the action of the current itself. If we suppose the core to be of soft
iron, and make a closer study of the action of the current as regards
the polarity that occurs under the influence of the poles, _s_, _n_,
_s_, we shall see that from _d_ to _a_ and from _b_ to _c_ the current
is contrary, while that from _a_ to _b_ and from _c_ to _d'_ it is
favorable to the development of such polarity. In short, with a spiral
moving from _d_ to _d'_ the resulting effect is _nil_, a fact,
moreover, that is self-evident. Under such circumstances, if we
suppose the shell, as well as the core, to be of soft iron, we shall
obtain a feeble current due to the presence of remanent magnetism; but
this magnetism will not be able to continue increasing under the
influence of the current. To solve this difficulty two means present
themselves: (1) to cause a, favorable magnetic current and act upon
the armature, and (2) to suppress such portions of the current in the
spirals as are injurious in effect. The first solution was thought of
by Gramme in 1871, and is represented diagramatically in Fig. 2. The
second is due to Prof. Pfaundler, and dates back to 1870. The core is
cut through the center (Fig. 3), and the portion to the right is
suppressed; the current is interrupted between _da_ and _cd'_, and is
closed only between _a_ and _c_ (_v_, Fig. 1). It results from this
arrangement that, under the action of the current, the polarity due to
remanent magnetism does nothing but increase. It suffices then for but
little remanent magnetism to prime the machine; the polarity of the
shell continues to increase, and the energy of the magnetic field, and
consequently of the current, has for a limit only the saturation of
the soft iron. If, now, we curve the core, the spirals, and the
armature into a circle, we have a Gramme or a Pfaundler machine,
according as we consider Fig. 2 or Fig. 3.

[Illustration: FIG. 2.]