system any two points of the coil situated at opposite ends of the
same diameter of the ring be connected respectively with the two poles
of a voltaic battery, the electric current having two courses open to
it, will divide into two portions traversing 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