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.]

[Illustration: FIG. 3.]

This latter apparatus has in this case the form shown in Fig. 4.

[Illustration: FIG. 4.]

The spiral, _s m b_, is movable, and the core, N _o s_, is kept in a
position of equilibrium by virtue of its weight, and is provided with
rollers. For the sake of greater clearness, the front part of the
armature is supposed to be removed. The current does not circulate in
the spirals to the right of the diameter, W O, which latter is not
absolutely vertical. The position of the rubbers and armature is
regulated once for all. We do not know just what were the means
devised by Kravogl to suppress the current in the spheres to the
right. At all events, it is probable that the system has grown old
since Gramme invented his collector. In the application of the Kravogl
motor to the generation of continuous currents, Professor Pfaundler
now proposes to ingeniously utilize the Gramme collector. In such a
case the arrangement shown in Fig. 5 would be ado