respect to the core, according as we suppose the solenoid or the
bobbin fixed. In the first case we have a Bessolo motor, and in the
second a Kravogl one.

In order to obtain a continuous motion it is only necessary to allow
the current to circulate successively in the different portions of the
solenoid. It is difficult to keep the core in place, since it is
unreachable, being placed in the interior of the bobbin. Kravogl
solved this difficulty by constructing a hollow core into which he
poured melted lead. This heavy piece, mounted upon rollers, assumed a
position of equilibrium that resulted from its weight, from friction,
and from magnetic attraction. But for a current of given intensity
this position, once reached, did not vary, and so necessitated a
simple adjustment of the rubbers. Under such circumstances, with a
somewhat large number of sections, the polarity of the core was nearly
constant. The spirals as a whole were attached to a soft iron armature
that had the effect of closing up the lines of forces and forming a
shell, so to speak.

Like Bessolo, Kravogl never thought of making anything but a motor,
and did not perceive that his machine was reversible. It results from
some correspondence between Dr. A. Von Waltenhofen and Mr. L.
Pfaundler at this epoch that the latter clearly saw the possibility of
utilizing this motor as a current generator. Under date of November 9,
1867, he wrote, in speaking of the Kravogl motor, which had just been
taken to Innsbruck in order to send it to Paris. "I regret that I
shall not be able to see it any more, for I should have liked to try
to make it act in an opposite direction, that is to say, to produce a
current or an electric light by means of mechanical work." A little
more than two years later these experiments were carried out on a