in which the magnetism is induced.

In the apparatus figured in Figs. 3 and 4, the armature rotates
between the two vertical limbs, A B, of a fixed electro-magnet
furnished with extended pole pieces, A A, B B (Fig. 4), each of which
embraces about six of the armature coils. The fixed electro-magnet is
constructed of two vertical iron cylindrical bars, A and B, united at
their lower extremities by a horizontal iron bar, F F, the one being
rigidly and permanently attached to it, while the other is fastened to
it by a screw, G, passing through a slot so that the distance of the
pole pieces from one another and from the armature ring is capable of
adjustment.

The connections of the machine, which are shown in Fig. 3, are made as
follows: The positive current, entering by the attachment screw, h,
passes by a wire to the right hand commutator screw, l, to the
right-hand roller, k, through the commutator to the ring, around
which it traverses to the left-hand roller, k