« PreviousContinue »
Voltaic Electricity - The Voltaic Battery - Intensity — Quantity
Static Electricity, and Electricity in Motion — Luminous Effects - Mr. Grove on the Electric Arc and Light-Decomposition of Water-Formation of Crystals by Voltaic Electricity — Photo-galvanic Engraving
Conduction — Heat of Voltaic Electricity — Electric Fish. VOLTAIC or Dynamic electricity is elicited by the force of chemical action. It is connected with some of the most brilliant periods of British science, from the splendid discoveries to which it led Sir Humphry Davy and Dr. Faraday.
In 1790, while Galvani, Professor of Anatomy in Bologna, was making experiments on electricity, he was surprised to see oonvulsive motions in the limbs of a dead frog accidentally lying near the machine during an electrical discharge. Though a similar action had been noticed long before his time, he was so much struck with this singular phenomenon, that he examined all the circumstances carefully, and at length found that convulsions take place when the nerve and muscle of a frog are connected by a metallic conductor. This excited the attention of all Europe ; and it was not long before Volta, Professor at Pavia, showed that the mere contact of different bodies is sufficient to disturb electrical equilibrium, and that a current of electricity flows in one direction through a circuit of three conducting substances. From this he was led, by acute reasoning and experiment, to the construction of the Voltaic pile, which, in its early form, consisted of alternate discs of zinc and copper, separated by pieces of wet cloth, the extremities being connected by wires. This simple apparatus, perhaps the most wonderful instrument that has been invented by the ingenuity of man, by divesting electricity of its sudden and uncontrollable violence, and giving in a continued stream a greater quantity at a diminished intensity, has exhibited that force under a new and manageable form, possessing powers the most astonishing and unexpected. The expression current has no relation to a fluid, which is now considered to be as inconsistent with the phenomena of dynamic as with static electricity.
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
he ck 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