648 ARAGO'S ROTATIONS. [314. magnetic needle or a bar magnet be set vibrating parallel to the surface of a disk of copper, it will come to rest much more speedily than if vibrating over paper or glass. These effects were first satisfactorily explained by Faraday ; he found that whenever a piece of conducting matter is made to pass either before a single pole or between the opposite poles of a magnet so as to cut the magnetic curves at right angles, electrical currents are produced across the metal, transverse to the direction of motion. For example, let the copper disk, c, fig. 258, be made to revolve, in the direction of the arrows on the circumference, between the poles, n s, of a horse FIG. 258. g shoe magnet, and let a wire w, which is connected with one end of the galvanometer, g, be pressed against the centre of the disk, whilst the other wire wo' from the galvanometer rests against the edge of the disk between the magnetic poles. Under these circumstances, a current will be found to flow from the centre towards the circumference of the disk, c, and then through the wires, as shown by the arrows. If the disk be made to revolve in the opposite direction, the current will flow from the circumference towards the centre of the disk. Currents may also be obtained from any of the forms of the apparatus which exhibit the rotation of magnets round a conducting wire, or of the wire round the magnet, if a galvanometer be substituted for the battery, and if the magnet or the wire be made to revolve by hand. FIG. 259. Now let us suppose that in Arago's experiment we are looking down upon the revolving disk, c, fig. 259; when the disk revolves beneath the magnet, it cuts the magnetic curves at right angles: currents are produced underneath the north pole, from the centre of the plate towards the circumference, a, beyond the pole. These currents occur in the opposite direction-viz., from the circumference to the centre, underneath the south pole, and thus traverse the diameter of the plate parallel to 315.] SAXTON'S MAGNETO-ELECTRIC MACHINE. 649 the magnet, returning by the more distant parts of the plate, as shown by the dotted arrows. Such currents necessarily exert a repulsive action upon the magnet in a direction which coincides with that in which motion is observed, and no currents are obtained until either the magnet or the plate is set in motion. (315) Magneto-Electric Machines.-Various machines have been contrived for the production of magneto-electric currents; one of the most convenient of which is Saxton's magneto-electric machine. It is represented in fig. 260, in perspecFIG. 260. tive; fig. 261 shows a section of the coils and armature on a larger scale. It consists of a powerful horse-shoe magnet, M, placed horizontally upon one of its sides in front of its ends or poles, and as close to them as is possible without producing actual contact, an armature of soft iron, a b, is made to revolve upon a horizontal axis, A, which admits of being turned by means of a strap passing over a multiplying wheel, w. This armature consists of two straight pieces of iron, about two inches (5 centimetres) in length, which, by means of a cross 650 MAGNETO-ELECTRIC MACHINES. [315. piece of iron, x, are connected together parallel to each other, at such a distance that they shall be opposite the middle of each pole of the horse-shoe magnet. Around each limb, c d, of the armature, a long fine copper wire, covered with silk to insulate the coils from each other, is wound in several successive layers. The corresponding ends of each of these helices are connected together; one pair, ef, is soldered to the spindle, s, on which the armature rotates, and through it is connected with a circular copper disk, i, the edge of which dips into a cup of mercury, m, whilst the other pair of wires, g h, is connected with a stout piece of copper which passes through the axis of the spindle, s, from which it is electrically insulated, and terminates in a slip of copper k, placed nearly at right angles to the crosspiece, x, which connects the two limbs of the soft iron armature. Beneath the slip of copper, k, is a second mercury cup, 7, which can be made to communicate with the cup, m, either by a wire, or by some other conductor of the current. The arms of the slip, k, alternately dip into the mercury, and rise above it, and the points of contact are so arranged that the circuit (which, when 7 and m are properly connected, is complete so long as k is beneath the mercury) shall be broken at the time that the armature loses its magnetism. Under these circumstances a bright spark is obtained each time that the slip k quits the mercury. Four currents are therefore produced in the wire surrounding the armature during each complete revolution, two successive currents being in one direction, and the two others being in the opposite direction. Suppose, for example, that the limb, c, of the armature is opposite the marked pole of the steel magnet: if now it be made to recede from this pole, a current will be produced in a given direction through the coil which surrounds this limb, and on the approach of the same limb towards the unmarked end of the magnet, a second current will be produced in the same direction through the coil; a third current will be produced, but in a reversed direction, as the limb e leaves the unmarked end of the magnet, whilst a fourth current will be produced on the approach of the limb c to the marked pole of the magnet, and will coincide in direction with the third current. If the connexion between the mercury cups, and m, be effected by grasping with the hands two copper cylinders, H, H, each of which by means of a wire is in connexion separately with one of the cups, a succession of powerful shocks will be experienced. Acidulated water and many saline solutions may be decomposed if these currents be transmitted through them; but in order to produce polar decomposition, it is necessary to suppress or turn up one of the points of the slip k, and thus to lose half the power of the machine: otherwise the currents at each half revolution are in opposite directions. In the construction of these magneto-electric machines, great care must be taken that the insulation of the coil is very perfect. Different effects are obtained from such a machine by varying the length and the diameter of the wire which is wound around the armature. When currents of high tension are required, such as those needed for giving shocks, or for the decomposition of electrolytes, a great length of thin wire is preferable; but a much smaller length of thicker wire will give the largest sparks, and will ignite the greatest length of fine platinum wire. A machine upon this principle has been contrived by Wheatstone for exploding charges of gunpowder, when provided with Abel's magnet fuse, which seems to leave little to be desired in simplicity, certainty, 315.] MAGNETO-ELECTRIC LIGHT. 651 and facility of application. (Report to Secretary for War, Wheatstone and Abel, Nov. 1860.) Wheatstone and others have contrived magneto-electric machines, by which a continuous electric current in a uniform direction may be kept up for any length of time. These batteries are, in fact, combinations of several simple machines, similar in principle to Saxton's; the coils are connected together so as to form a continuous circuit. The armatures are so arranged that each shall in turn become magnetic, just before the preceding armature has entirely lost its magnetism. By this contrivance, the current is made to commence in one coil before it has ceased in the coil which immediately precedes it. Magneto-electric machines are now used in Birmingham on a large scale, as a substitute for the voltaic battery, in processes of electro-silvering and electro-gilding. A single Saxton's machine will, if kept in continuous revolution, precipitate from 90 to 140 ounces (25 to 4 kilogrammes) of silver per week from its solutions; and machines have been constructed by which 17 ounces or 048 kilogr. of silver per hour have been deposited upon articles properly prepared for this mode of plating. Mr. Holmes has succeeded, by the use of a powerful magnetoelectric machine, in producing a light of great steadiness and intensity between two points of gas coke: this light can be maintained without interruption so long as the magents are kept in rotation, and the charcoal continues unconsumed. The machine consists of 48 pairs of permanent compound bar magnets, arranged in 6 parallel planes, so as to form a large compound wheel, between which the armatures, 160 in number, are arranged in 5 sets, the total amount of wire being about half a mile in length. The wires are insulated by cotton, and the contacts are so arranged as to maintain a continuous current in the same direction. This is accomplished thus: One half of the helices are arranged so as to arrive on the poles of the magnet at the instant that the other half are exactly midway between the poles. Thus there are two distinct currents; and what may be called the dead point, that is, the point when the current becomes inverted in one series, occurs exactly at the time when the other current is at its maximum, so that if now the inverted currents can be again inverted in both of these distinct currents, and that the two now flowing in one direction can be united as one compound current, it is evident that the result will be a current nearly as constant as that from a galvanic battery, with the advantage of equable continuity. This is done by the two commutators, which consist each of two insulated rings of metal, of such a form at the periphery that two rollers or rubbers change sides from one disk to the other at the same instant that the current is reversed. Then, by combining the two commutators, a compound current is obtained that will produce a constant white light or any of the ordinary effects of the galvanic current. The quantity and the intensity of the induced current depend first, on the amount of magnetism induced in the soft iron; secondly, on the 652 MAGNETO-ELECTRIC MACHINES. [315. facility with which the poles of the magnetized soft iron can be reversed; thirdly, on the velocity with which the change of polarity takes place; fourthly, on the length and diameter of the wire forming the helices. The amount of magnetism induced in the soft iron depends on the size and strength of the steel magnets employed, and on the weight and softness of the iron in the helices; but in practice, the weight of the soft iron is limited by the weight of the steel magnets, for, if too heavy, the steel magnets will be slowly deprived of their magnetism. The most advantageous proportion of iron in the armatures has been found by experiment to amount to of the weight of the steel magnets employed. To facilitate the change of the poles, the soft iron cores of the helices are not solid pieces of iron, but are tubes, single, double, or treble, as it is found by experiment that the same weight of iron, when divided in this manner, loses or takes magnetism in much less time than when in a solid form. There is a limit to the velocity to be employed when the maximum of electricity is required, for this reason. It has been already remarked that the amount of electricity depends on the amount of magnetism taken up, and that the soft iron takes time to become saturated, as it may be termed, with magnetism: hence, if the velocity with which the cores move from one pole of a magnet to another be too great, there will not be sufficient time for the cores to become saturated. But as again the quantity of electricity increases as the velocity increases, it is necessary to ascertain this maximum point exactl, which is easily done, either by experiment or calculation, based on certain data. The steel bars weigh about 1 ton, and the wheel is made, by the aid of a small steam engine, to revolve with a rapidity varying from 150 to 250 times per minute. This light was for several months in successful operation at the South Foreland Lighthouse, and subsequently at Dungeness, the actual expense of fuel in working the engine being about equal to that of the oil formerly used, while the light is far more brilliant. It was stated by Mr. Holmes, in a lecture given before the Society of Arts in December, 1863, from which the foregoing details are taken, that the light at Dungeness had then been in continuous use since the 6th June, 1862, and is equal photometrically to 14 of Fresnel's first-class lighthouse lamps. It is now again employed at the South Foreland. Mr. Wilde (Proc. Roy. Soc. 1866, xv. 107 and Phil. Trans. 1867, 89) has lately contrived an apparatus by means of which, with very moderate magnetic power, he can, by employing suffi cient mechanical power to produce rotation of his apparatus, obtain an extremely intense current of electricity. The foundation of the arrangement is a magneto-electric machine of a peculiar form :-A compound hollow magnet cylinder, composed of two bars of soft iron connected by pieces of brass, was constructed with an internal diameter of 1625 inches (4 centimetres). Upon this cylinder could be placed at pleasure I or more horse-shoe magnets. each weighing 1 lb., and capable of supporting a weight of about 10 lbs. These magnets were so arranged that all the poles of the same name rested upon the same soft iron bar of the magnet cylinder. the interior of this cylinder a soft iron armature was made to revolve in close proximity to the internal surface, but without actually touching it. The cross section of this armature was somewhat like that of an ordinary railway rail, or like a capital H. Along the length of the armature, in the hollows thus formed, about 183 feet (56 metres) of insulated copper wire was coiled o'03 inch In |