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Page 24
[Footnote 1: _Zeitschrift des Electrotechnischen Vereines_ in
_Wien_, July, 1883.]
The Kravogl motor that figured at the Universal Exhibition of 1867 is
but little known, and it is now very difficult to obtain drawings of
it. What is certain is that this motor is an application of the
properties of the solenoid, and, from this standpoint, resembles the
Bessolo motor that was patented in 1855. We may figure the apparatus
to our mind very well if we suppose that in the Gramme ring a half and
almost two-thirds of the core are removed, and the spirals are movable
around the said core. If a current be sent into a portion of the
spirals only, and in such a way that only half of the core be exposed,
the latter will move with respect to the bobbin or the bobbin with
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
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_ 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.
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