Electric Light, the light produced by a current of electricity in passing through a resisting medium, as a gas or a small wire. Like solar light, it also produces the combination of chlorine and hydrogen, acts chemically on chloride of silver in the photographic process, and when viewed through a slit in a screen by a prism it presents a spectrum crossed by very bright lines, differing in character with the substance of which the electrodes are formed, and with the gases through which the sparks are passed. Hence the electric light becomes an important means of chemical analysis. With carbon the lines are remarkable for number and brilliancy; with silver they are intensely green; with lead, of a violet tint; and so on, varying with each different substance. Being producible in nitrogen, the electric light is not the effect of combustion, but of the intense heating and volatilization of ponderable matter. We say ponderable matter, because the electric spark cannot pass through a vacuum. A most intense and steady electric light is evolved between two points of coke, forming the poles of a battery, and brought into close proximity.
From its great brilliancy and cheapness this light would seem to be well adapted for illumination, especially for lighthouses, and if introduced into mines it would apparently prove the most powerful illuminating agent, without tending, like other lights, to contaminate the purity of the air. But this light is deficient in the most penetrating ray of the spectrum, the red, and therefore does not produce the effect which was anticipated from it in the way of penetrating fog. It is also so very intense, a large quantity of radiation being given off from a single point, that the eye is dazzled, and vision becomes much more indistinct than with the same quantity of light given off from a lamp with large concentric wicks. It may however be used with much success in various optical experiments. But in this application one of the principal obstacles to be overcome is the continual separation of the charcoal or coke points as these are slowly consumed. Caution is required in experimenting with this light when of great intensity; a single moment of exposure to the radiation from a battery of 000 couples produces violent headache and inflammation of the eyes. For the purpose of illumination it is necessary to have the light constant and uniform.
The galvanic current as well as the distance between the carbon points must not change; and as the carbon slowly wears away, an apparatus is required to move them toward each other. The engraving represents Duboscq's regulator. Both the points move, but with unequal velocities, so as to keep the light at a fixed point, the positive point wearing away twice as fast as the negative. The motion is produced by a coiled spring in the drum placed on the axis x y. This turns two wheels, a and b, which move the racks C and C, and by these the carbon points, one up and the other down. The current passes into the apparatus by the wire E and leaves it by E', without however passing through the rackwork, which is insulated. Before leaving it passes through the coil B, surrounding a piece of soft iron, which is a magnet while the current passes, and constitutes the regulator. When the carbon points approach too near the strength of the magnet is increased, and by means of a lever the rod d is made to turn in such a way as to stop the motion of a series of toothed wheels which are moved by the drum. In this way the motion of the points is arrested until they have wasted enough to diminish the current, and consequently the strength of the magnet, when the stop is relieved, and the motion continues.
ELECTRO-MAGNETISM, that science which treats of the development of magnetism by means of voltaic electricity. In our article on electricity we have given an exposition of the facts of this branch of science, independent of any hypothesis as to the causes of the phenomena; but the present topic cannot be treated satisfactorily without giving some idea of the generalizations which have been invented to explain the phenomena, and to express the laws of their mutual connection and dependence. It must be recollected that science does not consist in an accumulation of facts, but in a knowledge of principles, and it is impossible to arrive at a full comprehension of these principles without expressing them by means of some hypothesis from which logical deductions can be made, which will enable us at any time, independently of mere memory, to say what result will be produced when the conditions are known, or in other words, which will not only present to us the relations of known phenomena, but enable us also to predict the occurrence of those which have not been observed. Without hypotheses of this kind no extended and definite progress can be made in science.
It should, however, always be borne in mind that they are the provisional expressions of the generalizations of our knowledge at a given time, and that we must hold ourselves in readiness to modify or even abandon them when we meet with facts with which they are decidedly inconsistent. Two hypotheses have been proposed to account for the phenomena of electricity: one, that of Du Fay, known by the name of the theory of two fluids, and the other by that of the Franklinian, of one fluid. According to the first, all bodies are pervaded by two elastic fluids, the atoms of each repelling those of the same kind and attracting those of the opposite kind. When the two fluids are together in equal quantities in the same body, they neutralize each other; but when separated by friction or other means, their attractions and repulsions are manifested by various electrical phenomena. The second hypothesis supposes that all the electrical phenomena are produced by the disturbance of one highly elastic fluid, which pervades the earth and all material bodies, and which is able to move with various degrees of facility or not at all through the pores of substances of different kinds of gross matter, which are hence considered either conductors or nonconductors; that the atoms of this fluid repel each other with a force varying inversely as the square of the distance; that the atoms of the same fluid attract the atoms of gross matter, or some ingredients in it, with a force varying in accordance with the same law; and that the atoms of gross matter devoid of electricity tend to repel each other with a force inversely as the square of the distance.
When any body has so much electricity combined with it that the self-repulsion of its atoms is just balanced by the attraction of the same atoms for the unsaturated matter, then the body is said to be in its natural state. So long therefore as all portions of space contain their natural share of the fluid, no electrical phenomena are exhibited; but if, by means of friction, chemical action, heat, or other agencies, together with the interposition of partial or non-conducting substances, the electricity is accumulated in one portion of space, and rendered to the same amount deficient in another, then two classes of phenomena are manifested : 1, those called statical, such as induction and the consequent attraction and repulsion of light bodies, due merely to the accumulation or deficiency of the fluid; 2, dynamical, or those which arise from the transfer of the fluid from the place where it is redundant to that where it is deficient. Franklin's claims to philosophic genius rest particularly upon his conception of this theory of electricity, which, with slight modifications and additions, is still sufficient to express the connection and relation of the multiplicity of facts which have been discovered since his day.
However different the two theories at first sight may appear, their mathematical expression and the deductions from them do not differ, provided that we adopt the modification of the latter proposed by AEpinus and Cavendish, that matter devoid of electricity repels matter; an assumption not inconsistent with the attraction of gravitation and chemical action, since we may refer even these to the same cause. The theory of Du Fay was generally adopted by German and French savants, because it was first discussed by them in a mathematical form. The theory of Franklin was afterward developed mathematically, and with the modifications we have mentioned is, we think, more readily applicable to the facts of the present state of the science than the other. It follows from the theory of Franklin that if electricity be communicated to a sphere of conducting matter, all the fluid will be found at the surface, because each atom repels the other, and the state of equilibrium will be that of an equal distribution at the circumference; the atoms are prevented from flying into space by the non-conducting medium of air in which the globe exists.
In like manner it follows from an application of the law of attraction inversely as the square of the distance, that when a body has less than its natural share of electricity the deficiency must exist at the surface. In charged conductors of elongated forms, the distribution of the fluid will be greater at the two extremities. The phenomena of the Leyden jar are readily deduced, and all the facts connected with it may be anticipated even with numerical exactness, by the application of this theory. When a redundancy of electricity is thrown on one side of a pane of glass, the repulsion acting through the glass will drive off a portion of the natural electricity on the other side, the unsaturated matter of which will attract the free electricity thrown on the first side and thus neutralize its repulsive energy; and in this way an immense amount of electricity can be accumulated in a small space. When the two surfaces are joined by a conducting circuit a discharge takes place with great intensity, because the fluid on the charged side is impelled through the circuit by the repulsion of its own atoms, and because it is attracted to the other side by the unsaturated matter.
If an insulated conductor in the form of a long cylinder with round ends be brought near a charged conductor, but not within striking distance, the natural electricity of the former will be repelled to the further end; the end nearer the charged body will be in a state of deficiency of electricity, or negatively electrified, while the further end will be in a state of redundancy, or positively electrified. Between the two ends there will be a point which will be neutral or in its natural state. The intensity of this action diminishes rapidly with the distance, particularly in the case where the cylindrical conductor is short and the excited body is small; but in the case of atmospherical electricity, in which the charge is on the surface of a large cloud, the inductive action takes place through several miles of intervening space. An attempt was made by AEpinus, Poisson, and others, to apply the same hypothesis to the phenomena of magnetism. Between these and those of electricity a striking analogy was observed. For example, bodies which are dissimilarly electrified attract each other; those which are similarly electrified repel each other. In like manner, two similar poles of a magnet repel, and two dissimilar poles attract each other.
Again, if the north pole of a magnet be brought near an unmagnetized bar of soft iron, the near end exhibits southern polarity and the further end northern polarity, apparently similar to the result of the action in the example we have just given of electrical induction. There is however this remarkable difference, that if we magnetize a piece of hardened steel in the same way by the induction of a powerful magnet, and afterward break the bar into two pieces, each half will exhibit a north and south pole of equal intensity; and if we continue to break each piece into two others, however far the division may be continued, the same result will be produced, namely, a pole at each end of each piece and a neutral point in the middle. From this experiment we infer that the polarity of magnetism results from the development of the magnetic power in each atom of the mass; while if the same experiment be made with an electrical conductor, that is, if it be separated into two parts while under the influence of the excited body, each half will exhibit a charge of only one kind of electricity.
By considering therefore that electrical induction is produced by a bodily transfer of fluid from one end of the conductor to the other, and limiting the disturbance in magnetism to the particles of gross matter, a mathematical expression of most of the phenomena known previous to the discovery of • Oersted was obtained. Still electricity and magnetism were so dissimilar in some particulars that they continued to be studied as distinct branches of science. The fact had long been noticed that discharges of lightning frequently gave polarity to bars of steel, and in some cases reversed the mariner's compass. A series of experiments to imitate these effects were made by Franklin and others by passing shocks through darning needles. The results were unsatisfactory, since the needle was sometimes magnetized in one direction and sometimes in the other, and frequently not at all, without any apparent change in the conditions. Indeed, ordinary electricity was not favorable to the study of the connection of electricity and magnetism, since the phenomena which belong to both are exhibited during the continuance of an electrical current; and in the case of the discharge of a Leyden jar the transfer is so instantaneous that we are only able to study effects which have taken place, without being able to make any observations as to the manner in which they have been produced.
This was the condition of the science up to the winter of 1819-'20, when Prof. Oersted of Copenhagen put a new interrogation to nature by asking what would take place in regard to a magnetic needle when a wire transmitting a current of galvanism was brought near it. He found that when the wire was brought parallel to and near the needle, the latter tended to turn at right angles to the former. This was a new result, unlike any phenomenon before discovered. Previous to this, the connection between electricity and magnetism had been sought in the analogy of the polarity of the two ends of a magnetic bar and the two extremities of a galvanic battery, both of which exhibited polarity. An account of this remarkable discovery was published in all parts of the civilized world, and everywhere excited the interest of men of science. It was repeated in England, France, and Germany. The additional fact was discovered by Arago in France and Davy in England, that the wire joining the two poles of a galvanic battery while the latter was in action was capable of imparting magnetism to iron filings; but the person who seized on the phenomenon with the greatest avidity, and who in the course of a few months developed the whole subject to such an extent as to elevate it to the rank of a new science, was Ampere, of the French academy.
He discovered an additional fact which gave a key to all that had previously been found by his contemporaries, namely, that two parallel wires transmitting currents of electricity in the same direction attract each other, while similar wires transmitting currents moving in opposite directions repel each other. On this fact, combined with the hypothesis that all magnetic action consists in the attraction or repulsion of electrical currents, he founded his celebrated theory of electro-magnetism, which gives in a single sentence a generalization from which all the known phenomena of electro-magnetism as well as ordinary magnetism can be deduced. This theory is based upon one fact and one hypothesis. The fact is this, that currents moving in the same direction attract, and moving in opposite directions repel, each other; the hypothesis is, that the magnetism of a bar of steel consists in currents of electricity revolving at right angles to the length of the bar around each particle of the metal, the resultant of which would be a current around the circumference of the bar.
Let us suppose a number of shillings or cents piled one on the other, and cemented together so as to form a cylindrical column or rod 8 or 10 in. in height; and let us further suppose that on account of some molecular action a current of electricity is perpetually circulating in the circumference of each piece of coin, and that the direction of the currents is the same in the whole series. If we further suppose that the column is standing on end, and that this motion is contrary to that of the sun and contrary to that of the hands of a watch when placed face upward, such arrangement will represent the hypothetical magnet of Ampere, in which the north end, or that which turns to the north, is uppermost, and consequently the south pole undermost. If these postulates be granted, instead of loading the memory with an almost infinite variety of disconnected facts, we shall have at once a generalization from which all the phenomena can be deduced at pleasure in a series of logical corollaries. If this theory be true, or if it be even an approximation to the truth, it will follow that if currents of electricity be transmitted through an arrangement of the kind we have described, the phenomena of ordinary magnetism will be exhibited; and this anticipation will be realized if we coil a piece of copper wire covered with silk into the form of a corkscrew spiral, forming a cylinder 8 or 10 in. long, and if the two projecting ends not included in the spiral be passed backward through the cylinder and made to project from the middle at right angles to the length of the cylinder on opposite sides.
If this cylinder, the several spires of which will represent the pieces of money, be supported horizontally, so as to turn freely as a magnetic needle moves on its pivot, it will take a north and south position when a powerful current of galvanism is transmitted through the wire. Nay, more, another cylinder formed of like spires through which a current of galvanism is passing will act upon the first precisely as a magnet would act upon another magnet. Indeed, so long as the galvanic current is passing through this helix or spiral, it exhibits all the properties of an ordinary magnet; but they immediately disappear when the current is interrupted. To deduce from his theory the almost infinite number of facts which it involves, Ampere first considered the action of currents on currents. Starting with the hypothesis that the attraction and repulsion were inversely as the square of the distance between the elementary parts or smallest portion of the currents, he deduced mathematically the consequence that the force of a current of considerable length acting on a single element of a current would vary inversely as the simple distance; and this he was enabled to verify by experiment by suspending a bent wire through which a current was passing so as to be free to oscillate under the influence of a single element, which was ingeniously effected by doubling a piece of covered wire in the middle of its length, thus >. When a current was passed through this double wire, the portion of it which went to the point of bending and that which came from it neutralized each other, and the residuary effect therefore was that of a single point, which gave a result exactly in conformity to the deduction from the theory.
After proving experimentally this fundamental principle, he was enabled by mathematical reasoning, principally of a simple character, to deduce the resultant action of the most complex forms of conductors upon conductors. Among many others, the following important deductions immediately flow from the premises assumed. If a current of electricity be sent in the direction from A to B through a straight conductor, A B, of indefinite length, placed for example horizontally, and a current be sent downward through a terminated conductor, C D, perpendicular to the former, the latter conductor will be impelled parallel to itself along the length of the horizontal conductor. This effect will be due to the fact that on the right side of the short conductor the elements of the two currents are moving in opposite directions; the current in the short wire is approaching the point F, while the current in the horizontal wire is moving from this same point, and hence on this side repulsion will take place; while on the left-hand side of the short wire the two currents are moving toward the same point, and therefore attraction will be exhibited; and under the influence of these two forces, the short conductor will move parallel to itself from right to left along the horizontal conductor.
If the direction of the current in either of the two conductors be reversed, the motion of the short conductor will also be reversed. If, instead of the short conductor, one in the form of a ring be freely suspended over the long conductor, with the plane of the latter across the former, the current passed through this will ascend on one side of the ring and descend on the other. Therefore, the one side will tend to move to the right and the other to the left, and the resultant action will be to bring the plane of the ring parallel to the horizontal current; in which case the current in the lower part of the ring will be moving in the same direction as the current in the long wire. Now, since, according to the theory of Ampere, magnetism depends upon currents of electricity, it follows that the magnetism of the earth results from currents of electricity moving continually from east to west. Hence, if a conductor be bent into the form of a ring or hoop, and freely suspended, it will arrange itself east and west. To insure the success of this experiment, the hoop should be formed of a long wire covered with silk and coiled into the form of a ring so as to multiply the actions.
Such a ring may be considered as one of the disks represented by the shillings in the hypothetical magnet; and since each disk making up the whole length of the rod would be similarly acted upon by the currents of the earth, the axis of the rod would assume a north and south direction if left free to move, thus affording an explanation of the fact, so long considered an ultimate one, of the directive property of the needle. Let us return again to the action of the long horizontal conductor on the short perpendicular one. If the former be bent into a horizontal circle, then it is evident, from the reasons we have before given, that the short conductor, moving perpetually round it parallel to itself or retaining its perpendicular position, will describe a circle. This may be shown experimentally by bending a piece of wire into the form of a n, and supporting it vertically on the point of a perpendicular wire which fits lightly into a socket on the under side of the middle of the arch. (See fig. 1.) If the two ends of this bent wire dip into a circular basin of mercury through the middle of which, surrounded by a glass tube, the supporting pointed wire passes, and if a powerful current of galvanism be sent down through this wire, it will descend through the legs of the n into the mercury; and if at the same time a powerful current be passed through a ring or hoop conductor placed horizontally around it, a rapid rotation of the bent wire will take place.
Now since magnetism, according to the theory which we have adopted, consists in currents of electricity revolving at right angles to the magnet, if a magnetized bar be introduced within the branches of the bent conductor, a similar rotary motion will be produced. This fact was first shown experimentally by Prof. Faraday. It is, however, a logical consequence of the theory of Ampere, and might have been deduced from it. A beautiful illustration of the phenomena of terrestrial magnetism was first exhibited by Prof. Barlow of Woolwich, England. He prepared a wooden globe, into the surface of which a long conductor was buried in a spiral groove extending with many turns from pole to pole. This globe was afterward covered with paper, on which were drawn the continents and oceans. When a small dipping needle was placed over this apparatus and a current of galvanism sent through the concealed conductor, the needle assumed a direction similar to that which would be due to an analogous position on the earth's surface; and since, in all cases, the needle tends to arrange itself at right angles with the direction of the current, by a proper adjustment of the conducting wire in the groove the variation of the needle at every point of the earth's surface could be accurately represented.
The explanation of all the phenomena of ordinary magnetism readily flows from the same principles. We have stated that if a magnet be broken in two, each half becomes a separate magnet, exhibiting north and south polarity. If the hypothetical magnet which we have illustrated by a pile of shillings be broken in the same way, each part will become a separate magnet, and the two ends of the two parts which were previously in contact will attract each other, because the currents will be revolving in the same direction; but if we turn the other end of one magnet to the same end of the other, repulsion will ensue, because the currents are revolving in different directions. By a little reflection it will not be difficult to explain or to anticipate the action of the two magnets on each other under any assumed con-dition. In adopting this hypothesis, it is not necessary to contend for the actual existence of electrical currents in the magnet or even in the earth. It is sufficient to assert that all the peculiarities of the known phenomena of magnetism are precisely such as would result from an assemblage of currents such as Ampere has supposed to exist.
It is probable that in the phenomena of magnetism a molecular distribution of the fluid takes place which is analogous •to that in a wire transmitting a current. Indeed, we know that at the moment of magnetizing a bar of iron, a molecular change is produced in the metal of sufficient intensity to cause a sensible sound; a fact which was first noticed by Prof. Charles G. Page of Washington. - It is an interesting fact in the history of science, that discoveries in one branch serve to throw light on other branches, and in many cases to furnish instruments by which actions too delicate to be appreciated by ordinary means may be exhibited and measured. Soon after the discovery of Oersted, Prof. Schweig-ger of Germany covered a long wire with silk and coiled it into the form of a rectangle, within which he suspended by means of a fibre of silk a magnetic needle. When a very feeble current of electricity was sent through this conductor, each turn of the wire acted on the needle to turn it at right angles to its own direction; and in this way an instrument called the galvanometer was produced, by which the most feeble galvanic action in the form of a current is exhibited. - It has been before stated that Arago and Davy discovered that the conducting wire through which a galvanic current is flowing is capable of inducing magnetism in iron filings.
They also showed that a discharge of ordinary electricity, when made above or below a sewing needle, gave it definite polarity; and in this way the reason of the failure of Franklin and others, who had attempted to magnetize steel wire by ordinary electricity, was explained. In these attempts the electricity was sent through the length of the needle, instead of across or around it, as the theory of Ampere would indicate. Mr. Sturgeon, in England, was the first to construct an electro-magnet, which consisted of a piece of iron wire bent in the form of a horseshoe, insulated with a coating of sealing wax, over which was loosely coiled a few feet of copper wire. (See fig. 2.) When the current was sent through the latter, the iron became magnetic, and exhibited in proportion to its size a very intense action. The first person, however, who exhibited the great power of the galvanic current in producing magnetic effects was Prof. Henry of Washington. He found that by surrounding a large bar of iron bent into the form of a horseshoe with a number of coils of wire, so connected with the battery of a single element that the current in each wire would move in the same direction, a magnetic power of astonishing magnitude could be produced with a comparatively small galvanic apparatus.
A magnet constructed on this principle, now in the cabinet of the college of New Jersey, at Princeton, will readily support 3,500 lbs. In order, however, to produce a maximum effect of this kind, it is necessary that great care be taken in the insulation of the wires, that there be no cutting across from one wire to another; and for this purpose the ends of two wires intended to be soldered to the positive pole of the battery should project together, while the two ends intended to be united to the negative pole of the battery should also be associated. If the magnetic power of the iron is to be developed by means of a compound battery, then a single long wire may be employed instead of a number of short ones. The power of the electro-magnet depends on the following conditions: on the energy of the current, the dimensions and form of the iron, the nature of the iron - the softer the better - the perfect insulation of the wire, and the proper adjustment of the length of the wire to the intensity of the battery. By means of an electro-magnet of the kind we have mentioned, the instantaneous development of an immense magnetic power is produced, by which discoveries have been made in regard to this mysterious agent, of the highest interest.
Prof. Faraday showed by the application of this instrument that magnetic property is possessed by all bodies, either in the direction of the greatest length of the body, the form in which it is ordinarily developed, or at right angles to this length. He found, for example, that when different substances are made into bars and suspended by means of a fibre of silk between the poles of a powerful electro-magnet, they arrange themselves with the longer axis in the directum of the pole or with the shorter axis in the same direction. Bodies of the former class are called magnetic; those of the latter class are called diamagnetic. This property is even possessed by gases. (See Diamagnetism.) An electro-magnet even of immense power can be magnetized, unmagnet-ized, and remagnetized in an opposite direction, by instantaneous changes in the direction of the current of the galvanic battery. The large magnet we have mentioned as at Princeton can be loaded with several hundred pounds, and while in this condition may be so rapidly unmagnetized and remagnetized with the opposite polarity that the weight has not time to commence its fall before it is arrested by the attraction of the reverse magnetism.
This sudden change of polarity affords a means of producing mechanical movements of considerable power through the agency of electro-magnetism, which have by some been considered as a rival to steam power. The first machine moved by this power was invented by Prof. Henry immediately after his experiments in developing electro-magnetism, and an account of it was published in the "American Journal of Science " in 1831. It consisted of an oscillating iron beam surrounded by a conductor of insulated copper wire. A current of electricity was sent through this in one direction, which caused one end to be repelled upward and the other attracted downward by two stationary magnets. The downward motion of the one end of the beam near its lowest point brought the conducting wires in contact with the opposite poles of the battery, which produced the reverse motion, and so on continually. In a subsequent arrangement, the velocity of motion was regulated by a fly-wheel, and electro-magnets substituted for the permanent magnets at first used.
Prof. Ritchie of the London university afterward produced a rapid rotatory motion between the two legs of an inverted horseshoe magnet in a piece of iron around which a current of electricity was made to revolve, and the magnetism reversed at each semi-revolution. Modifications of these two forms of the apparatus have since been made in almost every part of the civilized world. A large electro-magnetic engine was constructed by Prof. Jacobi of St. Petersburg, by which a small boat was propelled at the rate of several miles an hour. But the largest and most efficient engme of this kind was constructed by Prof. Page of Washington, at the expense of the government. It exhibited sufficient power to propel with considerable velocity a railway car, and afforded the best means which has yet been presented of estimating the comparative cost of the application of electricity as a motive power. From all the experiments which have been made, it appears that though the electro-magnetic power can be applied with less loss in the way of effective work than heat by means of the steam engine, yet the cost of the material by which it is generated is so great that it cannot be economically employed.
According to the experiments of Despretz, one pound of coal in burning develops as much heat as six pounds of zinc; consequently, under the same conditions, six times as much power is developed from the [ burning of an equal weight of the former as from that of the latter. Now the power of the steam engine is produced by the burning of coal in air, while that of the electro-magnetic engine is developed from the oxidation or burning of zinc in acid; and since coal and air are the simple products of nature, while zinc and acid require artificial preparation at the expense of power, it must be evident that electro-magnetism cannot compete with steam, although it may be applied in some cases where the expense of materials is of secondary consideration. Electro-magnetism, for example, is applied with much success in calling into operation power at a distance, as in the case of the electro-magnetic telegraph, in giving simultaneous motion to the hands of clocks situated in different parts of a city, in measuring very minute portions of time, and in bringing into action the power necessary to ring alarm bells. - For an exposition of the scientific principles of electro-magnetism, see De Montferrand's work on the subject, translated from the French by Prof. Cumming of Cambridge, England; and for various ingenious modifications of apparatus, and interesting facts of the science, Dr. Page's papers in the "American Journal of Science and Art." (See Galvanism, Magnetism, and Magneto-Electricity.)
Fig. 1. - Rotation of a Vertical Current.
Fig. 2. - Horseshoe Electro-magnet.