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.
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