THE FIXATION OF MAGNETIC PHANTOMS. 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.