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the wires recede, an induced current in the same direction as the inducing current.

§ 338. After Dr. Faraday's discovery, many ingenious contrivances were made for exalting the effects and facilitating experiments. The most complete arrangement now in use was the original combination of Mr. Saxton.

A very powerful horseshoe magnet, formed of numerous steel plates closely applied together, or an electro-magnet of soft iron of the same form, is placed in a horizontal position. An armature of the purest soft iron has each of its ends bent at a right angle, and is mounted in such a way that the surfaces of those ends are directly opposed, and close to the poles of the magnet; in this position it may be made to rotate rapidly in a vertical direction, by means of multiplying wheels and an endless band. Two series of copper wires, covered with silk, are wound round either end of this bar, as compound helices. The extremities of these wires having the same direction, are connected together, and with a circular disc rotating with the armature in a cup of mercury, with which it is, therefore, in metallic communication in every position of the disc. The other extremities of the wires are united together, and, passing without metallic contact through the spindle upon which the apparatus turns, terminate in a slip of copper, with two opposite points placed at right angles to the axis. These, in the act of rotation, alternately dip into and rise above the mercury in another cup, which may be connected with the first at pleasure by means of a copper wire. By the laws of magnetic induction, the armature becomes a temporary magnet whenever its bent ends are opposite the poles of the magnet, and ceases to be magnetic when they are at right angles to them. The momentary generation and destruction of the magnetic force, which will be oppositely directed in the bar as its

opposite ends become opposed to the same poles in the act of rotation, must, by the laws of magnetoelectric induction, induce corresponding opposite electric currents in the copper wire if the circuit be complete, by the immersion of the points at the moment of their passage. The points are so arranged that, standing nearly at right angles to the revolving bar, they just rise from the mercury as its ends become opposed to the poles of the magnet, and the circuit being thus suddenly broken at the moment of the electric wave, the current passes in the form of a brilliant spark. (84)

(84) This figure represents a section of Mr. Saxton's magneto-electrical machine. AB is the horseshoe magnet. CD the armature of soft iron, standing in a transverse direction to the poles of the magnet. EF is the spindle upon which it revolves

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by means of the multiplying wheels Ea. h and i are the metallic wheels and points by which connexion is made and broken between the mercury cup, K, and the ends of the coils of wire round the armature, c D.

339. By means of this magneto-electrical ma chine, all the well-known effects of voltaic currents may be very commodiously produced. When the communication is made between the spindle and the revolving disc, by means of a fine platinum wire instead of the dipping points, the wire may be maintained at a red heat; although the effect being produced by alternating currents in opposite directions, a kind of pulsation, or intermission of the light, may be discerned. Upon making the communication between the two mercury cups by means of copper cylinders grasped in the hands, a continued painful contraction of the muscles of the arms takes place, which destroys voluntary motion, and, under certain circumstances, is perfectly intolerable. If the currents be transmitted into liquid electrolytes by means of platinum electrodes, they are readily decomposed.

§ 340. However large the quantity of electricity may be which is thrown into circulation by a single voltaic circuit, its intensity is very low, and it is wholly incapable of effecting its passage by disruptive discharge. In consequence of this, when the conducting wire is very short, no spark is visible upon making or breaking contact between the generating and conducting surfaces; or, at most, a very minute one upon the latter occasion. Nevertheless, when the wire is long, a very bright spark is produced upon breaking contract. This spark is greatly increased when the same long wire is coiled up into a helix; and still more when it is wound round an iron core, so as to constitute an electromagnet. Under these last circumstances, if the ends of the wire near the battery be grasped by a person with moistened hands, he will receive a considerable shock at the moment of breaking the contact at a point between the place at which it is held and the battery, and the spark will be diminished. If a decomposing apparatus be interposed

296 SELF-INDUCTION OF CONDUCTING WIRE.

between the same extremities of the wire, the direct current will pass through the latter so long as the contact with the battery plates is maintained; but, on breaking contact, iodide of potassium will be decomposed in the former, and the situation of the evolved iodine will prove that the decomposition has been effected by a current in an opposite direction to that of the battery current. In this last experiment a bright spark occurs at the place of disjunction, indicating that only a small part of the extra current passes the decomposing apparatus because of its small conducting power. The most instructive set of results are, however, obtained when a galvanometer is introduced between the extremities of the conducting wire. When contact is then made, a portion of the primary current which passes through the principal wire is diverted through the galvanometric wire, and the needle is deflected. If, while in this situation, it be forced back by pins, applied upon opposite sides of its two extremities, to its natural position when uninfluenced by a current, and the battery contact be then broken, it will be strongly deflected in the opposite direction; thus showing that the extra current followed a course in the cross-wires contrary to the one produced by the direct action of the electromotor.

341. It is clear, then, that, at the moment of its return to its natural state, a reflex wave of electricity is generated in the conducting wire, which is analogous to that which it is capable of inducing at the same moment in a parallel independent wire. It is generated, also, by the same lateral or magnetic force, and it is increased, like the latter, by the reaction of the temporary magnetism of soft iron in the moment of its cessation, and by the juxtaposition of the coils of a helix.

In fact, if a second and independent wire be placed by the side of the primary conducting wire, as by coiling the two together in one helix, taking

care that they may be well insulated, so long as the circuit of the second wire is not complete, the primary wire will give a good spark; but, upon completing the secondary circuit by bringing its ends into metallic contact, the whole of the lateral force will be expended upon it, and little or no spark will appear when the primary circuit is broken. Then, again, if the ends of the second wire should happen to be disconnected at the moment when the battery contact is broken, a spark will there appear instead of occurring at the disjoined extremity of the principal wire. Even the weak current of the thermo-electric circuit (§ 284) may be made to yield a spark from this secondary action by the assistance of a helix.

342. In the wire of the helix of the magnetoelectric machine, these principles of action have an important influence. From the construction of the apparatus, the current is permitted to move in a complete metallic circuit of great length during the first instance of its formation: it gradually rises in strength, and is then suddenly stopped by the breaking of the metallic circuit; and thus great intensity is given by induction to the reverse current which at that moment passes. Different effects are produced by varying the length and diameter of the wire upon the armature. A great length (1500 yards) of fine insulated copper wire will decompose water with the greatest advantage, and give the severest shocks; but a less length (40 yards) of thicker wire will give the largest sparks, and ignite the largest quantity of platinum wire.

343. By a primary coil of thick wire of 120 feet in length, the connexion of which with a voltaic circuit may be rapidly made and broken by a spring pressing upon a toothed wheel and an inducteous coil of finer wire 120 yards long, the ends of which may be grasped by the hands, an electrodynamic machine may be constructed, the effects

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