At the origin of every science, of whatever nature it may be, there is
always a fruitless period, of greater or less length, characterized by
the warfare of a few superior minds against general apathy. The finest
discoveries pass unperceived, so to speak, since they cannot cross the
limits of a narrow circle; and it often happens that they fall into
oblivion before they have been seriously judged. Meanwhile, a slow
progress is imperceptibly made, and, in measure as theoretical
principles more clearly disengage themselves, a few industrial
applications spring up and have the effect of awakening curiosity. An
impulse is thus given, and from this moment a movement in advance goes
on increasing at a headlong pace from day to day.
With electricity this period has been of comparatively short duration,
since scarcely a century and a half separate us from the first
experiments made in this line of research. Now that it has truly taken
its place in a rank with the other sciences, we like to go back to the
hesitations of the first hour, and trace, step by step, the history of
the progress made, so as to assign to each one that portion of the
merit that belongs to him in the common work. When we thus cast a
retrospective glance we find ourselves in the presence of one strange
fact, and that is the simultaneousness of discoveries. That an
absolutely original idea, fertile in practical consequences, should
rise at a given moment in a fine brain is well; we admire the
discovery, and, in spite of us, a little surprise mingles with our
admiration. But is it not a truly curious thing that _several_
individuals should have had at nearly the same time that idea that was
so astonishing in one? This, however, is a fact that the history of
electrical inventions offers more than one example of. No one ignores
the fact that the invention of the telephone gave rise to a notorious
lawsuit, two inventors having had this ingenious apparatus patented on
the same day and at nearly the same hour. This is one example among a
thousand. In the history of dynamo-electric machines it is an equally
delicate matter to fix upon the one to whom belongs the honor of
having first clearly conceived the possibility of engendering
continuous currents.
We do not wish to take up this debate nor to go over the history of
the question again. Every one knows that the first continuous current
electric generator whose form was practical is due to Zenobius Gramme,
and dates back to July, 1871, an epoch at which appeared a memoir
(entitled "Note upon a magneto-electric machine that produces
continuous currents") that was read to the Academy of Sciences by Mr.
Jamin. Ten years previous, Pacinotti had had a glimpse of the
phenomenon, and of its practical realization, but was unfortunately
unable to appreciate the importance of his discovery and the benefit
that might be reaped from it. It is of slight consequence whether
Gramme knew of this experiment or not, for the glory that attaches to
his name could not be diminished for all that. But an interesting fact
that we propose to dwell upon now has recently been brought to light
in an electrical review published at Vienna.[1] It results from
documents whose authenticity cannot be doubted that, as far back as
1867, Mr. L. Pfaundler, a professor at Innsbruck, very clearly
announced the reversibility of a magneto-electric motor constructed by
Kravogl, a mechanician of the same place, and that he succeeded some
time before Gramme in obtaining continuous currents.
[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.
[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 adopted. Let us suppose
an ordinary collector having as many plates as there are sections in
the ring, these plates being connected as usual with the entrance and
exit wires of the sections. The diametrically opposite touches that
are in the line, W O, are divided, and one of the halves is connected
at the entrance, _c a'_ (Fig. 4), with the corresponding section,
while the other communicates with the exit, _c' a_, of the neighboring
section. Each of these halves is prolonged by a piece of metal bent
into the form of an arc of a circle and embracing a little less than a
semi-circumference. Between these prolongations there is an insulating
part. In the rotary motion of the spiral, at least one of the touches
is always outside of the arc comprised between the brushes, R. In
order to secure a continuity of the circuit in the effective arc, W S_ o_,
it is only necessary to arrange a rubber, M, in such a way as to
establish a communication between the two parts of the divided touch
as soon as this latter enters the arc under consideration.
In order to produce a current in the direction of the arrows shown in
Fig. 4, the spiral and axle must revolve from right to left. In this
case the rubber, M, occupies the position shown in the same figure,
the brushes embracing an arc of a little less than 180°. As soon as
the lower touch comes in contact with the brush, R, when the
revolution is being effected from left to right, the rubber, M,
establishes a communication between the two halves that have until now
been isolated, and the current is no longer interrupted. The second
touch during this time is at any point whatever of the arc, W N _o_,
and the spirals corresponding to the latter arc outside of the
circuit. In short, thanks to the rubber, M, we have an ordinary Gramme
collector in that portion of the circuit comprised between the
brushes, and a collector with a breakage of the circuit in the portion
to the right.
[Illustration: FIG. 5.]
This type of machine is entirely theoretical. In the apparatus used
for Prof. Pfaundler's experiments in 1870, the armature revolved with
the solenoid. The core and armature were of soft iron, and the core
was arranged in a manner analogous to the preceding, and remained in
place under the action of its weight, and the shell, forming a
complete circle, revolved with poles fixed in space.
Practically, the machine that we have just described would prove
inconvenient to realize, and would present serious inconveniences. In
the first place, it seems to us quite difficult to transmit the motion
of the solenoid to the axle, supposing the former to revolve within
the armature. In the second place, considerable friction would surely
occur between the spirals and core, and the axle, being submitted to a
lateral stress, would be placed in a poor condition for work. It is
even allowable to doubt whether such a type could be practically got
up. At all events, no trial has as yet been made of it.
Compared with the Gramme machine, from an absolutely theoretical point
of view, the Pfaundler apparatus presents undoubted advantages. A
theoretically perfect dynamo electric machine would be one in which
there was a complete reciprocity between the magnetizing action of the
current and the inductive action of the magnetic field. Now, such is
not the case in the Gramme machine. In this apparatus the soft iron
core is at the same time a magnet through favorable induction and a
disadvantageous electro-magnet. This double polarization is only
remedied to a certain extent by the adjustment of the brushes. In the
Pfaundler machine, on the contrary, the electro-magnetism and
magnetism through induction act in the same direction, and concur in
effecting a polarization that favors the production of the current.
Looked at it in this light, the latter machine more nearly approaches
the type of perfection than does that of Gramme.
But we must not forget that such qualities are purely theoretical. In
practice the best machine is that in which the copper is best
utilized, that is to say, that which with a given weight of this metal
furnishes the most work. Now, this is certainly not the case in the
Pfaundler machine, for here half or more than half of the ring is
inert--a defect which is apparent at first sight. It results from this
that as soon as we propose to obtain an electromotive force, however
slight it be, we must get it with machines of large dimensions. Now,
it is permissible to believe that under such circumstances (taking
into consideration the complication of mechanical means that the
construction of such apparatus necessitates, and the great friction
that occurs) it would be impossible to obtain practical rotary
velocities. Comparing his machine with Gramme's, Prof. Pfaundler
expresses the idea that between them there is the same analogy as
there is between a constant pressure and an expansion engine. With
cylinders of equal diameters the work performed by the former of these
is greater than that done by the second, but in the latter the
expansive force of the steam is better utilized. This comparison seems
to us to be more ingenious than exact. Would it not be coming nearer
to the truth if we were to suppose a case of a hydraulic motor whose
performance continued diminishing with the height of the fall, and
would it not be advantageous under such circumstances to utilize only
a portion of the fall for the purpose of increasing the motor's
performance?
This machine, however, as before stated, has never as yet been
constructed, so that experimental data relative to its mode of working
are wanting. It is especially interesting as regards its origin, which
dates back to an epoch at which researches on the dynamo electric
machine were at their heat. It is in its historical aspect that it is
proper to regard it, and it is from such a point of view that we have
deemed it well to say a few words about it in this place.--_La Lumiere
Electrique._
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