ELECTRICITY.

About sixty years ago a popular book was published having for its theme the advantages which would flow from the general diffusion of scientific knowledge. Great prominence was, of course, given to the utility of science in its direct application to useful arts, and many scientific inventions conducing to the general well-being of society were duly enumerated. Under the head of electricity, however, the writer of that book mentioned but few cases in which this mysterious agent aided in the accomplishment of any useful end. The meagre list he gives of the instances in which he says “even electricity and galvanism might be rendered subservient to the operations of art,” comprises only orreries and models of cornmills and pumps turned by electricity, the designed splitting of a stone by lightning, and the suggestion of Davy that the upper sheathing of ships should be fastened with copper instead of iron nails, with a hint that the same principle might be extended in its application. At the present day the applications of electricity are so numerous and important, that even a brief account of them would more than fill the present volume. Electricity is the moving power of the most remarkable and distinguishing invention of the age—the telegraph; it is the energy employed for ingeniously measuring small intervals of time in chronoscopes, for controlling time-pieces, 482and for firing mines and torpedoes; it is the handmaid of art in electro-plating and in the reproduction of engraved plates, blocks, letterpress, and metal work; it is the familiar spirit invoked by the chemist to effect marvellous transformations, combinations, and decompositions; it is a therapeutic agent of the greatest value in the hands of the skilful physician. Such an extension of the practical applications of electricity as we have indicated implies a corresponding development of the science itself; and, indeed, the history of electricity during the present century is a continuous record of brilliant discoveries made by men of rare and commanding genius—such as Davy, Ampère, and Faraday. To give a complete account of these discoveries would be to write a treatise on the science; and although the subject is extremely attractive, we must pass over many discoveries which have a high scientific interest, and present to the reader so much of this recently developed science as will enable him to comprehend the principles of a few of its more striking applications.

The science of electricity presents some features which mark it with special characters as distinguished from other branches of knowledge. In mechanics and pneumatics and acoustics we have little difficulty in picturing in our minds the nature of the actions which are concerned in the phenomena. We can also extend ideas derived from ordinary experience to embrace the more recondite operations to which heat and light may be due, and, by conceptions of vibrating particles and undulatory ether, obtain a mental grasp of these subtile agents. But with regard to electricity no such conceptions have yet been framed—no hypothesis has yet been advanced which satisfactorily explains the inner nature of electrical action, or gives us a mental picture of any pulsations, rotations, or other motions of particles, material or ethereal, that may represent all the phenomena. Incapable as we are of framing a distinct conception of the real nature of electricity, there are few natural agents with whose ways we are so well acquainted as electricity. The laws of its action are as well known as those of gravitation, and they are far better known than those which govern chemical phenomena or the still more complex processes of organic life.

Definite as are the laws of electricity, there is no branch of natural or physical science on which the ideas of people in general are so vague. Spectators of the effects of this wonderful energy—as seen violently and destructively in the thunderstorm, and silently and harmlessly in the Aurora—knowing vaguely something of its powers in traversing the densest materials, in giving convulsive shocks, and in affecting substances of all kinds—the multitude regard electricity with a certain awe, and are always ready to attribute to its agency any effect which appears mysterious or inexplicable. The popular ignorance on this subject is largely taken advantage of by impostors and charlatans of every kind. Electric and magnetic nostrums of every form, electric elixirs, galvanic hair-washes, magnetized flannels, polarized tooth-brushes, and voltaic nightcaps appear to find a ready sale, which speaks unmistakably of the less than half-knowledge which is possessed by the public concerning even the elements of electrical science.

Electricity has also a special position with regard to its intimate connection with almost every other form of natural energy. Evolved by mechanical actions, by heat, by movements of magnets, and by chemical actions, it is capable in its turn of reproducing any of these. It plays an important, but as yet an undefined, part in the physiological actions constantly going on in the organized body, and is, in fact, all-pervading in its influence over 483all matter, organic and inorganic—a secret power strangely but universally concerned in all the operations of nature. We are compelled to regard electricity not as a kind of force acting upon otherwise inert matter, but rather as an affection or condition of which every kind of matter is capable, although we are still unable to form a conjecture of the precise nature of the action.

We have now to address ourselves to the task of unfolding so much of the science as will enable the reader to understand the leading principles of such important applications as electro-plating, illumination, and the telegraph; and this will necessarily include an account of the grand discovery of the identity, or at least intimate connection, of magnetism and electricity.

ELEMENTARY PHENOMENA OF MAGNETISM AND ELECTRICITY.

The distinctive property of a magnet is, as everybody knows, to attract pieces of iron, and this property having been observed by the ancients in a certain ore of iron which was found near the city of Magnesia, in Asia Minor, the property itself came to be called Magnetism. A bar of steel, if rubbed with the natural magnet or loadstone, acquires the same property, and if the bar be suspended horizontally or poised on a pivot, it will settle only in one definite direction, which in this country is nearly north and south. If a narrow magnetized bar be plunged into iron filings, it will be found that these are attracted chiefly by the ends of the bar, and not at all by the centre. It appears as if the magnetic power were concentrated in the extremities of the bar, and these are termed its poles, the pole at the end of the bar which points to the north is called the north pole of the magnet, and the other is named the south pole. If a north pole of one magnet be presented to the north pole of another, they will repel each other, and the same repulsion will take place between the south poles, whereas the north pole of one magnet attracts the south pole of another. In other words, poles of the same name repel each other, but poles of opposite names attract each other, or still more concisely, like poles repel, unlike poles attract each other.

Magnetism acts through intervening non-magnetic matter with undiminished energy. Thus, the attractions and repulsions of magnetic poles manifest themselves just as strongly when the poles are separated by a stratum of wood or stone as when merely air intervenes, and the attraction of small pieces of iron by a magnet takes place through the interposed palm of one’s hand without diminution. A delicately suspended needle in even a remote apartment of a large building moves whenever a cart passes in the street. It is almost too well known to require mention here, that iron and steel are the only common substances which are capable of plainly exhibiting magnetic forces, and, indeed, there are no known substances capable of so powerful a magnetization as these. But the difference in the magnetic behaviour of iron and steel is not so well understood, and it is a point of importance for our subject, and connected with a fundamental law which governs all magnetic manifestations. A piece of pure iron is very readily cut with a file, whereas a piece of steel may be so hard that the file 484makes no impression upon it whatever; and hence a piece of pure iron, or rather iron holding no carbon in combination, and possessed of no steely quality, is often spoken of as soft iron. When a piece of soft iron is placed near the pole of a magnet, the iron becomes, for the time, a magnet. If iron filings be sprinkled over it, they will arrange themselves about the parts of the iron respectively nearest and farthest from the magnet, thus showing that the piece of soft iron has acquired magnetic poles. It will be found on examining these poles that the one nearest the magnet is of the contrary name to the pole of the magnet, and the farthest is of the same name. The conversion of the soft iron into a magnet by the influence of a magnetic pole is termed induction. It need hardly be said that the inductive effect is more powerful in proportion to the shortness of the distance separating the piece of soft iron from the magnetic pole, and, of course, the effect is at its maximum when there is actual contact. Induction thus explains, by aid of the law of the poles, the attraction which a magnet exercises over pieces of iron, for it is plain that the inductive influence is accompanied by attraction between the two contiguous oppositely-named poles of the magnet, and of the piece of iron. But attraction is not the only force, for the pole developed at the farthest portion of the piece of iron being of the same name as the inducing pole, these will be mutually repulsive. The attractive force will, however, be more powerful on account of the shorter distance at which it is exerted, and will predominate over the repulsive force, particularly at short distances, because then the difference will be relatively greater. At distances from the inducing pole relatively great to the distance between the two poles of the piece of iron, the difference may be so small that its effect in attracting the piece of soft iron will be imperceptible, and then the piece of iron acted on by two (nearly) equal parallel forces, will be subject to what is termed in mechanics a couple, the only effect of which is to turn the body into such a position that the opposing forces act along the same line. The definite direction assumed by a freely suspended needle may be explained by supposing that the earth itself is a magnet having a south pole in the northern hemisphere, and a north pole in the southern hemisphere, the line joining these poles being shorter than the axis of the earth, and not quite coinciding with it in position; and the fact of the needle being turned round but not bodily attracted is then easily accounted for, the attractive and repulsive forces being reduced to a couple in the manner just explained.

If the attempt be made to turn a piece of steel into a magnet, by the induction of a magnetic pole, the same results will be obtained as in the case of soft iron, but in a much feebler degree, and with this difference: the piece of steel does not lose its magnetism when the inducing magnet is withdrawn, whereas in the case of the soft iron every trace of magnetism vanishes the instant the inducing pole is removed. And if the pole of the magnet be not only put in contact with one end of the piece of steel, but rubbed on it, the piece will acquire permanent and powerful magnetism. Hence it will be noticed that a piece of soft iron can by the mere approximation of a magnetic pole be converted in an instant into a magnet, and by the removal of the magnet can as instantly be deprived of its magnetism, and made to revert into its ordinary condition; while steel is not so readily magnetized, but retains its magnetism permanently.

Fig. 253.—A simple Electroscope.

The elementary phenomena of electricity are extremely simple and easy of demonstration, and as the whole science rests upon inferences derived from these, the reader would do well to perform the following simple experiments 485for himself. Apparatus is represented in Fig. 253, but the only essential portion is a straw, B, suspended from any convenient support by a very fine filament of white silk. To one or both ends of the straw a little disc of gilt paper, or a small ball of elder-pith or of cork, should be attached, so that the straw may be balanced horizontally. Now rub on a piece of woollen cloth a bit of sealing-wax, or a stick of sulphur, or a piece of amber, or a penholder, paper-knife, or comb made of ebonite, and immediately present the substance to the ball at the end of the straw. It will be first attracted to the rubbed surface, but after coming into contact with it, repulsion will be manifested and the ball will separate, and may be chased round the circle by following it with the excited body. The attraction of light bodies by amber after it has been rubbed appears to be the one solitary electrical observation recorded by the ancients, but it has given its name to the science, ελεκτρον being the Greek name for amber. The cause, then, of this property is named electricity, and bodies which exhibit it are said to be electrified. The reader will remark that these words explain nothing: they are used merely to express a certain state of matter and the entirely unknown cause of that state. Let the pith or cork ball at the end of the straw be again charged with electricity, by bringing it into contact with a piece of sealing-wax or ebonite which has just been electrified by friction. In this condition it will, as we have just seen, be repelled by the substance which charged it, and on trial it will be found to be repelled also by all the substances we have named, after they have been excited by friction. But if, while still charged with the electricity communicated to it by contact with sealing-wax, sulphur, ebonite, or amber, we present to it a warm and dry glass tube which has just been rubbed with dry silk, we shall find that the ball will be strongly attracted. After contact with the glass, repulsion will take place, and the ball will refuse again to come into contact with the excited glass. In this condition, however, it will be immediately attracted by rubbed sealing-wax or ebonite, and so on alternately: the ball when repelled by the wax is attracted by the glass, and when repelled by the glass is attracted by the wax.

These simple experiments prove that, whatever electricity may be, there are two kinds of it, or, at least, it manifests two opposite sets of forces. The electricity evolved by the friction of glass with silk was formerly called vitreous electricity, and that shown by excited resin, sealing-wax, amber, &c., was named resinous electricity. These names have now been respectively replaced by the terms positive and negative. It must be understood that these terms imply no actual excess or defect, but are purely distinguishing terms, just as we speak of the up and down line of a railway, without implying an inclination in one direction or the other. A fact of great importance in electrical theory is discovered when the substances in which 486electricity is developed are carefully examined: it is found that one kind is never produced without the other simultaneously appearing. Thus, the silk which has been used for rubbing the glass in the above experiments will be found to exhibit the same electricity as sealing-wax or ebonite. And, further, the quantities of positive and negative electricity evolved are always found to be equal, or equivalent to each other; that is, if they are put together they completely neutralize or destroy each other’s effects. We have used the word “quantity,” implying that electricity can be measured. No doubt, whatever electricity may be, there may be more or less of it; but can we measure an imponderable, invisible, impalpable thing, incapable of isolation? What we really measure when we say that we measure electricity is the attractive or repulsive force: we balance this against some other force (that of gravitation, for example), and we say, so much weight lifted represents so much electricity.

If we try to electrify a piece of metal by holding it in the hand and rubbing it against woollen cloth, silk, or other substance, we shall fail in the attempt: no signs of electricity will thus be shown by the metal. Hence bodies were formerly divided into two classes—those which could be electrified by friction, and those which could not. It was afterwards found, however, that there was no real ground for this division, but that, on the contrary, no two bodies can be rubbed together, even if they are made of the same substances, without positive electricity appearing in one, and an equivalent quantity of negative electricity in the other. The real difference between bodies which prevents the manifestation of electricity in many cases depends upon the fact that electricity is able to traverse some substances with great facility, while others prevent its passage. Thus, if we suspend horizontally a hempen cord by white silk attached to the ceiling, so that the hempen cord comes in contact with nothing but the silk, we shall find, on presenting a piece of excited ebonite to one end of the cord, that electric attraction of light bodies will be manifested at the other. If a silk cord be substituted for the hempen one, no such effect will be observed. The hemp is, therefore, said to be a conductor, and the silk a non-conductor. Again, if we substitute for one of the silk threads suspending the cord a piece of twine, or a wire, we shall fail to obtain any electric manifestations at the remote end, because the electricity will be carried off into the earth by the conducting powers of these substances. On the other hand, filaments of glass or ebonite may be used, instead of the silk, with the same effect: they do not allow the electricity to run through them to the ground, and are therefore termed, like the silk, insulators of electricity. The distinction of bodies into conductors on the one hand, and into non-conductors or insulators on the other, is of paramount importance in the science and in all its applications. This distinction, however, is not an absolute one: there is no substance so perfect an insulator that it will not permit any electricity to pass, and there is no conductor so perfect that it does not offer resistance to the passage. Substances may be arranged in a list which presents a gradation from the best conductor to the best insulator. The metals are by far the best conductors, but there is great relative diversity in their conductive power. Silver, copper, and gold are much the best conductors among the metals, iron offering eight times, and quicksilver fifty times, the resistance of silver. Coke, charcoal, aqueous solutions, water, vegetables, animals, and steam are all more or less conductors, while among the substances called insulators may be named, in order of increasing insulating power, india-rubber, porcelain, leather, paper, wool, silk, mica, glass, wax, 487sulphur, resins, amber, gum-lac, gutta-percha, and ebonite. It will now be obvious why the electricity developed by the friction of a piece of metal fails to manifest itself under ordinary circumstances, as, for instance, when held in the hand: the metal and the body being both conductors, the electricity escapes. But if the piece of metal be held by an insulating handle of glass or ebonite, the electrified condition may easily be observed.

THEORY OF ELECTRICITY.

The few elementary facts which have been pointed out are absolutely necessary for the foundation of what is sometimes termed the theory of electricity, but which is properly no theory,—at least, not a theory in the same sense as gravitation is a theory explaining the motions of the planets, or even in the sense in which the hypothesis of the ether and its movements explains the phenomena of light. It is absolutely necessary to have a conception of some kind which may serve to connect in our minds the various phenomena of electricity, if it were only to enable us the more easily to talk about them. In default of any supposition which will shadow forth what actually occurs in these phenomena, we have recourse to what has been aptly termed a representative fiction: we picture to ourselves the actions as due to imaginary fluids—fluids which we know do not exist, but are as much creations of the mind as Macbeth’s air-drawn dagger; not, however, like his “false creation,” proceeding from “the heat-oppressed brain,” but intellectual fictions, consciously and designedly adopted for the purpose of enabling us the better to think of the facts, to readily co-ordinate them, and to express them in simple and convenient language. Non-scientific persons hearing this language usually mistake its purport, and imagine that the actual existence of an “electric fluid” is acknowledged. The accounts which appear in the newspapers of the damage done by thunderstorms are often amusing from the objectivity which the reporter attributes to the “electric fluid.” It is described, perhaps, as “entering the building,” “passing down the chimney,” then “proceeding across the floor,” “rushing down the gas-pipes,” “forcing its way through a crevice, and then streaming down the wall,” &c., in terms which imply the utmost confidence of belief in the existence of the “fluid.” With this intimation that the hypothesis of electric fluids is merely, then, a “façon de parler,” the reader will not be misled by the following brief explanation of the elementary facts in the language of the theory.

In the natural state all bodies contain an indefinite quantity of an imponderable subtile matter, which may be called “neutral electric fluid.” This fluid is formed by a combination of two different kinds of particles, positive and negative, which are present in equal quantities in bodies not electrified; but when there is in any body an excess of one kind of particles, that body is charged accordingly with positive or negative electricity. Both fluids traverse with the greatest rapidity certain substances termed conductors; but they are retained amongst the molecules of insulating substances, which prevent their movement from point to point. When one body is rubbed against another, the neutral electric fluid is decomposed—the positive particles go to one body, the negative with which these positive 488particles were before united pass to the other body. The particles of the same name repel each other, but particles of opposite names attract each other; and it is this attraction which is overcome when the electricities are separated by friction or in any other manner.

It will be observed that the above is nothing but the statement of the elementary facts in the language of the hypothesis. This system of the two fluids readily lends itself to the explanation of nearly all the phenomena presented in what is termed static electricity—that is, in those phenomena where the actions are conceivably due to a more or less permanent separation of the fluids. The grand discoveries in electricity turn, however, upon quite another condition, namely, one in which the two hypothetical fluids must be imagined as constantly combining, and here the utility of the hypothesis is less marked. Inasmuch, however, as there can be no doubt regarding the identity of the agent operating in the two sets of circumstances, the facts of dynamical electricity must still be expressed in the same language, with the aid of any additional conceptions which may give us more grasp of the subject.

ELECTRIC INDUCTION.

In all electrical phenomena an inductive action occurs, which resembles that which we have already indicated with regard to magnetism. Thus, if we take an insulated metallic conductor in the uncharged state, and bring it near an electrified body, we shall find that the conductor, while still at a considerable distance, will give signs of an electrical charge. Suppose we have a cylindrical conductor, and that we present one end of it to the electrified body, but at such a distance that no spark shall pass, we shall find, if the charge on the electrified body be strong and the conductor be brought sufficiently near, that on bringing the finger near the insulated cylinder, a spark passes. While the cylinder continues in the same position with regard to the electrified body, no further sparks can be drawn from it; but if the distance between the two bodies be increased, the insulated cylinder will be found to have another charge of electricity, which will again produce a spark. And by repeating these movements we may obtain as many sparks as we desire by these mechanical actions, without in the least drawing upon the charge on the original electrified body. The electrophorus is a device for obtaining electricity by this plan, and several rotatory electrical machines have lately been invented which yield large supplies of electricity by a similar inductive action.

Fig. 254. The Gold-leaf Electroscope.

It is found that in such a case as that we have above supposed, if the electrified body is charged with positive electricity, the uncharged conductor brought near it has its electricities separated—the negative attracted and held by the attraction of the positive charge in the parts of the cylinder nearest the inducing body; while the corresponding quantity of positive electricity is driven towards the most remote parts of the insulated conductor. It is this last which gives the spark in the first case, and if it be not thus withdrawn from the conductor, it re-combines with the negative electricity when the conductor is withdrawn from the neighbourhood of the electrified body, and the conductor then reverts to the natural or unelectrified state. 489But the contact of a conducting body with the conductor while it is under the influence of the electrified body withdraws only positive electricity, the negative—being held, as it were, by the attraction of the positive electricity of the charged body—is not thus removed, and in this condition it is sometimes called disguised or dissimulated electricity—a term the propriety of which is doubtful. The excess of negative “fluid” which the conductor thus acquires shows itself, however, only when the inducing body has been withdrawn. Precisely similar effects will take place, mutatis mutandis, if the electrified body has a negative charge. A demonstration of inductive effects is readily afforded in the action of the gold-leaf electroscope, Fig. 254, in which two strips of gold-leaf are suspended within a glass case from wire passing through the top, and terminated in a metal plate. This instrument is often used for showing the existence of very small electric charges. Let a stick of sealing-wax be rubbed and held, say, a foot or more from the plate of the electroscope, the leaves will diverge with negative electricity. The sealing-wax being retained in the same position, touch the plate for an instant with the finger. This will remove the negative charge, but the positive electricity will be retained on the plate by the attraction of the negative of the sealing-wax. Now remove the sealing-wax, when the dissimulated charge will spread itself over the whole insulated metallic portion of the electroscope, and the leaves will diverge with a strong charge of positive electricity. If an excited glass tube is brought near the electroscope, the leaves will now diverge still more; if the sealing-wax is replaced in its former position, the leaves will collapse. In all these cases the electrified body parts with none of its own electricity by developing electrical effects in the neighbouring bodies.

The inductive actions we have described take place through the air, which is a non-conductor, and such actions may be made to take place through any other non-conductor. With solid non-conductors, such as glass, gutta-percha, &c., the inducing body may be brought very near to the conductor on which it is to act; for the intervening solid substance, or dielectric, as it has been appropriately called, opposes a resistance to the combination of the opposite electricities, and the inductive effects are greatly intensified by the approximation. Faraday discovered that the amount of inductive action with a given charge is also dependent upon the nature of the dielectric, and that the electric forces act upon the particles of the dielectric, circumstances which are of the greatest importance, as we shall presently find, in practical telegraphy. The most familiar instance of induction is probably well known to the reader in the Leyden jar, Fig. 255, which is simply a wide-mouthed bottle of thin glass, covered internally and externally with tin-foil to within a few inches of the neck. The inner coating communicates by means of a rod and chain with a brass knob. Such a jar admits of the accumulation of a larger quantity of electricity than the conductor of a machine will retain. A very few turns of the machine will 490suffice usually to charge the conductor to the fullest extent; but if it be put in communication with the knob of a jar, a great many more turns will be required to attain the same charge in the conductor, and the excess of electricity represented by these additional turns will have accumulated within the jar—an effect due to the “dissimulated” electricity of its exterior.

Fig. 255.—The Leyden Jar.

Everybody knows the result when a metallic communication is established between the exterior and the interior of a charged Leyden jar. There is a very bright spark, a snap, and the jar is “discharged.” Everybody knows, also, the sensation experienced when his body takes the place of the metallic communication, or forms part of the circuit through which the communication takes place. Everybody knows that the shock then felt may also be experienced at the same moment by any number of persons who join hands, under such conditions that they also form a part of the line of communication. Such facts irresistibly suggest the notion of something passing through the whole chain, and this notion is in perfect harmony with the hypothesis of the “fluids,” for we have only to suppose that it is one or both of these which rush through the circuit the instant the line of communication is complete. It was one of Franklin’s discoveries that the electrical charges of the Leyden jar do not reside in the metallic coatings; for he made a jar with removable inside and outside coatings, which, properly taken from the glass, showed no signs of electrification, yet when replaced the jar was found to be again highly charged. This would seem to show that the charge clings to, or penetrates within, the glass.

DYNAMICAL ELECTRICITY.

Let us take a vessel containing water, to which some sulphuric acid has been added, Fig. 256, and in the liquid plunge a plate of copper, C, and a plate of pure zinc, Z, keeping the plates apart from each other. As it is not easy to obtain zinc perfectly free from admixture of other metals, an artifice is commonly resorted to for obtaining a surface of pure metal, by rubbing a plate of the ordinary metal with quicksilver, which readily dissolves pure zinc, but is without action on the iron and other metals with which the zinc is contaminated, while the quicksilver is not acted upon by the diluted acid, but is merely the vehicle by which the pure zinc is presented to the liquid. Under the conditions we have described, no action will be perceived, no gas will be given off, nor will the zinc dissolve in the acid. If the electrical condition of the portion of the copper-plate which is out of the liquid be examined by means of a delicate electroscope, 491it will be found to possess a very weak charge of positive electricity, and a similar examination of the zinc plate will show the existence on it of a feeble charge of negative electricity. If the two plates be made to touch each other, or if a wire be attached to each plate, as shown in the figure, and the wires be brought into contact outside of the vessel, an action in the liquid is immediately perceptible at the surface of the copper plate, when a multitude of small bubbles of hydrogen gas will at once make their appearance, and the gas will be given off continuously from the copper plate so long as there is metallic contact through the wires, or otherwise, between the two plates, or until the acid is saturated with zinc—for in this action the zinc is dissolving, and, in consequence, liberating hydrogen, which strangely makes its appearance, not at the place where the chemical action really occurs, namely, at the surface of the zinc which is in contact with the acid, but at the surface of the copper which is not acted upon by the acid.

Fig. 256.—A Voltaic Element.

It is known that when we establish a metallic communication between two bodies charged with equivalent quantities of positive and negative electricities respectively, these combine and neutralize each other, and all signs of electricity vanish. It is obvious that the contact of the two wires has this effect, as the signs of electric charge which were before discoverable in each of the plates are no longer found while the wires are in contact. But the charges reappear the instant the contact is broken, the chemical action ceasing at the same time. If the wire connecting the two plates outside of the vessel be carefully examined, it will be found, so long as the chemical action is going on, to be endowed with new and very remarkable properties. If this wire be stretched horizontally over a freely suspended magnetic needle, and parallel to it, the needle will be deflected from its position, and, if the wire be placed very near it, will point nearly east and west, instead of north and south. Now, this effect is produced by any part whatever of the wire, and it instantly ceases if the wire be cut at any point. These facts at once suggest the idea of its being due to something flowing through the wire, so long as metallic continuity is preserved. This idea is much strengthened when we find that the action of the connecting wire upon the magnetic needle is quite definite—or, in other words, there are indications which correspond with the notion of direction. For when the wire, which we shall still suppose to be stretched 492horizontally above the needle and parallel to its direction, is so connected with the plates immersed in the acid that the portion which approaches the south-pointing pole of the needle proceeds from the copper plate, while the portion above the north pole is in connection with the zinc plate, then the north end of the needle will always be deflected towards the west—whereas, if the connections be made in the contrary manner, the deflection will be in the opposite direction; and if the wire be below the needle, the contrary deflections will be observed with the same connections. The discovery of the action of such a wire on the magnetic needle was made by Œrsted in 1819, and it is a discovery remarkable for the wonderful extent of the field which it opened out, both in the region of pure science and in that of practical utility.

Fig. 257.—Ampère’s Rule.

Since by such experiments as those just mentioned the notion of a current is arrived at, the mind recurs to the fiction of the “fluids,” and pictures the “positive fluid” as rushing in one direction, and the “negative fluid” in the other, to seek a re-combination into “neutral fluid.” But we must never lose sight of the fact that these ideas are consciously adopted as representative fictions to help our thoughts—just as John Doe and Richard Roe, imaginary parties to an imaginary lawsuit, used to be named in legal documents, in order to explain the nature of the proceedings. Failing, then, to find anything really flowing along our wire, it is still absolutely necessary, seeing there is something definite in its action, to assign a direction to the supposed current; and it has been agreed that we shall represent the current as flowing from the positively charged body to the negatively charged body—that is, in the case we have been considering, from the copper to the zinc through the wire. When this conventional representation has been adopted, the action on a magnetic needle can easily be defined and remembered by an artifice proposed by Ampère. In Fig. 257, let N S represent the magnetized needle, N being the pole which points towards the north, and S the south pole. Let C be the end of the wire connected with the copper plate, and Z that connected with the zinc. The current is therefore supposed to flow in the direction indicated by the arrows in a wire above the needle and in the wire placed below. Now, suppose that a man is swimming in the current in the same direction it is flowing, and with his face towards the needle, then the north pole of the needle will always be deflected towards his left. With the direction of current represented in the figure, the pole, N, will be thrown forward from the plane of the paper, or towards the spectator.

The reader who desires to study the mutual action of currents and magnets will find it necessary to fix this idea in his mind. He will now be able to see that if the wire be coiled round the needle, as shown by the lines and arrows, Fig. 257, so that the same current may circulate in reverse directions 493above and below the magnet, its effects in deviating the needle will everywhere concur—that is, the action of each part will be to turn the north pole towards the left. It is, therefore, plain that if the wire conveying the current be passed several times round the magnetic needle, the deflecting force will be increased; and a current, which would, by merely passing above or below the magnet, produce no marked deflection, might be made to produce a considerable effect if carried many times round it. The arrangement for this purpose is shown in Fig. 258, where it will be perceived that the needle is surrounded by a coil of wire, so that the current circulates many times about it, and the effects of each part of the circuit concur in deflecting the needle. Such an arrangement of the wire and needle constitutes what is called the galvanometer, an instrument used to discover the existence and direction of electric currents.

Fig. 258.—Galvanometer.

The arrangement of metals and acid which we have described is termed a voltaic couple, element, or cell; and a great controversy has long been carried on among men of science as to the place at which the development of electricity has its origin. Three-quarters of a century ago, the effect was attributed by Volta to the mere contact of the two dissimilar metals. In the experiment we have described this contact, supposing the wires to be of copper, would occur at the junction of the wire and the zinc plate. Now, by joining the copper plate of such a cell to the zinc plate of another cell, the copper of that to the zinc of a third, and so on, it is evident that the number of dissimilar contacts might be indefinitely increased, and the electric power should be proportionately augmented. It is found that this is really the case, but Volta’s explanation has been opposed by another which regards the chemical action in the cells as the real origin of the electric manifestations. This last explanation, supported by many apparently conclusive experiments of Faraday and others, has been generally accepted. Galvanic batteries—as a series of cells joined together in a certain manner are termed—have been constructed, in which there is no contact of dissimilar metals; and no electric current can be obtained from an apparatus in which no chemical action takes place. The contact theory in a modified form has recently been revived by Sir W. Thomson and others. In this it is now maintained that some separation of electricities really does take place by contact of dissimilar substances, but that a current can be produced only when this separation is continually renewed by chemical actions. Be the true explanation what it may, the fact is undoubted that by joining cell to cell, we can really obtain vastly more powerful effects. If we take a single cell, such as that represented in Fig. 256, and connect the plates with a long and thin wire, we shall find 494that the current flowing through each part of the circuit is much weaker than when we connect the plates with a short and thick wire. In other words, the action in the latter case, when the wire is stretched over a magnetic needle, will be more powerful than in the former. By using a long and thin wire the current may be so weakened that it becomes necessary to surround the needle with many coils of the wire to produce a marked deflection. Again, much depends upon the material; thus a copper wire conveys a much more powerful current than a German silver one of the same dimensions. There thus appears to be a certain analogy between the flow of electricity along conductors to that of water through pipes. The longer and narrower are the pipes, the less is the quantity of water forced through them by a given head; and similarly, the resistance to the passage of a current increases with the length and narrowness of the conducting wire. When all other circumstances are the same, the electrical resistance of a conductor varies directly as its length and inversely as its sectional area. Hence the current flowing in the apparatus represented in Fig. 256 would be increased by making the wire thicker, and by making it shorter by bringing Z and C nearer together, and by making the area they expose to the liquid larger; for in the liquid also the current flows as indicated by the arrow, a fact which may be proved by the deflection of a magnetized needle suspended above the vessel. The magnitude of the current depends, then, upon two opposing forces, namely, that which continuously separates the electricities, or drives them apart to re-combine through the circuit, and that which opposes their passage. The former, which is termed the electromotive force, originates, according to some, from the mere contact of dissimilar materials, according to others from the chemical action. Now, we may increase the strength of the current in a given arrangement, either by increasing the electromotive force, or by diminishing the resistance. The increase of the strength of the current, produced by merely pouring more acid into the vessel, Fig. 256, is due, according to the chemical theory, to the former cause; according to the contact theory, to the latter. By multiplying the cells we increase the electromotive forces: the current receives, so to speak, an onward shove in each cell, but with each cell we introduce an additional resistance. Hence, it follows, that when the resistance of the circuit outside of the cells is extremely small, the current produced by a single cell is as powerful as that produced by a thousand. But when the external resistance is great, as when long thin wires are used, the united electromotive forces of a number of cells are needed to drive the current through the circuit. The strength of a current, C, is therefore expressible by the following simple formula, in which r stands for the internal resistance, and e for the electromotive force in each cell; n represents the number of cells in the battery, these being supposed exactly similar in every respect; R is the sum of the resistances in the circuit outside of the battery.

ne

C =

nr + R

It is easily seen that the smaller R is made, the more nearly does the strength of the current become independent of the number of cells.

Fig. 259.—Daniell’s Cell and Battery.

Fig. 260.—Grove’s Cell and Battery.

But many modifications have been made in the materials and form of the cells, by which greater power and duration of action have been attained. Our space permits a description of only two forms, and these must be described without a discussion of the principles upon which their increased 495efficiency depends. Daniell’s constant cell is represented in Fig. 259, where D is a battery of ten such cells, A is a cylindrical vessel of copper, C is a tube of porous earthenware, closed at the bottom, and within it is suspended the solid rod of amalgamated zinc, B. The copper vessel and the zinc rod are provided with screws by which wires may be attached. In the copper vessel is placed a saturated solution of sulphate of copper, and some crystals of the same substance are placed on the perforated shelf within the vessel. The porous tube is filled with diluted sulphuric acid. When the battery is in action the zinc is dissolved by the sulphuric acid, and metallic copper is continually deposited upon the internal surface of the copper vessel. Daniell’s battery, in some form or other, is much used for telegraphs and for electrotyping. Grove’s cell is shown in section in Fig. 260. The external vessel is made of a rectangular form in glazed earthenware or 496glass. It contains a thick plate of amalgamated zinc, A, A, bent upwards, and between the two portions a flat porous cell, C, C, is placed, filled with strong nitric acid, in which is immersed a thin sheet of platinum. The outside vessel is charged with water, mixed with about ⅛th of sulphuric acid. D represents a battery of four such cells, in which the mode of connecting the platinum of one to the zinc of the next may be noticed. The terminal platinum and zinc form the poles of the battery, and to them the wires are attached which convey the current. The substitution of plates of coke for the platinum gives the form of battery known as Bunsen’s, which is also sometimes made with circular cells. Gover’s and Bunsen’s are much more powerful arrangements than Daniell’s, but the latter has the advantage as regards the duration and uniformity of its action.

Fig. 261.—Wire ignited by electricity.

When the current produced by a battery of a dozen or more such cells is conveyed by a wire, it is observed that this wire becomes sensibly hot, and, if the wire be thin enough, the heat may be sufficiently great to heat the wire to redness. By stretching a piece of platinum wire between two separate rods which convey the current, as represented in Fig. 261, the length of wire through which the current passes may be adjusted so as to give any required amount of light, and the wire may even be heated to the fusing-point of platinum. This property of electricity has some interesting applications, as, for example, in firing mines and other explosive charges, and in some surgical operations. A still more interesting exhibition of heating and luminous effects is observed when the terminals of a battery of many cells are connected with two rods of coke, or gas-retort carbon. When the pointed ends of the rods are brought into contact, the current passes, and the points begin to glow with an intensely bright light, and if they are then separated from each other by an interval of ⅒th of an inch or more, according to the power of the battery, a luminous arc extends between them, emitting so intense a light that the unprotected eye can hardly support it. This luminous arc is called the voltaic arc, and it excels all other artificial lights in brilliancy, a fact due to the extremely high temperature to which the carbon particles are heated, the temperature being, perhaps, the highest we can attain. It must not be supposed that in this brilliant light we see electricity: the light is due to the same cause as the light of a candle or gas flame, namely, incandescent particles of solid carbon. These particles are carried from one carbon point to the 497other, and it is found that the positive pole rapidly loses its substance, which is partly deposited on the negative pole. But in order to obtain a steady light, it is requisite to keep the pieces of carbon at one invariable distance; and therefore the transference of the material from one pole to the other, and the loss by combustion, must be compensated by a slow movement of the carbons towards each other. Several kinds of apparatus are used for this purpose, but they all depend upon the principle of regulating the motions by the action of an electro-magnet, formed by the current itself, which becomes weaker as the carbons are farther apart. The movement is communicated to the apparatus by clockwork. Duboscq’s electric lantern is shown in Fig. 262, with enlarged images of the carbon points projected on a screen. The mechanism of the regulator is contained within the cylindrical box immediately below the lantern. The supports of both carbons are moved; that which bears the positive carbon pole being advanced twice as fast as the other, and thus the light is maintained at the same level, for the positive carbon wears away twice as fast as the other. The light is more brilliant when charcoal is used instead of coke, but then it is necessary to operate in a vacuum, to avoid the combustion of the charcoal. The voltaic arc has recently been applied to illuminate lighthouses, and for other purposes, and will probably soon be more widely employed, for a cheap and convenient mode of producing a uniform current of electricity has recently been discovered and will be presently described.

Fig. 262.—Duboscq’s Electric Lantern and Regulator.

Fig. 263.—Decomposition of Water.

The current which is maintained by the chemical action taking place in the cells of the battery can also be made to do chemical work outside of the battery. When the poles of the battery are terminated by wires or plates of platinum, and these are plunged into water acidulated with sulphuric acid, bubbles of gas are seen to rise rapidly from each wire, or electrode, 498as it is termed. Fig. 263 shows an arrangement by which these gases may be collected separately, and examined, by simply placing over each electrode an inverted glass tube, filled also with the acidulated water. The gases collect at the tops of the tubes, displacing the water, and it is found that from the wire connected with the zinc end of the battery, or negative electrode, hydrogen gas is given off, while at the positive electrode oxygen gas is liberated, in volume precisely equal to half that of the hydrogen. This being the proportion in which these two substances combine to produce water, it appears that in the passage of the current a certain quantity of water is decomposed; and the quantity thus decomposed is in reality a measure of the current, all the other effects of which are found to be proportional to this. When the electricity in a current is said to be measured, it is simply the power of the current to deflect a magnet, or the quantity of gas it can liberate, or some other such effect, which is in fact measured. The discharge of a Leyden jar through such an apparatus as that represented in Fig. 263 would present no perceptible decomposition of the water; yet such a discharge passed through the arms and body produces, as everybody knows, a painful shock, and is accompanied by a bright spark and a noise, while the simultaneous contact of the fingers with the positive and negative poles of the galvanic battery occasions neither shock nor spark. Thousands of discharges from large jars must be passed through acidulated water to liberate the amount of gas which a battery current of a second’s duration will produce. The electricity of the jar is often spoken about as having a higher tension than that of the battery, but the latter sets an immensely greater quantity of electricity in motion. The idea may be illustrated thus: Suppose we have a small cistern of water placed at a great height, and that this water could fall to the ground in one mass. The fall of the small quantity from a great height would be capable of producing very marked instantaneous effects, such as smashing, as with a blow, any 499structure upon which it might fall. This would correspond with the small quantity of electricity which passes in the discharge of a Leyden jar. Contrast this with the case in which we allow a very large quantity of water to descend from a very small height—as when the water of a reservoir is flowing down a gently inclined channel. It is plain that a different kind of effect might be produced in this case; the current might be made, for instance, to turn a water-wheel, which the more forcible impact of the small quantity of water in the case first supposed would have broken into pieces.

Fig. 264.—Electro-plating.

It is probable that the apparent decomposition of water by the electric current is in reality a secondary effect, and that it is the sulphuric acid which is decomposed. When, instead of acidulated water, we place in the apparatus a solution of sulphate of copper, it is found that metallic copper is deposited on the negative electrode, and sulphuric acid collects at the positive electrode. The metal is deposited in a firm and coherent state, and the useful applications of this deposition of metals are of great interest and importance. For, in a similar manner, gold, silver, lead, zinc, and other metals may be made to form thin uniform layers over any properly prepared surface. The immense advantages which the arts have derived from electro-plating illustrate in a convincing manner the benefits which physical science can confer on society at large.

The process of electro-plating may be practised by the aid of apparatus of very simple character. Fig. 264 shows all that is necessary for obtaining perfect casts in copper of seals, small medals, &c. A A is a section of a common tumbler; B B is a tube, made by rolling some brown paper round a ruler, uniting the edge with sealing-wax, and closing the bottom by a plug of cork, round which the paper may be tied by a string, or in any other convenient manner. The tumbler contains a solution of sulphate of copper, and the tube is filled with water, to which about one-twentieth of its bulk of sulphuric acid has been added. A strip of amalgamated zinc, or a piece of thick amalgamated zinc wire, is placed in the tube, and a piece of copper bell-wire is twisted round the top of it, and has attached to its other extremity, and immersed in the copper solution, the article which is to be covered with copper. We may suppose that this is to be a cast in white wax or in plaster of one side of a medal. The cast is carefully covered with black lead by means of a soft brush, and the copper wire is inserted in such a manner as to be in contact with the black lead at some part. When the apparatus has been left for some hours in the position represented, a deposit of copper will be found over the blackleaded surface, and it will be a perfect impression of the wax cast.

Such a copper cast, or any article in copper having a perfectly clean surface, can be readily covered by a film of silver by means of a similar arrangement, where a solution of cyanide of potassium, in which some chloride of silver has been dissolved, is made to take the place of the sulphate of copper. Electro-plating with the precious metals has become a 500commercial industry of great importance; and this process has completely superseded the old plan of covering the metallic article to be plated with an amalgam of silver or of gold, and then exposing it to heat, which volatized the mercury, leaving a thin film of gold or of silver adhering to the baser metal. On the large scale a battery of several cells is used for electro-plating, and the articles are immersed in the metallic solutions as the negative poles of the battery; any required thickness of deposit being given according to the length of the time they remain. At the works of Messrs. Elkington, of Birmingham, these operations are conducted on a grand scale. The liquid there employed for silvering is a solution of cyanide of silver in cyanide of potassium, and the positive pole is formed of a plate of silver, which dissolves in proportion as the metal is deposited on the negative pole. As the charging of batteries is a troublesome operation, and their action is liable to variations which affect the strength of the currents, the more uniform, more convenient, and more economical mode of producing currents by magneto-electricity, which will presently be described, has been to a great extent substituted for the voltaic battery.

Fig. 265.—A Current producing a Magnet.

The wire conveying a current not only affects a magnetic needle in the manner already described, but itself possesses magnetic properties, of which, indeed, its action on the needle is the result and the indication. If such a wire be plunged into iron filings, it will be found that the filings are attracted by it: they cling in a layer of uniform thickness round its whole circumference and along its whole length, and the moment the connection with the battery is broken they drop off. This experiment shows that every part of the wire conveying a current is magnetic, and it may be proved that the action is not intercepted by the interposition of any non-magnetic material. Thus the action of the wire upon the magnetic needle takes place equally well through glass, copper, lead, or wood. Consequently, if we cover the wire with a layer of gutta-percha, or over-spin it with silk or cotton, we shall obtain like results on our filings, and if we coil the covered wire round a bar of iron, while the non-conducting covering of the wire will compel the current to circulate through all the turns of the coil, it will not interfere with the magnetic action on each particle of the bar. Whenever this is done it is found that the iron is converted into a powerful magnet so long as the current passes. Fig. 265 represents in a striking manner the result when the current is made to circulate through numerous convolutions of the wire; and as each turn adds its effect to that of the rest, magnets of enormous strength may be formed by sufficiently increasing the 501number of the turns. The end of the iron bar is shown projecting from the axis of the coil, and below it is placed a shallow wooden bowl, containing a number of small iron nails. The instant the battery connection is completed these nails leap up to the magnetic pole, and group themselves round it in the manner shown in the cut; and again, when the current is interrupted, the iron reverts to its ordinary condition, the magnetism vanishes, and the nails drop down in an instant. These effects may be produced again and again, as often as the current flows and is broken. A magnet so produced is called an electro-magnet, to distinguish it from the ordinary permanent steel magnets. By coiling the conducting wire round a bar of iron which has been bent into the form of a horse-shoe, very powerful magnets may be produced, and enormous weights may be supported by the force of the magnetic attraction so evoked. Fig. 266 represents the apparatus for experiments of this kind, in which weights exceeding a ton can be sustained.

Fig. 266.—An Electro-magnet.

Here, then, we have a striking instance of the subtile agent electricity, evoked by the contact of a few pieces of zinc with dilute acid, showing itself capable of exerting an enormous mechanical force. Engines have been constructed in which this force is turned to account to produce rotatory motion as a source of power. Such engines have certain advantages for special purposes; but the money cost for expenditure of material for power so obtained is, at least, sixty times greater than in the case of the steam engine. It is, however, in producing mechanical effects at a distance 502that the electric current finds the most interesting practical application of its magnetic properties. These are the actions which are so extensively utilized in the construction of telegraphic instruments, of clocks regulated by electric communication with a standard time-keeper, and of many ingenious self-registering instruments. The telegraph will be described in the next article, and we shall also have occasion in subsequent articles to describe some of the other applications of electro-magnetic and electro-chemical force.

INDUCED CURRENTS.

These very remarkable phenomena were discovered by the illustrious Faraday, in 1830, and this discovery, and that of magneto-electricity, may be ranked among the most memorable of his many brilliant contributions to electric science. Let two wires be stretched parallel and very near to each other, but not in contact. Let the extremities of one wire, which we shall term A, be connected with a galvanometer (page 415), so that the existence of any current through the wire may be instantly indicated. Let the two extremities of the other wire, B, be put into connection with the poles of a battery. The moment the connection is complete, and the battery current begins to rush through B, a deflection of the galvanometer needle will be observed, indicating a current of very short duration through A in the opposite direction to the battery current through B. This induced current, which is called the secondary current, does not continue to flow through A: it occurs merely at the time the primary or battery current is established; and though the latter continues to flow through the wire, B, no further effect is produced in the other wire. When, however, the battery connection is broken, and the primary current ceases to flow, at that instant there is set up in the wire, A, another momentary secondary current, but this one is in the same direction as the battery current. This is termed the direct secondary current, in opposition to the former, which is called the inverse current.

These effects are much more powerful when, instead of lengths of straight wire, or single circles of wires, we use two coils of wire, one of which, namely, that which conveys the primary currents, is placed in the axis of the other. It must be distinctly understood that the secondary currents are of momentary duration only; they are not produced at all while the battery is flowing, but only at the time of its commencement and cessation. If, however, we make the primary coil so that it can be slid in and out of the axis of the other, then while the primary current is continuously flowing, we can produce secondary currents in the other coil, by causing the coils to approach or recede from each other. As we bring the coils near each other, and slide the primary into the secondary, the current in the latter is inverse; when the one coil is receding from the other, it is direct. These mechanical actions are not produced without expenditure of force, for the approaching coils repel each other and the receding coils attract each other. The setting up of the battery current in the primary coil when placed within the other is equivalent to bringing it, with the current flowing, from an immense distance in an extremely small time. Similarly, 503when the battery current is broken, it is equivalent to an instantaneous recession. The effects, therefore, are proportionately powerful. It is found, also, and this we shall presently refer to more fully, that when, instead of the primary coil, a magnet is similarly moved into, or removed from, the axis of the secondary coil, currents in opposite directions are set up in the latter without any battery being used at all. The direction of these currents is the same as would be produced by a primary current that would form, in a piece of iron placed in the axis of the coil, an electro-magnet with poles similarly situated to those of the magnet so introduced or withdrawn. Hence, by placing a bar of soft iron in the axis of the primary coil, the secondary currents will be produced with increased force. When a long secondary coil, having the turns of its wire well insulated from each other, surrounds a primary coil provided with a core of soft iron, or still better, with a bundle of annealed iron wires, a series of powerful discharges, like those of a Leyden jar, may be obtained between the terminals of the secondary coil, when the battery contact is made and broken in rapid succession.

Fig. 267.—Ruhmkorff’s Coil.

Such induction coils have been very carefully and skilfully constructed by Ruhmkorff, and are therefore often called “Ruhmkorff’s Coils.” One of these is represented in Fig. 267. A B is the coil, and the apparatus is provided with what is termed a condenser, which consists of layers of tin-foil placed between sheets of thick paper, and alternately connected so that one set communicates with one extremity of the primary coil, and the other with the other. This condenser is conveniently contained in the wooden base of the instrument. Its introduction has greatly increased the intensity of the secondary current, and sparks of 18 in. or 20 in. in length have been obtained in the place of very short ones.

It should be stated that of the two secondary currents, only one has sufficient intensity to traverse the secondary circuit when there is any break in its continuity. This is the direct secondary current, or that which is produced on breaking the primary circuit. The reason is that the commencing current in the primary circuit induces in the spires of its own coil an inverse current, and the battery current therefore attains its full strength gradually, but still in a very short time; while, on the cessation of the battery current, the same induction sends a wave of electricity through the primary coil in the same direction, and then the current ceases abruptly. Consequently, in the latter case, the induced electricity of the secondary coil is set in motion in much less time, and therefore possesses much greater intensity.

504The magnetism of the iron core is usually made use of to break and make the current, by the attraction of a piece of iron attached to a spring, which, by moving towards the end of the core, separates from a point in connection with the battery, and, the current no longer flowing, the magnetism ceases, and the spring again brings back the iron and renews the contact.

Fig. 268.—Discharge through Rarefied Air.

By means of such coils many surprising effects have been produced. Perhaps one of the most beautiful experiments in the whole range of physical science is made by causing the discharges of the secondary coil to take place through an exhausted vessel in the manner represented in Fig. 268. A beautiful light fills the interior of the vessel, and the terminals appear to glow with a strange radiance—one being surrounded with a kind of blue halo and another with a red. On reversing the direction of the currents, which is done by the little apparatus at the right-hand end of the coil in Fig. 267, the blue and the red radiance change places. Beautiful flashes of light may also be made to appear in the vessel, having the most marked resemblance to the streamers of the Aurora Borealis. When, instead of vessels almost free from common air, we repeat the experiment with tubes containing an extremely small residue of some other gas, such as hydrogen, carbonic acid, &c., the colour of the light and other appearances change 506Geissler’s tubes have already been spoken of in connection with the spectroscope; but, independently of that, the various beautiful appearances which such tubes have been made to present, by the introduction of fluorescent substances and other devices, render the induction coil an instrument of the highest interest to the scientific amateur. Then there are striking physiological and other effects which the coil is capable of producing. For instance, we are able by its instrumentality to produce from atmospheric air unlimited quantities of that singular modification of oxygen which is called ozone. The electricity of the coil has been used for firing mines, torpedoes and cannons, and for lighting the gas-burners of large buildings.

Fig. 268a.—Large Induction Coil at the old Polytechnic Institution, London.

The late Mr. Apps, who was well known as a skilful constructor of scientific apparatus, devoted much attention to improving the induction coil, and he made a very large one for the Polytechnic Institution in Regent Street, London, which Institution was at that time the home of popular science, under the direction of Mr. Pepper. This coil is represented in Fig. 268a, surrounded by the somewhat scenic accessories which were then supposed to be required for making science attractive to the multitude. Externally, the coil appeared as a cylinder, nearly 5 feet long and 20 inches in diameter. From each end projected smaller cylinders. All these and also the two upright pillars upon which the apparatus was supported were covered with ebonite. The large cylinder contained the primary coil, which was made of copper wire one-tenth of an inch in diameter and 3,770 yards long, covered with cotton thread, and making about 6,000 turns round the central core. This primary coil was inclosed in an ebonite tube ½-inch thick, and outside of the tube, occupying 4 feet 2 inches of its length, was the secondary coil, containing 150 miles of silk covered wire, ·015 inch diameter, and very carefully arranged for insulation, so as to resist the tension of the electricity when the coil was in action. The condenser contained 750 square feet of tin-foil, and 40 Bunsen cells supplied the current for the primary coil. The power of this instrument was very great, for it would give a spark through the air of more than two feet in length, and the discharge could perforate a certain thickness of glass. It would charge a battery of Leyden jars having 40 square feet of tin-foil by only three breaks of contact in the primary circuit, so that the discharge would deflagrate considerable lengths of wire. The appearance of the spark, with this, as with other large induction coils, may be described as a thick line of light, surrounded by a reddish halo of less brilliancy, and this halo, unlike the line of the spark, had a sensible duration. The reddish glow might be blown aside by a current of air when a series of discharges was taking place, and partly separated from the denser looking line of light. The latter is no doubt formed by intensely heated particles of the metals between which the discharge takes place, while the former is probably due to the incandescence of the oxygen and nitrogen gases in the air. The disc shown in our illustration behind the coil was for carrying six Geissler tubes, to display the pretty experiment of the various colours of the luminous discharge in different attenuated gases. When the coil was first mounted it was provided with an ordinary contact-breaker, but as the strong sparks were found to very soon destroy the contact points, a contact-breaker was substituted on Foucault’s plan. In this, the contacts are made by a platinum tipped wire dipping into mercury, that occupies the bottom of a strong glass vessel and forms part of the circuit. The vessel is filled with alcohol, which is a non-conductor, and it is therefore in the midst of this liquid 507that the contacts are made and broken. This apparatus is shown in the illustration, on the table at the left. A favourite experiment at the Polytechnic was to connect one of the discharging wires of the coil with the back of a large looking-glass, and bring the other wire to the front. In this case the sparks assumed a peculiar appearance, for they became thin and wiry-looking, and divided into many branches. They were very bright, and the noise of the discharges, was crackling and quite different from that produced by the blow of the flaming sparks taken through the air. Their appearance is represented in Fig. 269. The effects in this experiment were probably due to the spark taking a path on the surface of the glass determined by points of moisture or other inequalities.

Fig. 269.—Spark on the Looking-glass.

Ruhmkorff’s coil has been of great advantage to the electrician, for it supplies a stream of high tension electricity like that of the common machine, but more readily and conveniently. M. Ruhmkorff was the first person to obtain the great prize of £2,000, which the late Emperor of the French (Napoleon III.) directed, in 1852, should be awarded every five years for the most useful application of the voltaic battery. But no award had been made until 1864, when the inventor of the induction coil was properly considered worthy of it. This invention was the means of bringing into notice a new range of interesting phenomena, especially those attending the discharge passed through highly exhausted vessels. Investigations into the circumstances which modify the appearances, and especially into the nature of the stratified discharge in which the vessels are filled with bands or flakes of light separated by dark intervals, have long engaged the attention of some of our ablest physicists. Remarkable results were obtained by Mr. Crookes with very highly exhausted vessels. These showed not only beautiful fluorescent luminous effects, but in them the discharge could produce mechanical actions, and Mr. Crookes was led to regard it as a stream of radiant matter.

MAGNETO-ELECTRICITY.

When it had been shown that an electric current was capable of evoking magnetism, it seemed reasonable to expect that the reverse operation of obtaining electric currents by means of magnets should be possible. Faraday succeeded in solving this interesting problem in November, 1831, and one of his earliest, simplest, and most convincing experiments 508for the demonstration of the production of electricity by a magnet is represented in Fig. 270. A B is a strong horse-shoe magnet, C is a cylinder of soft iron, round which a few feet of silk-covered copper wire are wound; one end of the wire terminates in a little copper disc, and the other end is bent, as shown at D, so that it is in contact with the disc, but pressing so lightly against it that any abrupt movement of the bar causes the point of the wire and the disc to separate. When the bar is allowed to fall upon the poles of the magnet, the separation occurs, and again when it is suddenly pulled off; and on each occasion a very small but brilliant spark is observed where the contact of the wire and disc is broken. It was in allusion to this experiment that a contributor to “Blackwood’s Magazine” wrote:

Around the magnet, Faraday

Is sure that Volta’s lightnings play;

But how to draw them from the wire?

He took a lesson from the heart;

‘Tis when we meet, ‘tis when we part,

Breaks forth the electric fire.

Fig. 270.—Magneto-electric Spark.

Fig. 271.—A Magnet producing a Current.

Fig. 272.—Clarke’s Magneto-electric Machine.

If a coil of fine insulated wire be passed many times round a hollow cylinder, open at the ends, and the extremities of the wire connected with a galvanometer at some distance, then if into the axis of the coil, A B, Fig. 271, a steel magnet be suddenly introduced, an immediate deflection of the needle takes place; but after a few oscillations it returns to its former position. When the magnet is quickly withdrawn, the needle receives a momentary impulse in the opposite direction. The magnetization and demagnetization of the iron core in the induction coil would, therefore, of itself cause the induced currents already described, for these actions are equivalent to sudden insertion and withdrawal of a magnet. If we suppose 509C, in Fig. 271, to represent, not a magnet, but a piece of soft iron—the reader will remember that this soft iron can be, as often as required, magnetized and demagnetized by simply bringing near one end of it the pole of a permanent magnet (see page 484). Upon this principle many ingenious machines have been constructed for producing electric currents by the relative motions of magnets and of soft iron cores surrounded by wires. 510Clarke’s machine is shown in Fig. 272. A is a powerful steel magnet fixed to the upright. A brass spindle passing between the poles can be made to rotate very rapidly by the multiplying-wheel, E, on which a handle is fixed. There are two short cylinders of soft iron parallel to the spindle, united together by the transverse piece of iron, D, which turns with the spindle. Each bar is surrounded by a great length of insulated copper wire, and the ends of the wires are so connected with springs which press against a portion of the spindle, which is here partly formed of a non-conducting material, that the currents generated in the coils, although in different directions as they approach a pole and recede from it, are nevertheless made to flow in one direction in the external circuit. R R in the figure represent two brass handles, which are grasped by a person wishing to experience the shocks the machine can give when the wheel is turned. When the terminals of the coil are provided with insulating handles and connected with pointed pencils of charcoal, the electric light can readily be produced by expenditure of mechanical effort in turning the handle. The arrangement of the points for this purpose is shown in Fig. 273, and we shall presently see what advantage has been drawn from this experiment on a great scale as a source of light.

Fig. 273.—Magneto-electric Light.

It will be observed that during the revolution of the armatures, as the wire-covered iron cores are termed, there are two maximum and two minimum points at which the currents are strongest and weakest. These variations may be lessened by increasing the number of armatures and of magnets, and Mr. Holmes arranged a machine with eighty-eight coils and sixty-six magnets, and the connections were so contrived that the currents always flowed in the same direction in the external circuit. This machine required 1¼ horse-power to drive it when the currents were flowing, but much less when the circuit was interrupted, and it was designed for, and successfully applied to, the production of the electric light for lighthouse illumination. Instead of steel magnets which gradually lose their strength, it is obvious that electro-magnets might be employed, but this source of electricity is costly, troublesome, and inconstant. Mr. Wilde hit upon the idea of using a small magneto-electric machine with permanent steel magnets, to generate the current for exciting a larger electro-magnet, and the current from this produced a still more powerful electro-magnet, 511from which a magneto-electric current could be collected and applied. The same idea was subsequently applied in other forms, as by shunting off a portion of the current produced from the mere residual magnetism of an electro-magnet, to pass through its own coils and evoke a stronger magnetism, which again reacts by producing a more powerful current, and so on continually; the limit being dependent only on the mechanical force employed, and on the power of the wires to convey the electricity, for they become very hot, and, unless artificially cooled, the insulating material would be destroyed. The armatures used in Wilde’s, Ladd’s, and other machines of this kind, are quite different in arrangement from those of Clarke’s machine, and are far superior. They are formed of a long bar of soft iron, of a section like this, Ꮋ, and the wire is wound longitudinally between the flanges from end to end of the bar, up one side and down the other. This armature rotates about its longitudinal axis between the pairs of the poles of a file of horse-shoe magnets, either permanent, or electro-magnets excited by the magneto-electric currents. In this case opposite poles are induced along the edges of the bar, and these poles are reversed at each half-turn. The intensity of the induced currents increases with the velocity with which the armature is made to revolve up to a certain point; but because the magnetization of the soft iron requires a sensible time to be effected, and the poles are reversed at every half-turn, it is found that a speed increasing beyond the limit is attended by decrease of the intensity of the current. The intensity in such machines has, therefore, a definite limit. But in a modification of the magneto-electric machine, which has quite recently been invented by M. Gramme, the limit is vastly extended by the ingenious disposition of the iron core and armatures, and his machines appear to solve the problem of the cheap production of steady and powerful electric currents, so that electricity will soon be applied in processes of manufacture where the cost of electrical power has hitherto placed it out of the question. We shall now endeavour to explain the principle on which the Gramme machine depends, and describe some forms in which it is constructed.

THE GRAMME MAGNETO-ELECTRIC MACHINE.

Fig. 274.

Fig. 275.—Gramme Machine for the Laboratory or Lecture Table.

Let X, Fig. 274, be a coil of covered wire; then while a bar magnet, B A, is advancing towards it and passing through it, as at M, a current will flow through the coil and along a wire connecting its ends, s s. The current will change its direction as the centre of the magnet is leaving the coil 512to advance in the direction, B A. If A A´ be a bar of soft iron, with the coil fixed upon it, we can still excite currents in the coil by magnetizing the bar inductively. If the pole of a permanent magnet be carried along from A´ to M in a direction parallel to the bar, but not touching it, the part of the bar immediately opposite will be a pole of opposite name, and the advance of this induced pole towards M will be attended with a current in the coil, and its recession by an opposite current. It need hardly be mentioned that the same result is attained if the magnetic pole is stationary, and the bar with the coil upon it moved in proximity to it. Now imagine that the bar is bent into a ring, the ends, A A´, being united. If the ring be made to turn round its centre in its own plane, and near a magnetic pole, it is plain that when the coil is approaching this pole a current will be produced in it, and when it is receding, an opposite current. Let the number of coils be increased, and each coil in turn will be the seat of a current, or of the electrical state which tends to produce a current. In Fig. 275 the reader may see how this disposition is realized. The figure shows a form of the Gramme Machine adapted for the lecture-table or laboratory. A M´ B M is the soft iron ring, covered with a series of separate coils placed radially, O is a compound horse-shoe steel magnet, S its south pole, N its north pole, each pole being armed with a block of soft iron hollowed into the segment of a circle and almost completely embracing the circle of coils. The magnetism of each pole is strongly developed in the interior faces of these armatures. The inductive action tends to produce two equal and opposite currents, which, like the currents of two similar voltaic batteries joined by their like poles, neutralize 513each other in the connected coils, but flow together through an external circuit. Fig. 276 will make clear the manner in which the coils, B B, are placed on the ring, A. The length of wire in each coil is the same, and the extremities are attached to strips of copper, R R, which are fixed on the spindle of the machine. The two ends of each wire are connected with two consecutive strips, while the coils are insulated from each other, and thus each coil, like the element of a battery, contributes to the aggregate current. The currents are drawn off, as it were, from these axial conductors at two opposite points of the ring, by springs very lightly touching them on each side of the spindle, as may be seen in Fig. 275. In Fig. 277 is another arrangement of the table apparatus with the magnet vertical, and formed according to the new plan suggested by M. Jamin, who finds the best magnets are made by tying together thin strips of steel.

Fig. 276.—Insulated Coils surrounding an Annulus of Iron Wires.

Fig. 277.—Hand Gramme Machine, with Jamin’s Magnet.

514But the importance of this invention consists in the facility which it affords for cheaply producing electricity on a scale adapted for industrial operations, for the deposition of metals, for artificial light, and for chemical purposes. The great importance of a cheap electric light for lighthouses prompted the British Government to permit the inventor to exhibit the light thus produced from the Clock Tower of the Houses of Parliament; for the signal light during the sittings of the House had previously been produced by a gas-light. This electric light was produced by a powerful Gramme machine, such as that shown in Fig. 278, driven by a small steam engine in the vaults of the Houses of Parliament, and the ordinary carbon points, reflectors, &c., were used in the Clock Tower, where the light was exhibited; copper wire ½ inch diameter being used to convey the current from the machine to the carbons. The result of these experiments may be gathered from the following extract from an official report made by the engineers of the Trinity House:

“Pursuant to the instructions received from the Deputy Master to furnish you with my opinion on the relative merits of the electric and gas lights under trial at the Clock Tower, Westminster, I beg to submit the following report:—On the evening of the 1st ultimo I was accompanied by Sir F. Arrow (who kindly undertook to check my observations by his experience) to the Westminster Palace, where we met Captain Galton, R.E., Dr. Percy, and some gentlemen connected with the electric and gas apparatus under trial. I was informed that the stipulations under which the lights were arranged were, that they be fixed white to illuminate a sector of the town surface of 180°, having a radius of three miles. I first examined the Gramme magneto-electric machine, in use for producing the currents of electricity. This machine we found attached by a leather driving-belt to the steam engine belonging to the establishment. We then proceeded to the Clock Tower, where we found the electric lamp, at an elevation of 250 ft. The Wigham gas apparatus was placed at the same elevation, within a semi-lantern of twelve sides, about 8½ ft. in diameter, and 10 ft. 3 in. high in the glazing. Near the centre of the lantern were three large Wigham burners, each composed of 108 jets. After the examination of the apparatus, we proceeded to Primrose Hill, for the purpose of comparing the electric and gas lights at a distance of three miles. The evening, which was wet and rather misty, was admirably suited to our purpose, ordinary gas-lights being barely visible at a distance of one mile.”

The results of a photometric comparison of the electric and gas lights were as under, the machine making 389 revolutions per minute, and absorbing 2·66 horse-power; the illuminating power of the gas used being 25 candles, and the quantity consumed 300 cubic ft. per hour.

Electric Light. Wigham Gas Burner. 108 jets.

Relative intensity of lights 945·56 370·56

Or as 100 39·19

Illuminating power in standard sperm candles as units 3,066 1,199

515“Electric Light.—Total cost per session £174 5s. 0d., being equal to 5s. 7d. per hour of exhibition of the light. Details shown in the full report. Gas Light.—Total cost per session of one burner of 108 jets, £159 15s. 3d., equal to 5s. 1·4d. per hour of exhibition of light, and £296 3s. 4d., equal to 9s. 5·9d. per hour of exhibition of the light, when using three burners of 108 jets each. Details shown in the full report. It will be observed from the photometric measurements, before referred to, of the electric light and 108–jet gas burner, that in the case of the electric light we have at our disposal for distribution over the required area an illuminant radiating freely in space equal to 3,066 candles; with the gas light we have an illuminant radiating freely in space equal to 1,199 candles. It is to be remembered that in dealing with the small electric spark as the focus of a dioptric apparatus for distribution over the required area, the light can be more perfectly utilized than with the large gas flame of the Wigham burner, owing to its very small dimensions as compared with the latter. The relative cost and efficiency of the three modes of illumination may be summed up as follows:

Electric Light. Gas.

One 108–jet Three 108–jet

Burner. Burners.

Cost of light per hour, in pence 67 61·4 113·9

Or as 100 91·6 170

Cost of light per candle per hour in pence ·0219 ·0512 ·0317

Or as 100 233·8 144·7

Cost of light from a dioptric apparatus for fixed light per standard candle per hour expressed in pence ·00118 ·00310 ·00275

Or as 100 262·7 233·1

“Thus by adopting the electric light as a standard of intensity and cost, there is shown a superiority over the gas in intensity of 65·2 per cent. when using one 108–jet burner, and 27·1 per cent. when using three 108–jet burners. There is also shown a saving in cost per candle or unit of light per hour of 162·7 per cent. when using one 108–jet burner, and 133·1 per cent. when using three of these burners, forming a triform gas-light. It is further to be remembered that the triform gas-light actually represents the maximum power obtainable at present by gas; but no reference has been made to the power of increase capable in the electric light by the adoption of two magneto-electric machines. By having the machine and lamp in duplicate, as estimated, and which I consider a necessity to insure perfect confidence in the regular exhibition of the electric light, this light can be doubled in intensity during such evenings as the atmosphere is found to be so thick as to impair its efficiency. This double power would be obtained at the trifling additional cost of coals and carbons consumed during the time this increased power may be found to be necessary; this additional cost I estimate at 4d. per hour. With the arrangement proposed 516for the electric light, I consider this powerful illuminant, if manipulated by 517careful attendants, perfectly reliable: in proof of this I may state that the electric light at the Souter Point Lighthouse, on the coast of Durham, has now been exhibited two years and a half, and the light has never been known to fail for one minute.”

Fig. 278.—Gramme Machine, with Eight Vertical Electro-magnets.

Fig. 279.—Gramme Machine, with Horizontal Electro-magnets.

Fig. 278 represents one of the light-producing machines. The electro-magnets are excited by a portion of the currents they themselves produce, they retaining sufficient residual magnetism to develop the currents. There is a pair of current-collectors on each side. This machine weighs 1,540 lbs., its height is 3 ft., and width 2 ft. It will produce a light having the intensity of 500 Carcel lamps, which may be doubled by increasing the speed. Fig. 279 is another form which is also adapted for illuminating purposes, and, when made with fewer coils, for electrotyping purposes also. There are in this also two sets of current-collectors, and by means of a connecting cylinder (seen at the base of the machine) the currents can be combined for quantity and for tension as may be required. This machine is only about 2 ft. square, and it produces a light equal to 200 burners; but this may be increased, as the following table shows:

518

Number of revolutions per minute. Intensity of light in Carcel Lamps. Remarks.

650 77 No heating and no sparks.

850 125 No heating and no sparks.

880 150 No heating and no sparks.

900 200 No heating and no sparks.

935 250 A little heat, no sparks.

1,025 290 Heat and sparks.

The value of M. Gramme’s invention for electro-plating is proved by the fact of its adoption by Messrs. Christofle of Paris, whose electro-plating establishment is one of the largest in the world. This firm has no fewer than fourteen of these machines at work, and each is capable of depositing 74 ozs. of silver per hour. There is little doubt that the electric current will now soon be employed for reducing metals. Thus fine copper, which is worth 3s. or 4s. per lb., may perhaps be obtained at about the cost of ordinary copper; potassium, sodium, and aluminium at less than half their present price; and magnesium, calcium, and other rare metals at prices which will bring them into commercial use. The machine shown in Fig. 280 is intended for electro-plating and for general purposes: it supplies the means of readily and cheaply plating with copper, or with any other metal, such articles as steam pipes, boiler tubes, ship plates, guns, bolts, nails, marine engines, machinery, culinary vessels, cisterns, &c. The advantage of protecting iron or other material from corroding agents is obvious; and as iron coated with copper is available not only for useful, but also for artistic, purposes, as a cheap substitute for bronze, this invention will doubtless lead to a greatly extended application of bronzed iron in buildings and ornamental structures.

The machine well illustrates how mechanical work may be changed into electricity, and electricity caused to do work. The power required to drive the machine at a given speed is much less when no current is being drawn from it, than when the current is flowing. If the current from one machine is sent through the armature of another, the latter revolves, and may be made to do work. Thus power may be conveyed to a distance by electricity, with only the loss caused by the resistance of the conducting wires. If, when two machines are thus connected, the direction of rotation in the first one be suddenly reversed, the armature of the second will almost immediately stop, and then resume its motion in the opposite direction. A very interesting experiment can be performed when the circuit connecting the two machines is made to include a certain length of platinum wire. When both machines are in motion, the platinum exhibits no heating effects; but if the second machine be stopped by an assistant while the rotation of the first is continued, the wire is raised to a red heat. In this way it is shown that motion, electricity, and heat are related to each other, and are mutually convertible; for on the stopping of the second machine, the electricity being no longer used up, so to speak, in producing motion, has its power transformed into heat.