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It was shown by Grotthus that the transmission of Voltaic electricity through liquids consists of a series of chemical affinities acting in definite directions; and Mr. Grove, from an examination of its action on the various kinds of matter, has come to the same conclusion. Indeed it is now the generally received opinion that a current of electricity is merely a continuous transmission of chemical affinity from particle to particle of the substance through which it is passing, and consequently that it is a continuous transmission of force. As the Voltaic battery has become one of the most important engines of physical research, some account of its present condition may not be out of place.
The disturbance of electric equilibrium, and a development of electricity, invariably accompany the chemical action of a fluid on metallic substances, and the electricity is most plentiful when that action occasions oxidation. Metals vary in the quantity of electricity afforded by their combination with oxygen. But the greatest abundance is developed by the oxidation of zinc by weak sulphuric acid. And, in conformity with the law that one kind of electricity cannot be evolved without an equal quantity of the other being brought into activity, it is found that the acid is positively, and the zinc negatively electric. It has not yet been ascertained why equilibrium is not restored by the contact of these two substances, which are both conductors, and in opposite electrical states. However, the electrical and chemical changes are so connected, that, unless equilibrium be restored, the action of the acid will go on languidly, or stop as soon as a certain quantity of electricity is accumulated in it. Equilibrium, nevertheless, will be restored, and the action of the acid will be continuous, if a plate of copper be placed in contact with the zinc, both being immersed in the fluid; for the copper, not being acted upon by the acid, will serve as a conductor to convey the positive electricity from the acid to the zinc, and will at every instant restore the equilibrium, and then the oxidation of the zinc will go on rapidly. Thus three substances are concerned in forming a Voltaic circuit, but it is indispensable that one of them should be a fluid. The electricity so obtained will be very feeble in overcoming resistances offered by imperfect conductors interposed in the circuit, or by very long wires, but it may be augmented by increasing the number of plates. In the common Voltaic battery, the electricity which the fluid has acquired from
the first plate of zinc exposed to its action is taken up by the copper plate belonging to the second pair, and transferred to the second zinc plate, with which it is connected. The second plate of zinc, possessing equal powers, and acting in conformity with the first, having thus acquired a larger portion of electricity than its natural share, communicates a larger quantity to the fluid in the second cell. This increased quantity is again transferred to the next pair of plates; and thus every succeeding alternation is productive of a further increase in the quantity of the electricity developed. This action, however, would stop unless a vent were given to the accumulated electricity, by establishing a communication between the positive and negative poles of the battery by means of wires attached to the extreme plate at each end. When the wires are brought into contact, the Voltaic circuit is completed, the electricities meet and neutralize each other, producing the shock and other electrical phenomena; and then the electric current continues to flow uninterruptedly in the circuit, as long as the chemical action lasts. The stream of positive electricity flows from the zinc to the copper. The construction and power of the Voltaic battery have been much improved of late years, but the most valuable improvement is the constant battery of Professor Daniell. In all batteries of the ordinary construction, the power, however energetic at first, rapidly diminishes, and ultimately becomes very feeble. Professor Daniell found that this diminution of power is occasioned by the adhesion of the evolved hydrogen to the surface of the copper, and by the precipitation of the sulphate formed by the action of the acid on the zinc. He prevents the latter by interposing between the copper and the zinc, in the cell containing the liquid, a membrane which, without impeding the electric current, prevents the transfer of the salt; and the former, by placing between the copper and the membrane solution of sulphate of copper, which being reduced by the hydrogen prevents the adhesion of this gas to the metallic surface. Each element of the battery consists of a hollow cylinder of copper, in the axis of which is placed a cylindrical rod of zinc; between the zinc and the copper a membranous bag is placed, which divides the cell into two portions, the inner of which is filled with dilute acid, and the one nearer the copper is supplied with crystals of the sulphate of that metal. The battery consists of several of these elementary cells connected together by metallic wires, the zinc rod of one with the copper cylinder of thet
next to it. The zinc rods are amalgamated, so that local action, which, in ordinary cases, is so destructive of the zinc, does not take place, and no chemical action is manifested unless the circuit be completed. The rods are easily detached, and others substituted for them when worn out. This battery, which possesses considerable power, and is constant in its effects for a very long time, is greatly superior to all former arrangements, either as an instrument of research, or for exhibiting the ordinary phenomena of Voltaic electricity.
A battery charged with water alone, instead of acid, is constant in its action, but the quantity of electricity it develops is comparatively very small. Mr. Cross, of Broomfield in Somersetshire, kept a battery of this kind in full force during twelve months. M. Becquerel had invented an instrument for comparing the intensities of the different kinds of electricity by means of weights; but, as it is impossible to make the comparison with Voltaic electricity produced by the ordinary batteries, on account of the perpetual variation to which the intensity of the current is liable, he has constructed a battery which affords a continued stream of electricity of uniform power, but it is also of very feeble force. The current is produced by the chemical combination of an acid with an alkali.
Metallic contact is not necessary for the production of Voltaic electricity, which is entirely due to chemical action. The intensity of the Voltaic electricity is in proportion to the intensity of the affinities concerned in its production, and the quantity produced is in proportion to the quantity of matter which has been chemically active during its evolution. Dr. Faraday considers this definite production to be one of the strongest proofs that electricity is of chemical origin.
Galvanic or Voltaic electricity is manifested by two continuous forces or currents passing in opposite directions through the circuit: the zinc is the positive end or pole of the battery, and the copper the negative.
Voltaic electricity is distinguished by two marked characters. Its intensity increases with the number of plates, its quantity with the extent of their surfaces. The most intense concentration of force is displayed by a numerous series of large plates: light and heat are copiously evolved, and chemical decomposition is accomplished with extraordinary energy; whereas the electricity from one pair of plates, whatever their size may be, is so feeble
that it gives no sign either of attraction or repulsion. Common or static electricity is of greater intensity and has a greater power of overcoming resistance than Voltaic electricity, but it acts upon a smaller quantity of matter. However, by diminishing the size of the plates, and increasing their number, the intensity of a battery may be increased till it becomes equal to that of the electrical machine.
The action of Voltaic electricity differs in some respects materially from that of the ordinary kind. When a quantity of common electricity is accumulated, the restoration of equilibrium is attended by an instantaneous violent explosion, accompanied by the development of light, heat, and sound. The concentrated power of the electricity forces its way through every obstacle, disrupting and destroying the cohesion of the particles of the bodies through which it passes, and occasionally increasing its destructive effects by the conversion of fluids into steam from the intensity of the momentary heat, as when trees are torn to pieces by a stroke of lightning. Even the vivid light which marks the path of the electricity is probably owing in part to the sudden compression of the air and the rapidity of its passage. But the instant equilibrium is restored by this energetic action the whole is at an end. On the contrary, when an accumulation takes place in a Voltaic battery, equilibrium is restored the moment the circuit is completed. But so far is the electric stream from being exhausted, that it continues to flow silently and invisibly in an uninterrupted current supplied by a perpetual reproduction. And, although its action on bodies is neither so sudden nor so intense as that of common electricity, yet it acquires such power from constant accumulation and continued action, that it ultimately surpasses the energy of the other. The two kinds of electricity differ in no circumstance more than in the development of heat. Instead of a momentary evolution, the circulation of the Voltaic electricity is accompanied by a continued development of heat, lasting as long as the circuit is complete, without producing either light or sound. Its intensity from a very powerful battery is greater than that of any heat that can be obtained by artificial means, so that it fuses substances which resist the action of the most powerful furnaces. The temperature of every part of a Voltaic battery itself is raised during its activity. With the greater number of metals Mr. Grove
found that the positive terminal or pole is hotter than the negative.
According to Mr. Joule, the quantity of heat generated in a unit of time is proportional to the strength of the current, and when a galvanic current is employed in chemical analysis, the heat in the entire circuit generated in a unit of time is equal to the work expended in producing it, minus that employed in the analysis. In fact, a current of electricity cannot pass through a homogeneous conductor without generating heat in overcoming resistance, an effect proved by Mr. Joule to be proportional to the square of the force of the current, and the same in whatever direction the current may be flowing. Any other thermal action that can take place must depend upon the heterogeneousness of the circuit, and must be reversible with the current. For example, if a semicircle of bismuth be joined to a semicircle of antimony, an electric current in passing through it produces cold where it passes from the bismuth to the antimony by absorption, and heat where it passes from the antimony to the bismuth.
The transit of the electricity from pole to pole is accompanied by light, and in consequence of the continuous current sparks occur every time the contact of the wires is either broken or renewed; but considerable intensity is requisite to enable the electricity to force its way through atmospheric air or gas. Both its length and colour are affected by the density of the medium through which it passes. If the medium be gradually rarefied the discharge increases from a spark to a luminous glow, differing in colour in different gases, but white in air. When very
much attenuated a discharge may be made to pass across 6 or 7 feet of space, while in air of the ordinary density it will not pass through an inch. In rarefied gas it resembles the Aurora by its continuous flashes. When the battery is powerful the luminous effects are very brilliant.
The most splendid artificial light known is produced by fixing pencils of charcoal at the extremities of the wires, and bringing them into contact. This light is the more remarkable as it is independent of combustion, since the charcoal suffers no apparent change, and, likewise, because it is equally vivid in such gases as do not contain oxygen. It depends upon the molecular arrangement of the charcoal; for Mr. Grove observes that "carbon in a transparent crystalline