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, k¹, and screw, l¹, to
the magnet coil, A, and thence through the coil of the magnet, B, to
the terminal screw, h, on the right hand of the figure. This method
of coupling up is of very great historical interest, for it is the
first instance on record of the magnet coils and armature of a machine
being included in one circuit, giving to it the principle of
construction of a dynamo-electric machine, and antedating in
publication, by two years, the interesting machines of Siemens,
Wheatstone, and Varley, and preceding them in construction by a still
longer period.

With this apparatus Dr. Pacinotti made the following interesting
experiments with the object of determining the amount of mechanical
work produced by the machine (when worked as an electro-magnetic