When we place a thin sheet of cardboard or glass upon a magnet and scatter
iron filings over it, we observe the iron to take certain positions and
trace certain lines which Faraday has styled lines of magnetic force, or,
more simply, lines of force. The figure, as a whole, which is thus formed
constitutes a magnetic phantom. The forms of the latter vary with that of
the magnet, the relative positions of the magnet and plate, etc.
[Illustration: METHOD OF FIXING MAGNETIC PHANTOMS.]
The whole space submitted to the influence of the magnet constitutes a
_magnetic field_, which is characterized by the presence of these lines of
force, and the study of which is of the most important character as
regards electro-magnetic action and that of induction. In order to study
these phantoms it is convenient to fix them so that they can be preserved,
projected, or photographed. Fig. 1 shows how they may be fixed. To effect
this, we cover the plate with a layer of mucilage of gum arabic, allow the
latter to harden, and then place the plate over the magnet. Next, iron
filings are scattered over the surface by means of a small sieve, and,
when the curves are well developed,[1] the surface is moistened by the aid
of an ordinary vaporizer. The layer of gum arabic thus becomes softened
and holds the iron filings so that the particles cannot change position.
When the gum has hardened again, the magnet is removed, and the phantom is
fixed.
[Footnote 1: The curves are obtained by striking the plate lightly with a
glass rod.]
We thus have a tangible representation of the magnetic field produced by
the magnet in the plane of the glass plate or sheet of paper. The number
of these lines, or their density, is at every point proportional to the
intensity of the field, and the curves that are traced show their
direction. To finish the definition of the field, it remains to determine
the direction of these lines of force. Such direction is, by definition,
and conventionally, that in which the north pole of a small magnetic
needle, free to move in the field, would travel. It results from this
definition that the lines of force issue from the north pole of a magnet
and re-enter the south pole, since the north pole of a magnet repels the
north pole of a needle, and _vice versa._
These considerations relative to the direction and intensity of the
magnetic field are of the highest importance for the physical theory of
magneto-electric machines.
The following is another method of fixing phantoms, as employed by Prof.
Bailie, of the Industrial School of Physics and Chemistry of the City of
Paris. He begins by forming the phantom, in the usual way, upon paper
prepared with ferrocyanide, and exposes it to daylight for a sufficient
length of time. The filings form a screen which is so much the more
perfect in proportion as it is denser, and, after fixation, there is
obtained a negative phantom, that is to say, one in which the parts where
the field is densest have remained white.
The same processes of fixation apply equally well to galvanic phantoms,
that is to say, to the galvanic fields produced by the passage of a
current in a conductor, and which consists of analogous lines of force.
The processes may be employed very efficaciously and with certainty of
success.--_La Nature._
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