aversing the coil around each half
of the ring from one point of contact to the other, and the direction
of the current, in each portion will be such as to magnetize the iron
core, so that its magnetic poles will be situated at the points where
the current enters and leaves the helix, and a straight line joining
these points may be looked upon as the magnetic axis of the system.
From this construction it is clear that, by varying the position of
the points of contact of the battery wires and the coil, the position
of the magnetic axis will be changed accordingly, and can be made to
take up any diametrical position with respect to the ring, of which
the two halves (separated by the diameter joining the points of
contact of the battery wires with the coil) may be regarded as made up
of two semicircular horseshoe electro-magnets having their similar
poles joined. To this form of instrument the name "Transversal electro
magnet" (_Eletro calamita transversale_) was given by its inventor, to
whom is undoubtedly due the merit of having been the first to
construct an electro-magnet the position of whose poles could be
varied at will by means of a circular commutator.

[Illustration: PACINOTTI ELECTRO-MAGNETIC MACHINE.--MADE IN 1860.]

By applying the principle to an electro-magnetic engine, Dr. Pacinotti
produced the machine which we illustrate on the present page. The
armature consists of a turned ring of iron, having around its
circumference sixteen teeth of equal size and at equal angular
distance apart, as shown in Fig. 1, forming between them as many
spaces or notches, which are filled up by coiling within them helices
of insulated copper wire, r r r, in a similar manner to that adopted
in winding the Brush armature, and between them are fixed as many
wooden wedges, m m, by which the helices are firmly held in their
place. All the coils are wound round the ring in the same direction,
and the terminating end of each coil is connected to the commencing
end of the next or succeeding helix, and the junctions so made are
attached to conducting wires which are gathered together close to the
vertical shaft on which the armature ring is fixed, passing through
holes at equal distances apart in a wooden collar fixed to the same
shaft, and being attached at their lower extremities to the metallic
contact pieces of the commutator, c, shown at the lower part of Fig.
3, which is an elevation of the machine, while Fig. 4 is a plan of th