d 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
larger motor constructed by Kravogl in 1869, and Mr. Pfaundler was
enabled to write as follows: "Upon running the machine by hand we
obtain a current whose energy is that of one Bunsen element." This
letter is dated February 11, 1870, that is to say, it is a year
anterior to the note of Gramme.

[Illustration: FIG. 1.]

In the presence of the historic interest that attaches to the
question, we do not think it will be out of place to reproduce here
the considerations that guided Prof. Pfaundler in the researches that
led him to convert the Kravogl motor into a dynamo-electric machine.
Let us consider two magnetized bars, _db_ and _bd'_, placed end to end
and surrounded by a cylindrical armature forming a shell, this
armature being likewise supposed to be a permanent magnet and to
present poles of contrary direction opposite the poles of the bars.
For the sake of greater simplicity this shell is represented by a part
only in the figure, _s n n s_. If, into a magnetic field thus
formed, we pass a spiral from left to right, the spiral will be
traversed by a current whose direction will change according to the
way in which the moving is done. It is only necessary to apply Lenz's
law to see that a reversal of the currents will occur at the points,
_a_ and _c_, the direction of the current being represented by arrows
in the figure. If we suppose a continual displacement of the spirals
from left to right, we shall collect a continuous current by placing
two rubbers at _a_ and _c_. Either the core or the shell may be
replaced by a piece of soft iron. In such a case this piece will move
with the spiral and keep its poles that are developed by induction
fixed in space. From this, in order to reach a dynamo-electric machine
it is necessary to try to develop the energy of the magnetic field by
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_