includes all phenomena where magnetism gives rise to electricity. "Under Induction of Electric Currents (q.v.), it is stated that when a coil in which 11 current circulates is quickly placed within another coil unconnected with it, a contrary induced current in the outer coil marks its entrance, and when it is withdrawn, a direct induced current attends its withdrawal. While the primary coil remains stationary, in the secondary coil, though the current continues to flow steadily in the primary, no cur rent is induced in the secondary coil. It is also shown that if, while the primary coil is stationary, the strength of its current be increased or diminished, each increase and diminution induces opposite ciirrents in the secondary coil. Change, in fact, whether in the position or current strength of the primary coil, induces currents in the secondary coil, and the intensity of the induced current is in proportion to the amount and sud denness of the change. In singular confirmation of Ampere's theory, a permanent bar magnet may be substituted for the primary coil in these experiments, and the same results obtained with greater intensity-. When a bar magnet is introduced into the second ary coil, a current is indicated, and when it is withdrawn, a current in a contrary direction is observed, and these currents take place in the directions required by Ampere's theory. A change of position of the magnet is marked by a current, as in the. former case. If we had the means of increasing or lessening the magnetism of the bar, currents would be induced the same as those obtained by strengthening or weakening the current in the primary coil. It is this inductive power of iron at the moment that a, change takes place in its magnetism, that forms the basis of magneto-electric machines. The manner in which this is taken advantage of will be easily understood by reference: to flg. 3. N S is a permanent horse-shoe magnet, and let us suppose it to be fixed; C is a bar of soft iron, with coils A and B wound round its extremities, and may be looked upon as the armature of the magnet. C D is capable of rotation round the axis E F. So long as C D remains in the position indicated in the figure, no currents are induced iu the surrounding coils, for no change takes place in the magnetism induced in it by the action of N S. The moment that the poles of C D leave N S, the magnetism of the soft iron diminishes as its distance from N S increases; and when it stands at right angles to its former position, the magnetism has disappeared. During, the first quarter-revolution, therefore, the magnetism of the soft iron diminishes, and this is attended in the coil (for both coils act, in fact, as one) by an electric current, which becomes manifest when the ends e, e, of the coil are joined by a conductor. During the second quarter revolution,
the magnetism of the armature increases till it reaches a maximum, when its poles are in a line with those of N S. A current also marks this increase. and proceeds in the same direction as before; for though the magnetism increases instead of diminishes, which of itself would reverse the induced current, the polos of the revolving armature, in conse quence of their change of position with the poles of the permanent magnet, have also been reversed, and this double reversal leaves the current to move as before. For the second half-revolution the current also proceeds in one direction, but in the opposite way, corresponding to the reversed position of the armature. Thus, in one revolution of a soft iron armature in front of the poles of a permanent magnet, two currents are induced in the coils encircling it, in opposite directions, each lasting half a revolution, starting' from the line jotning the poles.
general construction of a simple magneto-electric machine is shown in fig. 14. S is a fixed permanent magnet. B B is a so-ft iron plate, which are attached two cylinders of sat iron, round Whieh the coils C and D are wound. CBBD is thus the revolving armature, corresponding to C D in fig. 13. A A is a brass rod rigidly connected with the armature, and also serving as the rotating ax4e. F is a cylindrical projection on A A, and is pressed upon by two fork-like springs, H and K, which are also the poles of the machine. The enus, 171, n, of the coils are soldered. to two metal rings on F, insulated from eacn other. When the armature revolves, A A and F move with it. F, II, and K are so constructed as to act as a commutator, revers ing the current at each semi-revolution. By this arrangement, the opposite currents. proceeding from the coil at each semi-revolution are transmitted to H and K in the same direction, so that these, which constitute the poles of the battery, so to speak, remain always of the same name. When the armature is made to revolve with sufficient rapidity, a very energetic and steady current is generated. Of late years immense progress has, been made in the construction of such machines. In 1866 Wilde of Manchester sur prised the scientific world by a machine. of unprecedented power; and more recently, Gramme of Paris has constructed another Still more astonishing. These are driven by steam-engines, and completely eclipse both in power and constancy the largest galvanie battery hithertoput together. See MAGNETO-ELECTRIC MACHINE. See also ARMATHRE, DECLLNATION NEEDLE, DIAMAGNETISM, DIPPING NEEDLE, and ROTATION, MAGNET