Part 35

A very powerful horseshoe magnet, formed of twelve steel plates in close approximation, is placed in a horizontal position. An armature, consisting of a bar of the purest soft iron, has each of its ends bent at right angles, so that the faces of those ends may be brought directly opposite and close to the poles of the magnet when required. Ten copper wires—covered with silk, in order to insulate them—are wound round one half of the bar of soft iron, as a compound helix: ten other wires, also insulated, are wound round the other half of the bar. The extremities of the first set of wires are in metallic connexion with a circular disc, which dips into a cup of mercury, while the ends of the other ten wires in the opposite direction are soldered to a projecting screw-piece, which carries a slip of copper with two opposite points. The steel magnet is stationary; but when the armature, together with its appendages, is made to rotate vertically, the edge of the disc always remains immersed in the mercury, while the points of the copper slip alternately dip in it and rise above it. By the ordinary laws of induction, the armature becomes a temporary magnet while its bent ends are opposite the poles of the steel magnet, and ceases to be magnetic when they are at right angles to them. It imparts its temporary magnetism to the helices which concentrate it; and, while one set conveys a current to the disc, the other set conducts the opposite current to the copper slip. As the edge of the revolving disc is always immersed in the mercury, one set of wires is constantly maintained in contact with it, and the circuit is only completed when a point of the copper slip dips in the mercury also; but the circuit is broken the moment that point rises above it. Thus, by the rotation of the armature, the circuit is alternately broken and renewed; and as it is only at these moments that electric action is manifested, a brilliant spark takes place every time the copper point leaves the surface of the mercury. Platinum wire is ignited, shocks smart enough to be disagreeable are given, and water is decomposed with astonishing rapidity, by the same means; which proves, beyond a doubt, the identity of the magnetic and electric agencies, and places Dr. Faraday, whose experiments established the principle, in the first rank of experimental philosophers.

A magneto-electric machine has been recently constructed by Mr. Henley, of enormous power. It consists of two permanent magnets, from which the induction is obtained; each of these is formed of thirty horseshoe steel magnets, two feet and a half long, and from four to five inches broad, and each is surrounded by a coil of wire six miles long, coated with silk to insulate the coils. A shock from these wires would be instantaneous death. This apparatus will ultimately be employed to send a stream of electricity through long submarine and subterraneous wires; but a Volta-electric machine has hitherto been used, in which the electricity is generated by a galvanic battery instead of magnets.

Induction, or the effect of the spiral wires in augmenting the power of Voltaic electricity, is admirably illustrated in the Atlantic telegraph.

Wires that are to convey electricity under ground, or through water, must be defended from injury and insulated to prevent the lateral escape of the electricity. For that purpose the cable that is laid at the bottom of the Atlantic, from near Valentia in Ireland to Trinity Bay in Newfoundland, is formed of seven fine copper wires which convey the electricity, bound together by a coating of gutta percha, over which there are layers of cloth dipped in pitch, and then the whole is covered by steel wires twisted together in strands and twined round in long close spirals, forming a cord or cable not more than an inch and a quarter in diameter, and 2100 miles long. The use of the gutta percha is to insulate the wires; the other coatings are merely for protection.

The Voltaic battery which generates the electricity consists of 40 cells, the plates of which are alternately of zinc and platinized silver, each about nine inches square, the exciting fluid being dilute sulphuric acid. Although the force developed by this battery is so great that a piece of iron three inches long and three eighths of an inch in diameter placed in contact with the poles may be consumed in a few minutes, it is absolutely incapable of sending a current of electricity through wires 2500 miles long, on account of their resistance, without the aid of Dr. Faraday’s inductive action. It is only the primary agent for inducing a current of sufficient strength.

To accomplish that, many thousand yards of fine copper wire coated with silk are wound round a hollow soft iron cylinder; the whole is then coated by gutta percha, and the end of the wire is joined to the wires in the cable so as to form a continuous line from Valentia to Newfoundland. A second copper wire, shorter but thicker than the preceding, and also insulated by a coating of silk, is wound round the cylinder above the gutta percha: when the ends of this thick wire are brought into contact with the poles of the battery, currents of electricity flow through it, between pole and pole, and in their passage temporarily convert the hollow iron cylinder into a powerful electro-magnet, which by its reaction induces a current of electricity in the fine wire of sufficient power to cross the Atlantic. The efficiency of the electric telegraph depends upon the power we possess of breaking and renewing the current at pleasure, since by that means distinct and successive signals are made from station to station. In the Atlantic cable positive and negative electricity are transmitted alternately; the electricity is sent to America from alternate poles, and the current returns again through the water, which completes the circuit.

The passage of electricity through a cable or telegraphic wire in air is sensibly instantaneous; that through a cable, whether extended in water or under ground, requires time on account of lateral induction through the gutta percha; for the electricity, in passing through the wires, induces the opposite electricity on the surface of the water or moist earth in contact with the cable, and in that respect it is precisely like a Leyden jar, the gutta percha representing the glass. As the power of induction is proportional to the tension of the electricity, and as the tension is continually diminished by the resistance of the wires, the induction is continually diminished and requires a longer time. Electricity took two seconds to pass through a cable 768 miles long, laid under ground from London to Manchester, and back again twice; while in air it was all but instantaneous, because the inductive capacity of air is very much less than that of water or moist earth. In the experiment with the cable under ground it took two-thirds of a second to overcome the resistance of the wires, and then the velocity of the electricity was 1000 miles in a second, and it was the same whatever the intensity of the electricity.

It has already been mentioned that the efficiency of the electric telegraph depends upon the breaking and renewing the current of electricity by means of which a succession of waves of electricity are sent through the conducting wires. Now it has been ascertained that three electric waves may travel simultaneously through the wires of the Atlantic telegraph with sufficient intervals between them to record the indications they are intended to convey; that is, three signals can be intelligibly and practically transmitted in two seconds.

The original design, structure, and difficulty of depositing the cable are only equalled by the talent and perseverance with which it has been done. The 5th of August, 1858, will be memorable for the accomplishment of the boldest enterprise that ever was undertaken by man, and which is only the beginning of a vast submarine communication that will ultimately encircle the globe. It has been granted to British genius thus to annihilate time and space, in order to connect all mankind into one great family for their moral and religious advancement; and, whatever may be the fate of the British Islands in the course of ages, to their energetic race the glory will remain of having been the chief instruments in the hands of Providence for the civilization of the world—a civilization which will extend with the development of their numerous colonies into great independent Christian states, like those of the Union in North America. The thunderbolt snatched from heaven by Franklin now passes through the depths of the Atlantic as a messenger of peace between the kindred nations.[16]

In telegraphs on land the intensity of the battery or magnets is increased by induction on the same principle. It is by intensity that the electric current is enabled to pass through the wires, and that is augmented by increasing the number of coils round the cylinder: however, it is only advantageous when the distance between the stations is great, for then the resistance in the additional coils bears a small proportion to the resistance offered by very long wires, but a very great proportion to that opposed in very short ones. The nice adjustment for each case has been determined by the experiments of eminent electricians, and all the arrangements have been brought to great perfection in this wonderful triumph of science, which is due to Volta, who called into existence the fiery stream, and to Faraday, who has given it the energy of the lightning.

When the length of the wire in the helices of an electro-magnet is very great, it offers increasing resistance to the passage of the electricity, so that the cessation of magnetism is not instantaneous when the contact with the Voltaic battery is broken. To remedy that defect an instrument has been invented which instantaneously deprives the apparatus of the remaining electricity. A great length of fine wire gives the severest shocks, while a shorter and thicker wire gives the longest sparks and ignites the greatest quantity of platinum wire.

Ruhmkorff’s electro-inductive apparatus has either been improved, or new machines constructed, by Messrs. Grove, Gassiot, and Joule, of intense energy. Indeed, so great is the energy of electro-induction, that hopes were entertained of its superseding steam as a motive power. For the current of electricity from an electro-magnet can be made to flow in opposite directions, so as to produce alternate attractions and repulsions, and consequently a continued motion, which might be applied as a motive force to machinery. However, Mr. Joule has proved that the power developed by one pound of coal in combustion is to that produced by one pound of zinc consumed in Mr. Grove’s powerful electro-magnetic apparatus as nine to one, so that, even if zinc were as cheap as coal, and a Voltaic battery as easily kept in order as an engine-furnace, electricity will not supersede steam as a motive power.

A current of electricity traversing a conductor gives out a quantity of heat determined by fixed laws, the amount of which is invariable as long as the machine to which it is applied remains at rest; but the instant the machine is set in motion a reaction takes place in the intensity of the current, causing a diminution in the quantity of heat, because the heat that disappears is converted into the mechanical force exerted by the engine.

Mr. Joule’s experiments prove that, whenever a current of electricity is generated by a magneto-electric machine, the quantity of heat evolved by that current has a constant relation to the power required to work the machine; and on the other hand, whenever an engine is worked by a Voltaic battery, that the power developed is at the expense of the calorific force of the battery for a given consumption of zinc, the mechanical effect produced having a fixed relation to the heat lost in the Voltaic current. The obvious conclusion Mr. Joule draws from these experiments is, that heat and mechanical power are convertible into one another, and it becomes evident, therefore, that heat is either the vis viva or living force of ponderable particles, or a state of attraction and repulsion capable of generating vis viva (N. 222).

SECTION XXXIII.

Electricity produced by Rotation—Direction of the Currents—Electricity from the Rotation of a Magnet—M. Arago’s Experiment explained—Rotation of a Plate of Iron between the Poles of a Magnet—Relation of Substances to Magnets of three Kinds—Thermo-Electricity.

M. ARAGO discovered a source of magnetism in rotatory motion. If a circular plate of copper be made to revolve immediately above or below a magnetic needle or magnet, suspended in such a manner that it may rotate in a plane parallel to that of the copper plate, the magnet tends to follow the circumvolution of the plate; or, if the magnet revolves, the plate tends to follow its motion; so powerful is the effect, that magnets and plates of many pounds weight have been carried round. This is quite independent of the motion of the air, since it is the same when a pane of glass is interposed between the magnet and the copper. When the magnet and the plate are at rest, not the smallest effect, attractive, repulsive, or of any kind, can be perceived between them. In describing this phenomenon, M. Arago states that it takes place not only with metals, but with all substances, solids, liquids, and even gases, although the intensity depends upon the kind of substance in motion. Experiments made by Dr. Faraday explain this singular action. He found that, if a piece of metal or a metallic wire forming a circuit of any form be moved from right to left across the lines of force proceeding from the pole of a bar magnet, these lines of force induce a current of electricity flowing in one direction; and when the motion of the metal or wire is reversed, the direction of the current is reversed also: the rotation of the magnet about its axis has no effect on these results, and no current is induced when the metal or wire is at rest. A plate of copper, twelve inches in diameter and one fifth of an inch thick, was placed between the poles of a powerful horseshoe magnet, consequently crossing the magnetic lines of force at right angles, and connected at certain points with a galvanometer by copper wires. When the plate was at rest no effect was produced; but as soon as the plate was made to revolve rapidly the galvanometer needle was deflected sometimes as much as 90°, and by a uniform rotation the deflection was constantly maintained at 45°. When the motion of the copper plate was reversed, the needle was deflected in the contrary direction, and thus a permanent current of electricity was evolved by an ordinary magnet. The intensity of the electricity collected by the wires, and conveyed by them to the galvanometer, varied with the position of the plate relatively to the poles of the magnet.

The motion of the electricity in the copper plate may be conceived by considering that, merely by moving a single wire, like the spoke of a wheel, before a magnetic pole, a current of electricity tends to flow through it from one end to the other. Hence, if a wheel be constructed of a great many such spokes, and revolved near the pole of a magnet in the manner of the copper disc, each radius or spoke will tend to have a current produced in it as it passes the pole. Now, as the circular plate is nothing more than an infinite number of radii or spokes in contact, the currents will flow in the direction of the radii if a channel be open for their return; and, in a continuous plate, that channel is afforded by the lateral portions on each side of the particular radius close to the magnetic pole. This hypothesis is confirmed by observation; for the currents of positive electricity set from the centre to the circumference, and the negative from the circumference to the centre, and _vice versâ_, according to the position of the magnetic poles and the direction of rotation; so that a collecting wire at the centre of the copper plate conveys positive electricity to the galvanometer in one case, and negative in another; that collected by a conducting wire in contact with the circumference of the plate is always the opposite of the electricity conveyed from the centre. It is evident that, when the plate and magnet are both at rest, no effect takes place, since the electric currents which cause the deflection of the galvanometer are only induced by motion across the magnetic lines of force. When the plate is placed edgewise so as to be parallel to these lines of force, no revolution of it with the most powerful magnet produces the slightest signs of a current at the galvanometer. The same phenomena may be produced by electro-magnets. The effects are similar when the magnet rotates and the plate remains at rest. When the magnet revolves uniformly about its own axis, electricity of the same kind is collected at its poles, and the opposite electricity at its equator.

The phenomena which take place in M. Arago’s experiments may be explained on this principle. When both the copper plate and the magnet are revolving, the action of the induced electric current tends continually to diminish their relative motion, and to bring the moving bodies into a state of relative rest; so that, if one be made to revolve by an extraneous force, the other will tend to revolve about it in the same direction, and with the same velocity.

When a plate of iron, or of any substance capable of being made either a temporary or permanent magnet, revolves between the poles of a magnet, it is found that dissimilar poles on opposite sides of the plate neutralize each other’s effects, so that no electricity is evolved; while similar poles on each side of the revolving plate increase the quantity of electricity, and a single pole end-on is sufficient. But when copper, and substances not sensible to ordinary magnetic impressions, revolve, similar poles on opposite sides of the plate neutralize each other; dissimilar poles on each side exalt the action; and a single pole at the edge of the revolving plate, or end-on, does nothing. This forms a test for distinguishing the ordinary magnetic force from that produced by rotation. If unlike poles, that is, a north and south pole, produce more effect than one pole, the force will be due to electric currents; if similar poles produce more effect than one, then the power is not electric. These investigations show that there are really very few bodies magnetic in the manner of iron. Dr. Faraday therefore arranges substances in three classes, with regard to their relation to magnets:—those affected by the magnet when at rest, like iron, steel, and nickel, which possess ordinary magnetic properties; those affected when in motion, in which electric currents are evolved by the inductive force of the magnet, such as copper; and, lastly, those which are perfectly indifferent to the magnet, whether at rest or in motion.

It has already been observed that three bodies are requisite to form a galvanic circuit, one of which must be fluid. But, in 1822, Professor Seebeck, of Berlin, discovered that electric currents may be produced by the partial application of heat to a circuit formed of two solid conductors. For example, when a semicircle of bismuth, joined to a semicircle of antimony, so as to form a ring, is heated at one of the junctions by a lamp, a current of electricity flows through the circuit from the antimony to the bismuth; and such thermo-electric currents produce all the electro-magnetic effects. A compass needle, placed either within or without the circuit, and at a small distance from it, is deflected from its natural position, in a direction corresponding to the way in which the electricity is flowing. If such a ring be suspended so as to move easily in any direction, it will obey the action of a magnet brought near it, and may even be made to revolve. According to the researches of M. Seebeck, the same substance, unequally heated, exhibits electrical currents; and M. Nobili observed, that in all metals, except zinc, iron, and antimony, the electricity flows from the hot part towards that which is cold. That philosopher attributes terrestrial magnetism to a difference in the action of heat on the various substances of which the crust of the earth is composed; and, in confirmation of his views, he has produced electrical currents by the contact of two pieces of moist clay, of which one was hotter than the other.

M. Becquerel constructed a thermo-electric battery of one kind of metal, by which he has determined the relation between the heat employed and the intensity of the resulting electricity. He found that, in most metals, the intensity of the current increases with the heat to a certain limit, but that this law extends much farther in metals that are difficult to fuse, and which do not rust. The experiments of Professor Cumming show that the mutual action of a magnet and a thermo-electric current is subject to the same laws as those of magnets and galvanic currents; consequently all the phenomena of repulsion, attraction, and rotation may be exhibited by a thermo-electric current. M. Botto, of Turin, has decomposed water and some solutions by thermo-electricity; and the Cav. Antinori of Florence succeeded in obtaining a brilliant spark with the aid of an electro-dynamic coil.

The principle of thermo-electricity has been employed by MM. Nobili and Melloni for measuring extremely minute quantities of heat in their experiments on the instantaneous transmission of radiant heat. The thermo-multiplier, which they constructed for that purpose, consists of a series of alternate bars, or rather fine wires of bismuth and antimony, placed side by side, and the extremities alternately soldered together. When heat is applied to one end of this apparatus, the other remaining at its natural temperature, currents of electricity flow through each pair of bars, which are conveyed by wires to a delicate galvanometer, the needle of which points out the intensity of the electricity conveyed, and consequently that of the heat employed. This instrument is so delicate that the comparative warmth of different insects has been ascertained by means of it.

The conservation of force is strictly maintained throughout the whole science and different forms of electricity. In static electricity the positive and negative forces exactly balance one another; they are always simultaneous, and related often by curved lines of force; there is no defect or surplus, and the existence of one kind without the other is utterly impossible—it is absolutely a dual force. The very same may be said of electric currents, whether produced by the Voltaic battery or in any other way—the current in one part of the circuit is absolutely the same in amount and dual character as the other; and in the insulated Voltaic battery, where the sustaining power is internal, not the slightest development of the forces of either of these can occur till the circuit is completed or induction allowed at the extremities; for if when there is no circuit the induction be prevented, not merely no current, but no quantity of electricity at the poles ready to produce a current, can be evolved in the slightest degree.[17]

SECTION XXXIV.