CURRENT ELECTRICITY.

CHAPTER LIII.

GALVANIC BATTERY.

695. Voltaic Electricity.-Towards the close of last century, when the discovery of the various phenomena of frictional electricity had been followed by Coulomb's investigations, which first reduced them to an accurate theory, a new instrument was brought 'to light destined to effect a complete revolution in electrical science. In place of an element difficult to manage, capricious and uncertain in its behaviour, and constantly baffling investigation by the rapidity of its dissipation, the galvanic battery furnished a steady source of electricity, constantly available in all weathers, and requiring no special precautions to prevent its escape. Moreover, the electricity thus developed exhibited an entirely new set of phenomena, and opened up the way to such various and important applications, that frictional electricity at once fell into the second place, and the new agent became the main object of interest with all electrical investigators.

696. Galvanic Element. If two plates, one of zinc and the other of copper (Fig. 446), are immersed in water acidulated by the addition of sulphuric acid, and are not allowed to touch each other within the acid, but are connected outside it, either by direct contact, or by a metallic wire M and binding screws, as in the figure, a continuous current of electricity flows round the circuit thus formed, the direction of the positive current being from copper to zinc in the portion external to the liquid, and from zinc to copper through the liquid. Chemical action at the same time takes place, the zinc being gradually dissolved by the acid, and hydrogen being given out at the copper plate.

If, instead of employing two metals and a liquid, we form a circuit with any number of metals alone, no current will be gena

rated, provided that the whole circuit be kept at one temperature. If, however, some of the junctions be kept hot and others cold, a current will in general be produced.

The principles which underlie these phenomena appear to be as follows:

(1). When two dissimilar substances touch each other, they have not exactly the same potential at the point of contact. For instance, when zinc is in contact with copper, it is at higher potential than the copper.

(2). The difference is in general greater for two metals than for a metal and a non-metal or two non-metals.

(3). The difference depends not only on the nature of the two substances, but also on their temperatures.

(4). The difference of potentials between two metals is the same when they are in direct contact as when they are connected by one or more intervening metals: all the metals being still supposed to be at the same temperature.

(5). When two metals are connected by a conducting liquid which is susceptible of decomposition, their difference of potential is much smaller than when they are in direct contact. Thus, if the connecting wire M (Fig. 446) be of copper, and we break its connection with the copper plate, the difference of potential between the two plates will be less than the difference between the zinc plate and the copper wire. The zinc plate is positive with respect to the copper wire; hence the copper plate is positive with respect to the copper wire. On completing the circuit, positive electricity accordingly flows from the

copper plate into the copper wire. As the difference of potentials at the junction of the dissimilar metals is permanent, the current is permanently maintained. Chemical combination at the same time goes on; and the potential energy of chemical affinity which thus runs down, is the source of the energy of the current.

Every electric current may be regarded as a flow of positive electricity in one direction, and of negative electricity in the opposite

direction. The direction in which the positive electricity flows is always spoken of as the direction of the current.

697. Galvanic Battery. By connecting the plates of successive elements in the manner represented in Fig. 447, we obtain a battery. The copper of the first cell on the left hand is connected with the

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zinc of the second; the copper of the second with the zinc of the third; and so on to the end of the series.

If two wires of the same metal be connected, one with the first zinc and the other with the last copper, the difference of potential between these wires is independent of the particular metal of which they are composed, and is called the electro-motive force of the battery. Its amount can be measured by means of Thomson's quadrant electrometer; and in applying this test, it is not necessary that the wires which connect the battery with the electrometer should be of the same metal; for, whatever metals these wires may be composed of, the quadrants of the electrometer will (by law (4) above) assume the same potentials as if in direct contact with the plates of the battery.

The zinc of the first and the copper of the last cell (or wires proceeding from them) are called the electrodes or poles of the battery, the zinc being the negative and the copper the positive electrode. The current flows through the connecting wire from the positive to the negative electrode, and is forced through the battery from the negative to the positive.

698. Galvani's Discoveries.-About the year 1780, Galvani, professor of anatomy at Bologna, had his attention called to the circumstance that some recently skinned frogs, lying on a table near an electrical machine, moved as if alive, on sparks being drawn from the machine.

GALVANI'S EXPERIMENTS.

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Struck with the apparent connection thus manifested between electricity and vital action, he commenced a series of experiments on the effects of electricity upon the animal system. In the course of these experiments, it so happened

that, on one occasion, several dead frogs were hung on an iron balcony by means of copper hooks which were in contact with the lumbar nerves, and the legs of some of them were observed to move convulsively. He succeeded in obtaining a repetition of these movements by placing one of the frogs on a plate of iron, and touching the lumbar nerves with one end of a copper wire, the other end of which was in contact with the iron plate. Another mode of obtaining the result is represented in Fig. 448, two wires of different metals being employed which touch each other at one end, while their other ends touch respectively the lumbar nerves and the crural muscles. Every time the contact is completed, the limb is convulsed.

Fig. 448.-Experiment with Frog.

Galvani's explanation was, that at the junction of the nerves and muscles there is a separation of the two electricities, the nerve being positively, and the muscle negatively electrified, and that the convulsive movements are due to the establishment of communication between these two electricities by means of the connecting metals.

Volta, professor of physics at Pavia, disproved this explanation by showing that the movements could be produced by merely connecting two parts of a muscle by means of an arc of two metals; and he referred the source of electricity not to the junction of nerve and muscle, but to the junction of the two metals. Acting on this belief, he constructed in the year 1800 a voltaic pile.

699. Voltaic Pile.-This consisted of a series of discs of copper, zinc, and wet cloth, c, z, d, Fig. 449, arranged in uniform order, thus— copper, zinc, cloth, copper, zinc, cloth. .. the lowest plate of all being copper and the highest zinc. The wet cloth was intended

merely to serve as a conductor, and prevent contact between cach zinc and the copper above it. All the contacts between zinc and copper were between a copper below and a zinc above, so that they all tended, according to Volta's theory, to produce a current of electricity in the

Fig. 450.-Complete Pile.

same direction. The effects obtained from the pile were so powerful as to excite extraordinary interest in the scientific world.

700. Couronne de Tasses. He shortly afterwards invented the