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The author therefore has endeavored to clear up this uncertainty by experiment. His apparatus consisted of a small compound electro-magnet, immersed in water, revolving between the poles of a powerful stationary magnet.
To Faraday these experiments explained the phenomenon of Arago's rotating disk, the disk inducing the current from the magnet, and, in reacting, deflecting the needle. To prove this, he constructed a disk that revolved between the poles of an electro-magnet, connecting the axis and the edge of the disk with a galvanometer.
Among them I can pause to mention only those of Volta, the Italian, to whose discoveries the battery is due; Oersted, the Dane, who first discovered the magnetic properties of the electric current; Ampere and Arago, the Frenchmen, who prosecuted still further and most successfully similar researches; then Sturgeon, the Englishman, who may be said to have made the first electro-magnet; next, and not least illustrious among these illustrious men, our countryman Henry, who first showed the practicability of producing electro-magnetic effects by means of the galvanic current at distances infinitely great; and finally Steinheil, the German, who, after the invention of the Telegraph in all its material parts was complete, taught, in 1837, the use of the ground as part of the circuit.
To make the results of induction effective in practice, we must have great length of wire, and to this end, as in the case of the electro-magnet, we will adopt the spool form. We will suppose two wires, insulated so as to keep them from actually touching, held together side by side, and wound upon a core in several layers.
"Lydia told me last night over the telephone and very much surprised she was, too, when I swore her to secrecy that the jewel had been lost when the lamp was shipped from New York," Dundee explained. "There's a blank cartridge in the gun now, of course, but Miles, in his panic, took my words literally.... See the electro-magnet strapped to the gun butt?
These currents after traversing the line were passed through the receiver, which consisted of a tubular electro-magnet, having one end partially closed by a thin circular disc of soft iron fixed at one point to the end of the tube. This receiver bore a resemblance to a cylindrical metal box with thick sides, having a thin iron lid fastened to its mouth by a single screw.
This instrument consists essentially of an electro-magnet of horseshoe form with its two poles close together, and with its armature, a bar of iron, maintained in close proximity to the poles, but kept normally in a retracted position by a spring. Its attractive power draws the armature toward the poles.
Tubes of exceedingly thin glass, filled with various gases, and hermetically sealed, were suspended in pairs from the two ends of the cross-piece. The position of the rotating torsion-head was such that the two tubes were at opposite sides of, and equidistant from, the magnetic axis, that is to say from the line joining the two closely approximated polar points of an electro-magnet.
Professor Henry, by his experiments five years earlier, had demonstrated that many turns of small wire made the electro-magnet far more sensitive. Morse made this improvement in his own apparatus. In 1832 Henry had devised a telegraph very similar to that of Morse by which he signaled through a mile of wire. His receiving apparatus was an electro-magnet, the armature of which struck a bell.
Soon after he had commenced his researches on diamagnetism, Faraday noticed a remarkable phenomenon which first crossed my own path in the following way: In the year 1849, while working in the cabinet of my friend, Professor Knoblauch, of Marburg, I suspended a small copper coin between the poles of an electro-magnet.
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