Page:The American Cyclopædia (1879) Volume XI.djvu/21

 MAGNETO-ELECTRICITY 13 be suddenly brought into contact with the two poles of a horse-shoe magnet, an induced cur- rent will be developed for a moment in the sur- rounding wire ; and when the same rod is sud- denly detached from the poles, a current in an opposite direction will take place ; and in this way a continued series of alternate currents may be developed by alternately making and severing the contact of the poles of the magnet and the ends of the rod. A still greater effect may be produced by causing the rod to revolve on an axis at right angles to the middle of its length, before the poles of the magnet, so that each end in rapid succession may be brought in contact first with the K and then with the S. pole, and so on. Shortly after the discovery by Faraday of the laws we have stated, Mr. Joseph Saxton of this country, then a tem- porary resident of London, afterward attached to the United States coast survey, invented (1832) the first machine for giving sparks and shocks in accordance with the arrangement we have just described. Instead of a single bob- bin of wire on the middle of a straight bar, he employed two, one on each leg of a bar of soft iron bent into the form of a horse shoe, which were made rapidly to revolve by means of a multiplying wheel before the poles of a magnet. At each half revolution the mag- netism of the soft iron was entirely reversed, and in this way a series of currents was in- duced, of sufficient intensity to decompose water, fire combustible bodies, and powerfully to affect the nervous system. An instrument maker in London, who was employed to con- struct these machines, made a slight change in the arrangement, which principally consisted in placing the inducing horse-shoe magnet in a vertical position, and in causing the spools of wire to revolve in a plane parallel to its flat side, instead of parallel to its poles. This change, instead of improving the instrument, produced an opposite effect, since the strength of the induction was much diminished. The author of it, however, succeeded by advertise- ments, and an actual exhibition of it in France, in attaching his name to the invention, to the exclusion of that of Saxton. It is, however, gratifying to see that in the German works on the subject, and also in the better class of Eng- lish publications, justice is done to the original inventor. The next important series of inves- tigations on this subject, after the original dis- covery of Faraday, was by Professor Henry of Princeton, now secretary of the Smithsonian institution at Washington. He found that at the beginning and ending of the galvanic cur- rent in a long wire, an induced current was produced by an action which has sometimes been called the induction of a current on itself. To illustrate this, let the circuit of a small battery of a single element be closed by a short wire of about a foot in length, dipping into a cup of mercury. When the circuit is broken, no spark, or but a very feeble one, will be ob- served ; but if we now substitute for the short wire one of say 100 feet in length and of con- siderable thickness, a vivid spark will be ex- hibited when the circuit is interrupted. To obtain this result in the most striking manner, we should employ a copper ribbon at least an inch and a half wide and 100 ft. long, well covered with two thicknesses of silk, and rolled into the form of a flat spiral. At the rupture of a battery circuit of which this forms a part, a loud snap and deflagration of the metal will be produced, when with a short wire, the bat- tery remaining the same, scarcely any but a very feeble spark would be observed. By this arrangement several spires of ribbon react on each other, and increase the effect. By coiling a bell wire covered with silk of >600 or TOO ft. in length into a spiral ring, the intensity will be so much increased that shocks may be ob- tained by means of a small galvapic battery of a single element. If the same wire be coiled into the form of an elongated spiral, and in the centre of this a rod of soft iron be placed, or what is better, a bundle of iron wire, the intensity is still more exalted. In this case the magnetic reaction is combined with that of the current of galvanism, and the two actions be- ing in the same direction conspire to increase the effect. To produce, however, the most powerful inductive apparatus, a bundle of var- nished iron wires of about 15 in. in length, and together forming a diameter of about an inch, is surrounded with a coil of thick copper wire well covered with silk of 300 or 400 ft. in length. Around this, but separated from it by a cylinder of glass or pasteboard soaked in shell lac, is coiled a fine copper wire of 4 or 5 m. in length, care being taken that each spire be well insulated from every other. When a current of galvanism from a battery of even a single element is transmitted through the thick copper wire which surrounds the inner core or bundle of iron wire, the latter becomes magnetic; and at the instant the rupture is made in the battery current, a sudden cessation of the magnetism, as well as that of the cur- rent itself, induces a current of great intensity, though of small quantity, in the outer sur- rounding fine wire. Each spire of the long wire in this arrangement is subjected to the inductive influence ; and the rapidity of mo- tion of the electricity of the wire, were it not for the increased resistance, would be in pro- portion to the number of spires, or in other words to the length of the wire. This appa- ratus has received various ingenious improve- ments, the principle in all cases remaining the same. Dr. Page vas the first to invent an ap- paratus on this plan by which the rupture of the battery current was rendered automatic ; the magnetization of the iron core caused the attraction of a small magnet attached to one end of a lever which broke the circuit, and the consequent disappearance of the same magnet- ism allowed the end of the lever to fall into a cup of mercury and thus again complete the circuit. This instrument was much enlarged