INDUCTION OF ELECTRIC CURRENTS. The discovery of the power of electric currents to induce currents in neighboring conducting circuits is due to Faraday. His researches on the subject, named by him rolta-elect•ic induction, were published in the Philosophical Transactions (1831-32). Henry (1832) observed that when contact was broken in a long galvanic circuit a bright spark occurred, which did not occur when the Circuit was short. This was shown by Faraday (1834) to be due to the extra current induced by the various parts of the circuit in each other. 13achhoffner and Sturgeon (1837) showed the superior action, in induction apparatus, of a bundle of iron - :hes to that of a solid bar of iron. Henry (1841) studied the inductive action of induced. cur rents of different orders. De In Hive designed, in 1843, an electro-chemical condenser, consisting of a primary coil, which, by means of the extra current, could enable a single galvanic cell to decompose water. The same decomposition, however, had been effected by Wright in 1840. Ruhmkorff constructed (1850 or 1851) the first so-called induction coil, the excellence of which was chiefly attained by the proper insulation of the second ary coil. Fizeau (1853) increased, immensely the power of the coil, by providing it with a condenser. Of late years coils of great power have been constructed, rivaling, if not exceeding, the most powerful electric machines iu length and power of spark.
The fundamental law of current induction may be thus shown: Two long cop per wires are fixed so as to be parallel and close to each other. The extremities of the one are in connection with the poles of a galvanic battery, and those of the other, with the binding-screws of a galvanometer. The instant the circuit of the battery is com pleted, and the current sent along one wire, a current in the opposite direction is induced in the other wire, which is shown by the deflection of the needle of the gal vanometer. This induced current is only momentary, for though the current continues to circulate in the first wire, the needle soon falls back to its original position of rest, and the wire then gives free passage to other currents, and appears to be in no way affected. If, now, when the needle is at rest, the battery circuit be broken, and the
current stopped, another momentary current is indicated by the galvanometer needle, but in this case in the same direction as the inducing current. The inducing wire and current are called primary, and are so distinguished from the induced wire and current, which are termed secondary. The passive. condition of the wire while this under induction has been described by Faraday as electro-tonic. An electric throb, so to speak, marks the setting in of this state, and another its vanishing; the former in the oppo site direction to that of the inducing current, and the latter in the same direction. If the primary wire be movable, so that it can be suddenly brought near to, and with drawn from the secondary, while the battery current passes steadily, currents are induced as in the former case, the approach of the wire being marked by an inverse current, and its withdrawal by a direct one. As long, however, as the primary wire remains in any one position, all evidence of electricity in the secondary wire disap pears; but if in this position the strength of the primary current should be increased or diminished, momentary currents in the secondary wire would again mark the changes in the primary, the increase causing an inverse, and the decrease a direct current. Hence we conclude, that a current which begins, a current which approaches, or a current which increases in strength, induces an inverse momentary current in a neighboring conduct ing circuit, and that a current which stops, a current which retires, or a current which decreases in strength, induces a direct momentary current in a neighboring circuit. For inverse, the word negative, and for direct, the word positive, are frequently employed in reference to induced currents.