Part 33

Mr. Grove considers that the colour of the voltaic arc, or flame, which appears between the poles of a very powerful battery, depends upon the substance of the metal from whence it proceeds and on the medium through which it passes. The spark from zinc is blue, from silver it is green, from iron it is red and scintillating—precisely the colours afforded by these metals in their ordinary combustion. But the colour varies also with the medium through which the light passes, for when the medium is changed a change takes place in the colour, showing an affection of the intervening matter. A portion of the metal terminals or poles is actually transmitted with every electrical or Voltaic discharge, whence Mr. Grove concludes that the electrical discharge arises, at least in part, from an actual repulsion and severance of the electrified matter itself, which flies off at the points of least resistance. He observes that “the phenomena attending the electric spark or Voltaic arc tends to modify considerably our previous idea of the nature of the electric force as a producer of ignition and combustion. The Voltaic arc is perhaps, strictly speaking, neither ignition nor combustion. It is not simply ignition; because the matter of the terminals is not merely brought to a state of incandescence, but is physically separated, and partially transferred from one terminal to another, much of it being dissipated in a vaporous state. It is not combustion; for the phenomena will take place independently of atmospheric air, oxygen gas, or any of the bodies usually called supporters of combustion; combustion being in fact chemical union attended with heat and light. In the Voltaic arc we may have no chemical union, for if the experiment be performed in an exhausted receiver, or in nitrogen, the substance forming the terminals is condensed and precipitated upon the interior of the vessel, in, chemically speaking, an unaltered state. Thus, to take a very striking example, if the Voltaic discharge be taken between zinc terminals in an exhausted receiver, a fine black powder of zinc is deposited on the sides of the receiver; this can be collected, and takes fire readily in air by being touched with a match, or ignited wire, instantly burning into white oxide of zinc. To an ordinary observer the zinc would appear to be burned twice—first in the receiver, where the phenomenon presents all the appearance of combustion, and, secondly, in the real combustion in air. With iron the experiment is equally instructive. Iron is volatilized by the Voltaic arc in nitrogen, or in an exhausted receiver; and when a scarcely perceptible film has lined the receiver, if it be washed with an acid, it then gives, with ferrocyanide of potassium, the Prussian-blue precipitate. In this case we readily distil iron, a metal by ordinary means _fusible_ only at a very high temperature.”

Another strong evidence that the Voltaic discharge consists of the material itself of which the terminals are composed, is the peculiar rotation which is observed in the light when iron is employed, the magnetic character of this metal causing its particles to rotate by the influence of the Voltaic current. In short, Mr. Grove concludes that, although it would be hasty to assert that the electrical disruptive discharge can in no case take place without the terminals being affected, yet he had met with no instance of such a result, provided the discharge had been sufficiently prolonged, and the terminals in such a state as could be expected to render manifest slight changes![15]

Some years ago Mr. Grove discovered that the electrical discharge possesses certain phases or fits of an alternate character, forming rings of alternate oxidation and deoxidation on metallic surfaces. A highly polished silver plate in an air-pump was connected with the pole of a powerful inductive battery, while a fine metallic wire, or even a common sewing needle, was fixed at the other pole, and so arranged as to be perpendicular to the silver plate, and very near, but not touching it. By means of this apparatus the electrical discharge could be sent through any kind of rarefied media. In some of the experiments a series of concentric coloured rings of oxide alternating with rings of polished or unoxidated silver were formed on the plate under the point of the needle or wire. When the plate was previously coated with a film of oxide, the oxide was removed in concentric spaces by the discharge, and increased on the alternate ones, showing an alternate positive and negative electricity, or electricity of an opposite character in the same discharge.

When the silver plate was polished the centre of the rings formed on it was yellow-green surrounded by blue-green; then a ring of polished silver, followed by a crimson ring with a slight orange tint on the inner side and deep purple on the outer; lastly the indication of a polished one. When the air-pump was filled with attenuated olefiant gas the rings were precisely the same with those seen in thin plates; hence the effect is the same as that produced by the interference of light. In these experiments the luminous appearance extended from three quarters of an inch to an inch round the point of the needle or wire.

When the silver plate was connected with the negative pole of the battery a polished point appeared upon it opposite the needle, surrounded by a dusky ill-defined areola of a brown colour tinged with purple when viewed in one direction, and greenish-white when seen in another.

In the present year Mr. Gassiot, Vice-President of the Royal Society, has shown that the stratified character of the electric discharge is remarkably developed in the Torricellian vacuum. Among the various experiments made by that gentleman two may be selected as strongly illustrative of this new and singular property of electrical light.

In a closed glass tube about an inch internal diameter and 38 inches long, in which a vacuum had been made, two platinum wires were hermetically sealed, 32 inches apart, and connected with the poles of an inductive battery. The luminous appearance at the two poles was very different when electricity passed through the wires. A glow surrounded the negative pole, and in close approximation to the glow a well-defined dark space appeared, while from the positive pole or wire the light proceeded in a stream; but unless the charge be great or the tube short, the stream will not extend to the black band, which is totally different from the intervening space. When discharges of electricity were sent through this vacuum tube a series of bands or stratifications were formed which were concave towards the positive pole; and as in the changes in making and breaking the circuit the electricity emanates from the different terminals or wires, their concavities were in opposite directions.

When instead of platinum wires narrow tinfoil coatings were placed round the exterior of the glass tube and connected with the wires of the battery, brilliant stratifications filled the interior of the tube between the foil coatings, but no dark band appeared. At present Mr. Gassiot is inclined to believe that the dark band is due to interference; but that the stratifications arise from pulsations or impulses of a force acting in a highly attenuated but resisting medium, for even with the best air-pumps it is impossible to make a perfect void; he is still occupied with experiments on this new subject, and no doubt will obtain very remarkable results, of which none can be more extraordinary than his discovery of the powerful influence of the magnet on this electric light. The stratifications are formed in rapid succession in the tube with platinum wires and are turned different ways, but they can be separated at any part of the tube by the pole of a magnet round which the whole stratifications have a tendency to revolve. In the second experiment, where the tinfoil was used, the discharge was divided in two by the pole of a magnet, and the two parts had a tendency to rotate round the magnet in opposite directions.

Voltaic electricity is a powerful agent in chemical analysis. When transmitted through conducting fluids, it separates them into their constituent parts, which it conveys in an invisible state through a considerable space or quantity of liquid to the poles, where they come into evidence. Numerous instances might be given, but the decomposition of water is perhaps the most simple and elegant. Suppose a glass tube filled with water, and corked at both ends; if one of the wires of an active Voltaic battery be made to pass through one cork, and the other through the other cork, into the water, so that the extremities of the two wires shall be opposite and about a quarter of an inch asunder, chemical action will immediately take place, and gas will continue to rise from the extremities of both wires till the water has vanished. If an electric spark be then sent through the tube, the water will reappear. By arranging the experiment so as to have the gas given out by each wire separately, it is found that water consists of two volumes of hydrogen and one of oxygen. The hydrogen is given out at the positive wire of the battery, and the oxygen at the negative. The oxides are also decomposed; the oxygen appears at the positive pole, and the metal at the negative. The decomposition of the alkalies and earths by Sir Humphry Davy formed a remarkable era in the history of science. Soda, potash, lime, magnesia, and other substances heretofore considered to be simple bodies incapable of decomposition, were resolved by electric agency into their constituent parts, and proved to be metallic oxides, by that illustrious philosopher. All chemical changes produced by electricity are accomplished on the same principle; and it appears that, in general, combustible substances, metals, and alkalies go to the negative wire, while acids and oxygen are evolved at the positive. The transfer of these substances to the poles is not the least wonderful effect of the Voltaic battery. Though the poles be at a considerable distance from one another, nay, even in separate vessels, if a communication be only established by a quantity of wet thread, as the decomposition proceeds the component parts pass through the thread in an invisible state, and arrange themselves at their respective poles. According to Dr. Faraday, electro-chemical decomposition is simply a case of the preponderance of one set of chemical affinities more powerful in their nature over another set which are less powerful. And in electro-chemical action of any kind produced by a continuous current, the amount of action in a given time is nearly, if not rigorously, proportional to the strength of the current. The great efficacy of Voltaic electricity in chemical decomposition arises not from its tension, but from the quantity set in motion and the continuance of its action. Its agency appears to be most exerted on fluids and substances which by conveying the electricity partially and imperfectly impede its progress. But it is now proved to be as efficacious in the composition as in the decomposition or analysis of bodies.

It had been observed that, when metallic solutions are subjected to galvanic action, a deposition of metal, sometimes in the form of minute crystals, takes place on the negative wire. By extending this principle, and employing a very feeble Voltaic action, M. Becquerel has succeeded in forming crystals of a great proportion of the mineral substances, precisely similar to those produced by nature. The electric state of metallic veins makes it possible that many natural crystals may have taken their form from the action of electricity bringing their ultimate particles, when in solution, within the narrow sphere of molecular attraction. Both light and motion favour crystallization. Crystals which form in different liquids are generally more abundant on the side of the jar exposed to the light; and it is well known that still water, cooled below 32°, starts into crystals of ice the instant it is agitated. A feeble action is alone necessary, provided it be continued for a sufficient time. Crystals formed rapidly are generally imperfect and soft, and M. Becquerel found that even years of constant Voltaic action were necessary for the crystallization of some of the hard substances. If this law be general, how many ages may be required for the formation of a diamond!

The deposition of metal from a metallic solution by galvanic electricity has been most successfully applied to the arts of plating and gilding, as well as to the more delicate process of copying medals and copper plates. Indeed, not medals only, but any object of art or nature, may be coated with precipitated metal, provided it be first covered with the thinnest film of plumbago, which renders a non-conductor sufficiently conducting to receive the metal. Photo-galvanic engraving depends upon this. Gelatine mixed with bichromate of potash, nitrate of silver, and iodide of potassium, is spread over a plate of glass, and when dry a positive print is laid upon it with its face downwards, which, when exposed to the sun, leaves its impression. When soaked in water the gelatine swells around all those parts where the light had fallen, thus forming an intaglio, a cast of which is taken in gutta-percha, which is then coated with copper by the electro process, whence a copper plate in relief is obtained.

Static electricity, on account of its high tension, passes through water and other liquids as soon as it is formed, whatever the length of its course may be. Voltaic electricity, on the contrary, is weakened by the distance it has to traverse. Pure water is a very bad conductor; but ice absolutely stops a current of Voltaic electricity altogether, whatever be the power of the battery, although static or common electricity has sufficient power to overcome its resistance. Dr. Faraday has discovered that this property is not peculiar to ice; that, with a few exceptions, bodies which do not conduct electricity when solid acquire that property, and are immediately decomposed, when they become fluid, and, in general, that decomposition takes place as soon as the solution acquires the capacity of conduction, which has led him to suspect that the power of conduction may be only a consequence of decomposition.

Heat increases the conducting power of some substances for Voltaic electricity, and of the gases for both kinds. Dr. Faraday has given a new proof of the connexion between heat and electricity, by showing that, in general, when a solid, which is not a metal, becomes fluid, it almost entirely loses its power of conducting heat, while it acquires a capacity for conducting electricity in a high degree. M. Becquerel regards the production of heat and that of electricity to be concomitant; their dependence being such, that when one is increased the other diminishes, and _vice versâ_, so that one may altogether disappear with the increase of the other. For instance, when electricity circulates in a metallic wire, the greater the heat produced, the less the quantity of electricity which passes, and the contrary, so that the affair proceeds as if electricity were converted into heat, and heat into electricity. Again, in a closed galvanic circuit the sum of the heat produced in the chemical action of the acidulated water upon the zinc and in the conducting wire is constant, so that the quantity of heat disengaged in the reaction is greater in proportion as less electricity passes through the wire. These, and other circumstances, prove such an intimate connexion between the production of heat and electricity, that in the change of condition of substances the electrical effects might disappear or be annulled by the calorific effects.

The galvanic current affects all the senses: nothing can be more disagreeable than the shock, which may even be fatal if the battery be very powerful. A bright flash of light is perceived with the eyes shut, when one of the wires touches the face, and the other the hand. By touching the ear with one wire, and holding the other, strange noises are heard; and an acid taste is perceived when the positive wire is applied to the tip of the tongue, and the negative wire touches some other part of it. By reversing the poles the taste becomes alkaline. It renders the pale light of the glow-worm more intense. Dead animals are roused by it, as if they started again into life, and it may ultimately prove to be the cause of muscular action in the living.

Several fish possess the faculty of producing electrical effects. The most remarkable are the gymnotus electricus, found in South America; and the torpedo, a genus of ray, frequent in the Mediterranean. The electrical action of the torpedo depends upon an apparatus apparently analogous to the Voltaic pile, which the animal has the power of charging at will, consisting of membranous columns filled throughout with laminæ, separated from one another by a fluid. The absolute quantity of electricity brought into circulation by the torpedo is so great, that it effects the decomposition of water, has power sufficient to make magnets, gives very severe shocks and the electric spark. It is identical in kind with that of the galvanic battery, the electricity of the under surface of the fish being the same with the negative pole, and that in the upper surface the same with the positive pole. Its manner of action is, however, somewhat different; for, although the evolution of the electricity is continued for a sensible time, it is interrupted, being communicated by a succession of discharges.

SECTION XXX.

Discovery of Electro-magnetism—Deflection of the Magnetic Needle by a Current of Electricity—Direction of the Force—Rotatory Motion by Electricity—Rotation of a Wire and a Magnet—Rotation of a Magnet about its Axis—Of Mercury and Water—Electro-Magnetic Cylinder or Helix—Suspension of a Needle in a Helix—Electro-Magnetic Induction—Temporary Magnets—The Galvanometer.

THE disturbing effects of the aurora and lightning on the mariner’s compass had been long known. In the year 1819 M. Oersted, Professor of Natural Philosophy at Copenhagen, discovered that a current of Voltaic electricity exerts a powerful influence on a magnetized needle. This observation has given rise to the theory of electro-magnetism—one of the most interesting sciences of modern times, whether it be considered as leading us a step farther in generalization, by identifying two agencies hitherto referred to different causes, or as developing a new force, unparalleled in the system of the world, which, overcoming the retardation from friction, and the obstacle of a resisting medium, maintains a perpetual motion as long as the action of a Voltaic battery is continued.

When the two poles of a Voltaic battery are connected by a metallic wire, so as to complete a circuit, the electricity flows without ceasing. If a straight portion of that wire be placed parallel to, and horizontally above, a magnetized needle at rest in the magnetic meridian, but freely poised like the mariner’s compass, the action of the electric current flowing through the wire will instantly cause the needle to change its position. Its extremity will deviate from the north towards the east or west, according to the direction in which the current is flowing; and, on reversing the direction of the current, the motion of the needle will be reversed also. The numerous experiments that have been made on magnetism and electricity, as well as those on the various relative motions of a magnetic needle under the influence of galvanic electricity, arising from all possible positions of the conducting wire, and every direction of the Voltaic current, together with all the other phenomena of electro-magnetism, are explained by Dr. Roget in some excellent articles on these subjects in the Library of Useful Knowledge.

All experiments tend to prove that the force emanating from the electric current, which produces such effects on the magnetic needle, acts at right angles to the current. The action of an electrical current upon either pole of a magnet has no tendency to cause the pole to approach or recede, but to rotate about it. If the stream of electricity be supposed to pass through the centre of a circle whose plane is perpendicular to the current, the direction of the force exerted by the electricity will always be in the tangent to the circle, or at right angles to its radius (N. 223). Consequently, the tangential force of the electricity has a tendency to make the pole of a magnet move in a circle round the wire of the battery.

Rotatory motion was suggested by Dr. Wollaston. Dr. Faraday was the first who actually succeeded in making the pole of a magnet rotate about a vertical conducting wire. In order to limit the action of the electricity to one pole, about two-thirds of a small magnet were immersed in mercury, the lower end being fastened by a thread to the bottom of the vessel containing the mercury. When the magnet was thus floating almost vertically with its north pole above the surface, a current of positive electricity was made to descend perpendicularly through a wire touching the mercury, and immediately the magnet began to rotate from left to right about the wire. The force being uniform, the rotation was accelerated till the tangential force was balanced by the resistance of the mercury, when it became constant. Under the same circumstances the south pole of the magnet rotates from right to left. It is evident, from this experiment, that the wire may also be made to perform a rotation round the magnet, since the action of the current of electricity on the pole of the magnet must necessarily be accompanied by a corresponding reaction of the pole of the magnet on the electricity in the wire. This experiment has been accomplished by a vast number of contrivances, and even a small battery, consisting of two plates, has performed the rotation. Dr. Faraday produced both motions at the same time in a vessel containing mercury; the wire and the magnet revolved in one direction about a common centre of motion, each following the other.