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To the same category belongs the 'aerophone,' which may be described as a gigantic tympanum, vibrated by a piston working in a cylinder of compressed air, which is regulated by the vibrations of the sound to be magnified. It was designed to call out fog or other warnings in a loud and penetrating tone, but it has not been successful.

The 'magnetic ore separator' is an application of magnetism to the extraction of iron particles from powdered ores and unmagnetic matter.

The ground material is poured through a funnel or 'hopper,' and falls in a shower between the poles of a powerful electro-magnet, which draws the metal aside, thus removing it from the dress.

Among Edison's toys and minor inventions may be mentioned a 'voice mill,' or wheel driven by the vibrations of the air set up in speaking.

It consists of a tympanum or drum, having a stylus attached as in the phonograph. When the tympanum vibrates under the influence of the voice, the stylus acts as a pawl and turns a ratchet-wheel. An ingenious smith might apply it to the construction of a lock which would operate at the command of 'Open, Sesame!' Another trifle perhaps worthy of note is his ink, which rises on the paper and solidifies, so that a blind person can read the writing by passing his fingers over the letters.

Edison's next important work was the adaptation of the electric light for domestic illumination. At the beginning of the century the Cornish philosopher, Humphrey Davy, had discovered that the electric current produced a brilliant arch or 'arc' of light when passed between two charcoal points drawn a little apart, and that it heated a fine rod of charcoal or a metal wire to incandescence--that is to say, a glowing condition. A great variety of arc lamps were afterwards introduced; and Mr. Staite, on or about the year 1844-5, invented an incandescent lamp in which the current passed through a slender stick of carbon, enclosed in a vacuum bulb of glass. Faraday discovered that electricity could be generated by the relative motion of a magnet and a coil of wire, and hence the dynamo-electric generator, or 'dynamo,' was ere long invented and improved.

In 1878 the boulevards of Paris were lit by the arc lamps of Jablochkoff during the season of the Exhibition, and the display excited a widespread interest in the new mode of illumination. It was too brilliant for domestic use, however, and, as the lamps were connected one after another in the same circuit like pearls upon a string, the breakage of one would interrupt the current and extinguish them all but for special precautions. In short, the electric light was not yet 'subdivided.'

Edison, in common with others, turned his attention to the subject, and took up the neglected incandescent lamp. He improved it by reducing the rod of carbon to a mere filament of charcoal, having a comparatively high resistance and resembling a wire in its elasticity, without being so liable to fuse under the intense heat of the current. This he moulded into a loop, and mounted inside a pear-shaped bulb of glass.

The bulb was then exhausted of its air to prevent the oxidation of the carbon, and the whole hermetically sealed. When a sufficient current was passed through the filament, it glowed with a dazzling lustre. It was not too bright or powerful for a room; it produced little heat, and absolutely no fumes. Moreover, it could be connected not in but across the main circuit of the current, and hence, if one should break, the others would continue glowing. Edison, in short, had 'subdivided' the electric light.