The action of magnets upon the voltaic arc has been known for a long
time past. Davy even succeeded in influencing the latter powerfully
enough in this way to divide it, and since his time Messrs. Grove and
Quet have studied the effect under different conditions. In 1859, I
myself undertook numerous researches on this subject, and experimented
on the induction spark of the Ruhmkorff coil, the results of these
researches having been published in the last two editions of my notes on
the Ruhmkorff apparatus.
[Illustration: FIG. 1]
These researches were summed up in the journal _La Lumière Electrique_
for June 15, 1879. Recently, Mr. Pilleux has addressed to us some new
experiments on the same subject, made on the voltaic arc produced by a
De Meritens alternating current machine. Naturally, he has found the
same phenomena that I had made known; but he thinks that these new
researches are worthy of interest by reason of the nature of the arc in
which he experimented, and which, according to him, is of a different
nature from all those on which, up to the present time, experiments have
been made. Such a distinction as this, however, merits a discussion.
With the induction spark, magnets have an action only on the aureola
which accompanies the line of fire of the static discharge; and this
aureola, being only a sort of sheath of heated air containing many
particles of metal derived from the rheophores, represents exactly the
voltaic arc.
[Illustration: FIG. 2]
Moreover, although the induced currents developed in the bobbin are
alternately of opposite direction, the galvanometer shows that the
currents that traverse the break are of the same direction, and that
these are direct ones. The reversed currents are, then, arrested during
their passage; and, in order to collect them, it becomes necessary to
considerably diminish the gaseous pressure of the aeriform conductor
interposed in the discharge; to increase its conductivity; or to open to
the current a very resistant metallic derivation. By this latter means,
I have succeeded in isolating, one from the other, in two different
circuits, the direct induced currents and the reversed induced ones.
As only direct currents can, in air at a normal pressure, traverse
the break through which the induction spark passes, the aureola that
surrounds it may be considered as being exactly in the same conditions
as a voltaic arc, and, consequently, as representing an extensible
conductor traversed by a current flowing in a definite direction. Such
a conductor is consequently susceptible of being influenced by all the
external reactions that can be exerted upon a current; only, by reason
of its mobility, the conductor may possibly give way to the action
exerted upon the current traversing it, and undergo deformations that
are in relation with the laws of Ampère. It is in this manner that I
have explained the different forms that the aureola of the induction
spark assumes when it is submitted to the action of a magnet in the
direction of its axial line, or in that of its equatorial line, or
perpendicular to these latter, or upon the magnetic poles themselves.
Experiments of a very definite kind have not yet been made as to the
nature of the arc produced by induced currents developed in alternating
current machines; but, from the experiments made with electric candles,
we are forced to admit that the current reacts as if it were alternately
reversed through the arc, since the carbons are used up to an equal
degree; and, moreover, Mr. Pilleux's experiments show that effects
analogous to those of induction coils are produced by the reaction of
magnets upon the arc. There is, then, here a doubtful point that it
would be interesting to clear up; and we believe that it is consequently
proper to introduce in this place Mr. Pilleux's note:
"Having at my disposal," says he, "a powerful vertical voltaic arc of 12
centimeters in length, kept up by alternately reversed currents, and one
of the most powerful permanent magnets that Mr. De Meritens employs for
magneto-electric machines, I have been enabled to make the following
experiments:
"1. When I caused one of the poles of my magnet to slowly approach the
voltaic arc, I ascertained that, at a distance of 10 centimeters, the
arc became flattened so as to assume the appearance of those gas jets
called 'butterfly.' The plane of the 'butterfly' was parallel with the
pole that I presented, or, in other words, with the section of the
magnet. At the same time, the arc began to emit a strident noise, which
became deafening when the pole of the magnet was brought to within a
distance of about 2 millimeters. At this moment, the butterfly form
produced by the arc was _greatly spread out, and reduced to the
thickness of a sheet of paper_; and then it burst with violence, and
projected to a distance a great number of particles of incandescent
carbon.
"2. The magnet employed being a horseshoe one, when I directed it
laterally so as to present successively, now the north and then the
south pole to the arc, the 'butterfly' pivoted upon itself so as not to
present the same surface to each pole of the magnet."
By referring to the accompanying figure, which we extract from our note
on the Ruhmkorff apparatus, it will be seen that the aureola which
developed as a circular film from right to left at D, on the north pole
of the magnet, N.S. (Fig. 1), projected itself in an opposite direction
at C, upon the south pole, S, of the same magnet; but, between the two
poles, these two contrary actions being obliged to unite, they gave rise
in doing so to a very characteristic helicoid spiral whose direction
depended upon that of the current of discharge through the aureola,
or upon the polarity of the magnetic poles. On the contrary, when the
discharge took place in the direction of the equatorial line, as in Fig.
2, the circular film developed itself in the plane of the neutral line
above or below the line of discharge, according to the direction of the
current and the magnetic polarity of the magnet.
There is, then, between Mr. Pilleux's experiments and my own so great an
analogy that we might draw the deduction therefrom that induced currents
in alternating machines have, like those of the Ruhmkorff coil, a
definite direction, which would be that of currents having the greatest
tension, that is to say, that of direct currents. This hypothesis seems
to us the more plausible in that Mr. J. Van Malderem has demonstrated
that the attraction of solenoids with the currents, not straight,
of magneto-electric machines is almost as great as that of the same
solenoids with straight currents; and it is very likely that the
difference which may then exist should be so much the less in proportion
as the induced currents have more tension. We might, then, perhaps
explain the different effects of the wear of the carbons serving as
rheophores, according as the currents are continuous or alternating, by
the different calorific effects produced on these carbons, and by the
effects of electric conveyance which are a consequence of the passage of
the current through the arc.
We know that with continuous currents the positive carbon possesses a
much higher temperature than the negative, and that its wear is about
twice greater than that of the latter. But such greater wear of the
positive carbon is especially due to the fact that combustion is greater
on it than on the negative, and also to the fact that the carbonaceous
particles carried along by the current to the positive pole are
deposited in part upon the other pole. Supposing that these polarities
of the carbons were being constantly alternately reversed, the effects
might be symmetrical from all quarters, although the only current
traversing the break were of the same direction; for, admitting that the
reverse currents could not traverse the break, they would exist none the
less for all that, and they might give rise (as has been demonstrated
by Mr. Gaugain with regard to the discharges of the induction spark
intercepted by the insulating plate of a condenser) to return discharges
through the generator, which would then have, in the metallic part of
the circuit, the same direction as the direct currents succeeding,
although they had momentarily brought about opposite polarities in the
electrodes. What might make us suppose such an interpretation of the
phenomenon to have its _raison d'etre_, is that with the induced
currents of the Ruhmkorff coil, it is not the positive pole that is
the hottest, but rather the negative; from whence we might draw the
deduction that it is not so much the direction of the current that
determines the calorific effect in the electrodes, as the conditions of
such current with respect to the generator. I should not be
surprised, then, if, in the arc formed by the alternating currents of
magneto-electric machines, there should pass only one current of the
same direction, and which would be the one formed by the superposition
of direct currents, and if the reverse currents should cause return
discharges in the midst of the generating bobbins at the moment the
direct currents were generated.--_Th. Du Moncel_.
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