As shown in the article Electro-Magnetism, great magnetic power is developed by passing a current of galvanism around a bar of soft iron; and since in all cases a mechanical action is accompanied by an equal amount of reaction, it is reasonable to suppose that electricity ought to be evolved by magnetism. Various fruitless attempts were however made to obtain tins result; the form in which the effect was to appear was unknown, and it was not till 1881 that Faraday succeeded in exhibiting currents of electricity in a wire by means of magnetic reaction. It bas also been stated in the same article that, in accordance with the theory of Ampere, all the mechanical properties of an ordinary mag-net may be exhibited by currents of electricity transmitted through spiral conductors; and hence, in order to present the phenomena of this class in the simplest form, we shall begin with stating the fundamental facts of what is called electro-dynamic induction, or electricity induced by a galvanic current. 1. Let a portion of a copper wire be extended in a straight line horizontally, and the two ends at a distance be connected with a galvanometer so as. to form a closed circuit in which a current may be induced.
Let also a portion of another wire, connected with a galvanic battery, be placed parallel to the first, and a current sent through it. If the wire transmitting the battery current be suddenly brought near the wire connected with the galvanometer, during the approach of the second wire toward the first a current of the natural electricity of the latter will pass through the galvanometer in a direction adverse to that of the inducing current. 2. The induced current continues only during the motion of the inducing conductor; when the motion of this is stopped, the induced current ceases, and while the current of the battery remains stationary and continues the same in quantity and intensity, no perceptible effect is exhibited in the adjoining wire. 3. When the inducing current is suddenly moved away from the first wire, a current is observed to pass through the galvanometer in the opposite direction to the former induced current, or in the same direction as the battery current. 4. Let the two wires be placed parallel and near to each other, while the circuit of the battery current is interrupted.
If in this condition the current from the battery be suddenly established through the inducing conductor, an induced current of electricity will pass through the galvanometer in a direction adverse to that of the battery current; or in other words, the effect will be the same as that of the approach of the battery current to the inducing wire, as in case 1. 5.. During the continuance of the battery current of unimpaired strength and intensity, no disturbance of the natural electricity of the adjoining win- is perceived; but at the moment the current of the battery is stopped by a rupture of the circuit, a current passes through the galvanometer in the same direection as thatof the current of the battery.
All these phenomena are in accordance with the hypothesis that during the transmission of a current of electricity through a wire, there is exerted in space on every side an inductive action diminishing with the distance which disturbs the natural electricity of any conducting matter which may be brought within its influence; that while the conductor remains at rest within this influence an abnormal equilibrium exists; and when the conductor is removed from this influence, or when the latter ceases, the usual equilibrium is established by a reverse motion. Since, according to the theory of Ampere, magnetism consists of currents of electricity revolving at right angles to the length of the magnetized bar, it follows, that analogous results ought to be produced by magnetism; and for this purpose, instead of the battery current in the last series of experiments, let there be substituted a magnetized bar held at right angles to the wire connected with the galvanometer. 1. If this bar be suddenly brought down upon the wire perpendicular to its length, the galvanometer will indicate a current in an opposite direction to the hypothetical current in the lower side of the magnet.
If the wire be E. and W. and the magnet be held across it with its N. pole toward the north, the current in the lower side of the magnet will be from the E. to the W., while the induced current will be in an opposite direction, i. e., from W. to E. 2. When the motion of the magnet toward the wire is stopped, the induced current ceases, and no sign of electricity is exhibited so long as the magnet remains at rest. 3. When the magnet is suddenly removed from its proximity to the wire, a current in the opposite direction to that of the first, that is, in the same direction as the current in the lower side of the magnet, is indicated by the galvanometer. 4. When a bar of soft iron is placed across the wire at right angles, and this is suddenly magnetized, either by a galvanic current or by touching its ends to the poles of a horseshoe magnet, a momentary current is produced in the wire in a direction opposite to that of the hypothetical currents of the near side of the magnet. 5. So long as the soft iron bar remains at rest and its magnetism suffers no change, no current is indicated by the galvanometer; but the moment the bar is unmag-netized a reverse current takes place.
The two series of results we have given above are precisely analogous; the latter being merely a case of the former, in which the hypothetical currents of the magnet are substituted for the real current of the battery. - All the effects that we have described are produced with much more intensity, when, instead of using extended wires parallel to each other, we employ wires in the form of spirals, either flat or cylindrical. For example, to obtain an induced current of considerable intensity by means of magnetism, we place on a rod of iron, say four inches long, a spool of long wire covered with silk, which may occupy two inches of the length of the middle of the iron. If the two ends of this rod projecting beyond the spool be suddenly brought into contact with the two poles of a horse-shoe magnet, an induced current will be developed for a moment in the surrounding wire; and when the same rod is suddenly detached from the poles, a current in an opposite direction will take place; and in this way a continued series of alternate currents may be developed by alternately making and severing the contact of the poles of the magnet and the ends of the rod.
A still greater effect may be produced by causing the rod to revolve on an axis at right angles to the middle of its length, before the poles of the magnet, so that each end in rapid succession may be brought in contact first with the E". and then with the S. pole, and so on. - Shortly after the discovery by Faraday of the laws we have stated, Mr. Joseph Saxton of this country, then a temporary resident of London, afterward attached to the United States coast survey, invented (1832) the first machine for giving sparks and shocks in accordance with the arrangement we have just described. Instead of a single bobbin of wire on the middle of a straight bar, he employed two, one on each leg of a bar of soft iron bent into the form of a horse shoe, which were made rapidly to revolve by means of a multiplying wheel before the poles of a magnet. At each half revolution the magnetism of the soft iron was entirely reversed, and in this way a series of currents was induced, of sufficient intensity to decompose water, fire combustible bodies, and powerfully to affect the nervous system.
An instrument maker in London, who was employed to construct these machines, made a slight change in the arrangement, which principally consisted in placing the inducing horse-shoe magnet in a vertical position, and in causing the spools of wire to revolve in a plane parallel to its flat side, instead of parallel to its poles. This change, instead of improving the instrument, produced an opposite effect, since the strength of the induction was much diminished. The author of it, however, succeeded by advertisements, and an actual exhibition of it in France, in attaching his name to the invention, to the exclusion of that of Saxton. It is, however, gratifying to see that in the German works on the subject, and also in the better class of English publications, justice is dorie to the original inventor. The next important series of investigations on this subject, after the original discovery of Faraday, was by Professor Henry of Princeton, now secretary of the Smithsonian institution at Washington. He found that at the beginning and ending of the galvanic current in a long wire, an induced current was produced by an action which has sometimes been called the induction of a current on itself.
To illustrate this, let the circuit of a small battery of a single element be closed by a short wire of about a foot in length, dipping into a cup of mercury. When the circuit is broken, no spark, or but a very feeble one, will be observed; but if we now substitute for the short wire one of say 100 feet in length and of considerable thickness, a vivid spark will be exhibited when the circuit is interrupted. To obtain this result in the most striking manner, we should employ a copper ribbon at least an inch and a half wide and 100 ft. long, well covered with two thicknesses of silk, and rolled into the form of a flat spiral. At the rupture of a battery circuit of which this forms a part, a loud snap and deflagration of the metal will be produced, when with a short wire, the battery remaining the same, scarcely any but a very feeble spark would be observed. By this arrangement several spires of ribbon react on each other, and increase the effect. By coiling a bell wire covered with silk of 600 or 700 ft in length into a spiral ring, the intensity will be so much increased that shocks may be obtained by means of a small galvanic battery of a single element.
If the same wire be coiled into the form of an elongated spiral, and in the centre of this a rod of soft iron be placed, or what is better, a bundle of iron wire, the intensity is still more exalted. In this case the magnetic reaction is combined with that of the current of galvanism, and the two actions being in the same direction conspire to increase the effect. To produce, however, the most powerful inductive apparatus, a bundle of varnished iron wires of about 15 in. in length, and together forming a diameter of about an inch, is surrounded with a coil of thick copper wire well covered with silk of 300 or 400 ft. in length. Around this, but separated from it by a cylinder of glass or pasteboard soaked in shell lac, is coiled a fine copper wire of 4 or 5 m. in length, care being taken that each spire be well insulated from every other. When a current of galvanism from a battery of even a single element is transmitted through the thick copper wire which surrounds the inner core or bundle of iron wire, the latter becomes magnetic; and at the instant the rupture is made in the battery current, a sudden cessation of the magnetism, as well as that of the current itself, induces a-current of great intensity, though of small quantity, in the outer surrounding fine wire.
Each spire of the long wire in this arrangement is subjected to the inductive influence; and the rapidity of motion of the electricity of the wire, were it not for the increased resistance, would be in proportion to the number of spires, or in other words to the length of the wire. This apparatus has received various ingenious improvements, the principle in all cases remaining the same. Dr. Page was the first to invent an apparatus on this plan by which the rapture oi the battery current was rendered automatic; the magnetization of the iron core caused the attraction of. a small magnet attached to one end of a lever which broke the circuit, and the consequent disappearance of the same magnetism allowed the end of the lever to fall into a cup of mercury and thus again complete the circuit. This instrument was much enlarged and improved by Ruhmkorff of Paris, and was still further perfected by an ingenious Amencan artisan, E. S. Ritchie of Boston. The essential desideratum in the construction of this instrument is the perfect insulation of the several spires of wire, so that the intense electricity which is produced may not strike across from one spire to another; and Mr. Ritchie effected this by means of an ingenious process of winding, together with an improved insulation.
An appreciable time is required to overcome the resistance of the wire and to give it a full charge of the current of electricity, and also to magnetize iron; hence in the instrument we have described, when a single battery is employed, the induced current, which gives the intense spark, is that which is produced at the rupture of the battery current. We can however increase the intensity at the beginning of the current, by employing a battery of a number of elements, which, producing electricity of greater intensity, more suddenly establishes the current in the wire, and more rapidly develops the magnetism of the iron. - The improvements that have been made of late in the construction of magneto-electric or induction machines have been so striking as to warrant the hope that we shall eventually derive great advan-tages from the powerful electric currents that can thus be instantaneously generated, producing light, heat, or other effects in any locality whither the conducting wires are led. The accompanying figures illustrate the forms of the most notable machines that have been constructed.
The first is the machine constructed by the compagnie d'aalliance of Paris on the plans of Clarke and Nollet. In Clarke's machine, which is but a slight modification of Saxton's, two soft iron cores, connected by copper and iron bars, revolve rapidly in front of the poles of a powerful horse-shoe magnet. Around these cores is coiled an insulated copper wire, whose ends are so connected with a "commutator " that the alternating currents of electricity circulate always in the same direction through the external circuit. In the machines of the Alliance company the use of the commutator may be omitted if the currents are designed only for the production of light, since in this case the rapid reversals of the current are an advantage. In Siemens's machine, fig. 2, invented in 1854, a peculiar core replaces the double iron armature of Saxton and Clarke; this is a long cylinder around which a wire is wound lengthwise. The cylinder is made to revolve rapidly between the opposite poles of a series of horseshoe magnets; the perpetually reversing magnetism induced in the core by the magnets is carried in successive currents by the insulated wire coil to the commutator, and thence through the external circuit.
In Wilde's machine, fig. 3, the external current from a small Siemens machine, M, is made to pass through a large coil, A B, enclosing a soft iron horse-shoe bar, which is thereby magnetized and acts as a permanent magnet on a second revolving core, F, larger than but similar to that of the smaller apparatus. The latter core collects a much more powerful current than that first produced, and this can be used to generate a third or higher order of current; but with each such increase of current we increase the power required to turn the cores; and though the heat and light are magnificent, yet in no case can we convert into electrical energy more than a certain per cent, of the mechanical energy consumed. In the machine devised by Ladd in 1867, as shown in fig. 4, a principle has been introduced suggested a short time previously by both Siemens and Wheatstone. Two plates of soft iron, B B', are so placed that if they possess the least initial magnetism, as is ordinarily the case, then the rotation of the Siemens armature, a', collects the currents, which are at once led into the coils about B and B', and thus elevate the original magnetism of the plates to a high degree of intensity.
Between the opposite poles of the magnets rotates a second Siemens armature, a, which collects the current for the external circuit. Gramme's machine, invented in 1871, two views of which arc given in figs. 5 and 6, differs materially from its predecessors in that it offers a really continuous current instead of rapid alternations. This is effected by using a circular ring of soft iron, A A, for the core in which the magnetism is to be induced. The coil of wire around the core offers a continuous metallic circuit, divided into numerous sections, the ends of the wires in each so connected with radial metallic arms, R R, that as the ring rotates the induced current flows continuously from these arms to certain fixed metallic pieces in frictional contact with them, and thence to the external circuit. By dividing the current, one half may be led back to the exciting magnets, SON, and be used to increase the power of the machine. The effect produced by these machines increases proportionately to the velocity of rotation up to an unknown limit; it also increases with the number of coils encircling the ring core.
The machines of the Alliance company have been employed for illuminating purposes at some French lighthouses, and those of Wilde have been similarly employed in Great Britain. The Gramme machine has been used for the illumination of the Victoria tower in London, and in the galvano-plastic works of M. Christofle in Paris. - Currents of different Orders. An induced current, by its action on a third conductor, may produce another current, and this another, and so on. If we call the current of the battery a current of the first order, the first induced current is named that of the second order, and so on. The discovery and investigation of tin-principle and properties of currents of the different orders is mainly due to Prof. Henry. On reflecting a little, it will be evident that these currents cannot be produced immediately by placing several straight wires parallel to each other and passing a current of electricity through one of them; in this case the battery current would act on the surrounding wires, and simply produce in each of them an induced current of the second order. To obtain, therefore, currents of the different higher orders, we employ a number of flat spirals, through one of which placed horizontally on a table is transmitted the current from the battery.
Immediately above this, and separated from it by a stratum of air or a plate of glass is a second flat spiral, the ends of which are connected with a third spiral placed at such ft distance as to be entirely out of the influence of the battery current. Placing on the third a fourth (the two being separated as before by a plate of glass), and joining the ends of this with the ends of a fifth spiral, and so on, we shall have a series of successive currents. The current of the first order induced by the battery current induces a secondary current in the second spiral, which passes through the third spiral, and, thus free from the influence of the battery current, induces a current of the third order in the fourth sj.iral. which in turn, passing through the fifth spiral, induces a current of the fourth order in the sixth, and so on. Since each induced current must have a beginning and an ending, the current of the third order must in reality con-sist of two currents in immediate succession and in opposite directions, one produced at the beginning and theother attheending; and for a similar reason a current of the fourth order must consist of four currents in immediate suc-cession and opposite directions.
On this account currents of the higher orders do not definitely deflect the needle of the galvanometer, but merely give it a slight tremor; the impulses in opposite directions follow each other bo rapidly that the inertia of the needle is not overcome in the interval between the two. The existence therefore of currents of different higher orders could not be determined by the galvanometer; they however give intense shocks, and also permanently magnetize steel needles. This latter effect will be understood when it is recollected that, although the series of waves in different directions are the same in quantity, they differ very much in intensity; that at the beginning of the agitation they have much the greatest energy. Hence the currents of different orders exhibit dominant impulses in definite directions. If the direction of the battery current be represented by + , the current of the second order at the beginning of the battery current will be represented by -; the dominant current of the third order + , of the fourth - , and so on; while the series of dominant impulses at the ending of the battery current will be +, +, - , + , - , +. When a circular plate of copper or any other conducting substance is interposed between two spirals placed one above the other, and a current from the battery is transmitted through, for example, the lower one, the induced current at the ending of the current of the battery, in the upper spiral, will affect the galvanometer as if no plate were interposed, while the physiological effect, or the power of giving shocks, will bo entirely neutralized,this remarkable effect is due to an induced current in the interposed conductor which is rendered evident by cutting out a slip of the metal extending from the centre to the circumference of the plate; or in other words by removing one of the radii of which the circular plate maybe conceived to be made up and thus interrupting the circuit, in which an induced current otherwise could be produced; the shocks with the plate thus cut will be nearly as intense as when the plate is entirely removed.
The same effect takes place when instead of the plates a third flat spiral is introduced between the first and second spirals; so long as the ends of this spiral are separated, its presence produces apparently no effect; but if the ends be closed so as to form a perfect circuit which can be traversed by the induced current, the power-of giving shocks is neutralized. But the question naturally arises as to how the current in the plate affects the current in the upper spiral so as to destroy its power of giving shocks. The explanation of this is to be found in the fact, that while the current in the battery tends to induce a current both in the plate and in the spiral above it, each of these currents tends to induce an opposite current in the conductor of the other; we may therefore consider the upper spiral as being under the + influence of the current from the battery, and the - influence of the current of the plate; but as the current in the plate produces an equal inductive action in opposite directions at its beginning and ending, the only effect of it will be to prolong the action of the induced current in the upper spiral, or in other words, to diminish its intensity, and hence to neutralize its power to give shocks without perceptibly diminishing its effects on the galvanometer.
These facts are of importance in the construction of the inductive apparatus previously described; for if two points of two adjacent spires of the long wire happen to be in metallic contact, so as to form a closed circuit, the effect is the same as that of the interposition of a plate or spiral between the battery current and the induced current; the intensity of the latter will be neutralized, and hence the necessity of the perfect insulation of the several spires of the long wvire. For the same reason, if the iron core be enclosed in a hollow cylinder of copper or any other conducting metal so as to separate it from the outer coil of long wire, the great inductive power of the instrument will be neutralized; and it is also on this account that a bundle of varnished iron wires is employed for the core instead of a solid rod of iron. If however the copper cylinder we have just mentioned be interrupted by sawing out a thin slip parallel to its axis, and the solid iron core sawed down from its circumference to its centre, forming a saw-gash in the direction of the radius and in the plane of the axis, the interfering induced currents will be prevented.
We have stated that an induced current of considerable intensity is generated in the conductor of the battery itself at the moment of the rupture of the circuit. This also produces, on the principle of the interposed plate, an adverse action which tends to diminish the energy of the induction apparatus; a defect in the instrument which M. Fesso has remedied by causing the rupture to take place in a cup of mercury the surface of which is covered with 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 produced by suffering the extra current to expend itself on a large sheet of metal called a condenser. The facts we have here stated have been confirmed and extended by Masson, Verdet, 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 currents analogous to those we have described are generated of great intensity, and under favorable 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 stretching two long wires parallel to each other, and transmitting a discharge from a Leyden jar through one of them. He also obtained inductive 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 a part of the circuit, is subject to very complex variations. For example, when the two conductors are near each other, the direction indicated by the magnetization of the needle is opposite to that of the current 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 produced 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 experiments, was enabled to refer them all to the peculiarity 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, decreasing in intensity on account of the resistance of the wire, until the normal electrical equilibrium, 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. direction; 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 revolving around it from W. to E. on the N. 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 magnet; 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 system 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 in 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 contact 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 indicated by the instrument. If the direction of the revolution of the disk be changed, an opposite 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 expertinent be one of soft iron and suddenly excited by a galvanic current, the copper disk previously put in rapid motion will instantly be stopped. The current in the radii of the plate which arc approaching the magnetic pole, being in an opposite direction to those in the magnet, will be repelled; while those in the radii on the other side of the pole, being in the same direction with the current in the magnet, will be attracted; and hence the resultant action of all the induced currents will be to stop the plate. A similar result is produced when a cube of copper of about an inch in diameter is suspended between the poles of a powerful electro-magnet, and caused rapidly to revolve, from the untwisting of a thread by which it is suspended; when the magnet is suddenly excited, the revolution of the cube is instantaneously arrested, ami brought to rest without the least oscillation, as if the momentum and consequently the inertia of the mass were instantly annihilated.
If, in the case of the arrangement of the revolving disk we have mentioned, a rapid motion be communicated to it by a train of wheels in opposition the resistance between the induced currents and the magnet, a considerable exertion will be required to continue the motion; and since, according to the principle of the conservation of force, the muscular power expended must produce some effect, and no change is found in the condition of the metal after the experiment, the conclusion was drawn that the energy exerted was expended in generating heat, the truth of which was established by Foucault. The disk thus made to revolve in opposition to the force of the magnet increases in temperature, and soon becomes sufficiently hot toset tire to an ordinary match. - The Magnetism induced from the Earth and the Sun. The earth being a great magnet, currents of elec-tricitv must he induced in all conducting ma-tenal in which motion takes place at its surface. These currents are, however, of feeble intensity, but their existence may be shown by connecting the ends of a copper wire several hundred yards in length, covered with silk and wound around a wooden cylinder of about 2 ft. in length, with a galvanometer, and by suddenly turning the axis of the former from ahorizontal position into the direction of the dipping needle.
During the downward motion of the N. end of the cylinder, the galvanometer will indicate an induced current in an opposite direction to that of the hypothetical current of the earth, and, when the motion is reversed, an induced current in the same direction as that of the current in the earth. From this result it must be inferred that electrical currents are constantly produced by the magnetism of the earth, since no change in the direction and position of a conducting body can take place without developing the inductive action. Moreover, since the sun has been proved to he a great magnet exerting a powerful action on the earth, the daily rotation of the latter must subject it to an inductive action, similar to that we have described in the revolving plate of copper. There can be no doubt, in the present state of science, that such currents actually do take place, but their direction and intensity have not yet been ascertained. But from the association of the magnetic storms we have previously described with the occurrence of the aurora borealis, and also with that of the maximum number of spots on the sun, we are led to the conclusion that the three classes of phenomena are intimately connected, and that they furnish a subject of cosmical research of perhaps as great interest as any which have ever occupied the attention of the scientific world.
Fig. 1. - Lighthouse Machine.
Fig. 2. - Siemen's Armature.
Fig. 3. - Wilde's Machine.
Fig. 4. - Ladd's Machine.
Fig. 5. - Gramme's Machine.
Fig. 6. - Gramme's Machine.