Page:The American Cyclopædia (1879) Volume XI.djvu/25

 MAGNETO-ELECTRICITY 17 oil ; the current of the battery is interrupted by drawing the end of the conductor out of the mercury while it still remains in the oil, which being a bad conductor stops in part the induced current. A similar effect is pro- duced by suffering the extra current to expend itself on a large sheet of metal called a conden- ser. The facts we have here stated have been confirmed and extended by Masson, Yerdet, and Acre of France, Dove, Wartmann, Riess, and Lentz of Germany, Marianini of Italy, and De la Rive of Geneva. Induced Currents from Discharges of ordinary Electricity. When a discharge from a Leyden jar is transmitted through two spiral conductors separated by a pane of glass or a stratum of air, induced cur- rents analogous to those we have described are generated of great intensity, and under favor- able circumstances the effect may be exhibited at a great distance. Prof. Henry succeeded in magnetizing needles with induced currents at the distance of several hundred yards, by stretch- ing two long wires parallel to each other, and transmitting a discharge from a Leyden jar through one Of them. He also obtained induc- tive effects of the same kind from the discharges of the thunder cloud at a distance of several miles. The direction of induced currents from discharges of the Leyden jar is apparently very capricious ; they do not deflect the needle of the galvanometer, and the direction indicated by the magnetization of needles, enclosed in a small helix which forms apart of the circuit, is subject to very complex variations. For exam- ple, when the two conductors are near each other, the direction indicated by the magnetiza- tion of the needle is opposite to that of the cur- rent from the jar. If the two parallel wires or flat spirals be separated to a greater distance, the magnetization of the needle will indicate either a feeble current or one in an opposite direction ; and if the distance be still further increased, the opposite polarity of a greater intensity will be exhibited. A change also in the direction of the magnetization of the needle will be pro- duced by an interruption in the circuit of the induced current, or by the proximity of another closed circuit. These results have led European physicists to attempt to ascertain the direction of the current by chemical decomposition and other effects, but the results do not settle the question or throw much additional light on the character of the phenomena. Prof. Henry, however, after a very extended series of experi- ments, was enabled to refer them all to the pe- culiarity of the electrical discharge from the Leyden jar. This does not consist of a single discharge from the inside to the outside of the jar, as has been generally supposed, but in a series of discharges forward and backward alternately, until an equilibrium, as it were, is established by a series of oscillations, decreas- ing in intensity on account of the resistance of the wire, until the normal electrical equi- librium is attained. Induction in Masses of Metal in motion. Arago in 1824 discovered that when a copper plate -is made to revolve rapidly immediately under a magnetic bar freely suspended by an untwisted thread, the motion will be communicated to the latter even through a plate of glass ; and also that when a magnetic needle is made to vibrate immediately over a plate of copper, it will come to rest much sooner than when the metal is removed. These facts remained entirely isolated until Faraday showed that they were the results of currents induced in the plate by the action of the magnet. We have seen that when a wire is made to approach at right angles to a magnetized bar, a current is produced in the former opposite to that of the hypothetical current in the near side of the magnet. A similar result must be produced when a plate of metal is moved in the vicinity of a magnetic pole. To illustrate this, let the N". pole of a strong magnetic bar be placed perpendicularly on the middle of an oblong plate of copper, extended in a N. and S. direc- tion ; while the bar retains this position, let the plate be drawn in the direction of its length, say southward, under the magnetic pole. A magnetic bar thus placed with its N. pole downward has hypothetical currents re- volving around it from W. to E. on the K side, and from E. to W. on the S. side. If the plate therefore be moved southward, the N. part, which is approaching the pole, will have induced in it a current in an opposite direction to that of the current in the magnet, which will in this case be a current directed toward the west, while the S. part of the plate receding from the magnet will have currents produced in it in the same direction as those in the mag- net; but the currents on the S. side of the magnet are moving toward the west, and hence we shall have on both sides of the magnetic pole of the bar currents directed toward the west during the time the plate is drawn from the north toward the south. If we reverse the motion of the plate, the direction of the sys- tem of currents will also be reversed. If the poles of a horse-shoe magnet be furnished with two pieces of iron so as to form acting poles at a small distance from each other, and nearly jn the same line, and between these a circular disk of copper be made to revolve on an axis parallel to the line joining the poles, so that the latter shall be near the outer circumference, a system of currents from the centre to the circumference of the plate will be produced ; the radii of the plate which are approaching and 'those which are receding from the line joining the magnetic poles will both conspire to produce this effect. If one end of a galvanometer be brought in con- tact with the axis of the circular plate, and the other made to touch the circumference while it is thus revolving, a constant current will be in- dicated by the instrument. If the direction of the revolution of the disk be changed, an oppo- site current will be produced ; or if the velocity of the rotation be increased, a corresponding increase will be observed in the intensity of the current. If the magnet employed in this ex-