.G.UXANIC PAL:. —When two plates of copper and amalgamated zinc (zinc whose surface has been rubbed over with mercury) are placed in a vessel containing water to which a small of sulphuric acid has been added, so long as they are kept from touching, either within or without the liquid, they remain apparently unaffected. If, however, they be made to touch, bubbles of hydrogen gas are formed in abundance at the copper plate, and their formation continues until the plates are again separated. If the contact be maintained for some time, and the plates and liquid he afterwards exam ined, it is found that the copper plate weighs exactly the same as before, that the zinc plate has lost in weight, and that the liquid cantains the lost zinc in solution in the form of the sulphate of that metal. The contact need not be affected by the plates themselves. If wires of copper, or any other conductor of electricity, be soldered to the plates. or fixed to them by binding screws, and be made to touch, the changes just mentioned take place as if the plates were in contact. When the wires are thus joined, and so to speak, form one connecting wire between the plates, they exhibit very peculiar proper ties. If a portion of the connecting wire he placed parallel to. a magnetic needle, and the needle brought near, its north end no longer points to the north. but to a point either to the east or west of it. and this deviation ceases with the separation of the wires. It is not even necessary that -the wires be1n, contact, fur if their ends- be put•into'a yes sel containing a conducting liquid, the same changes occur, though to a diminished extent, the contact being completed through the liquid. The ends of the wires, when so immersed, show strong chemical affinities. If the conducting liquid were a solution of the sulphate of .copper, the wire from the zinc becomes coated with the copper of the solution whilst the other attracts its oxygen and sulphuric acid, and wastes away in entering into combination with them: The connecting wires are found, therefore, in actual or virtual combination, to possessvery marked magnetic and chemical properties. The arrangement just described constitutes a galvanic pair, which may be generally defined to be two dissimilar conducting plates immersed in a liquid which can get chemically on one ,of them, and capable of being placed in conducting connection; and the -properties just referred to, form the characteristic powers of galvanic electricity. These properties arise from the wires in connection being the seat of a constant discharge or flow of electricity, for they are posseosed, though to a very feeble extent, by the electricity of the friction electric machine. If the prime conductor' onductor Of a powerful electric machine (see ELECTRICITY) be connected with one of the binding screws of an insulated galvano meter, and a wire connected with the ground be fixed into the other, the plate on being turned causes a current of electricity to pass from the machine to the ground through the coil of the galvanometer; the needle of which will then show a deviation of one or two degrees. The deviation, so far as direction is concerned, is the same as that which would be produced•by placing the wires coming from the copper and zinc respectively in the same binding screws as those connected with the machine and the ground. This would indicate that the copper plate stands electrically in the same relation to the zinc plate as the prime conductor of the machine to the ground. , The electricity of the con ductor is positive, and that of the ground by induction negative; so that in the galvanic pair the copper plate, by analogy, gives of positive electricity, and the zinc plate nega tive. Again, let the wire from the machine end in an insulated vessel containing a solu tion of the sulphate of coptter, and let the end of a fine platinum wire connected with the ground be made to dip &low the surface of the solution, and let the machine be kept in action so as to send a current of electricity through the wires and liquid, at the end of some minutes the point of tho platinum wire will be covered with a minute quantity of copper. The wire connected with the zinc in the galvanic pair and that connected with the ground, are thus shown to display the same chemical power; and this, again, shows us that the zinc plate, like the ground in the above experiment, is the seat of negative electricity. Tao electric condition of the plates before contact reveals, with
the hid of a delicate electrometer, positive electricity in the copper plate and negative in the zinc plate. If the wire joined to the zinc plate, or as we may write it shortly, zinc wire (not,.however, necessarily a zinc wire), be connected with the ground, and the insulated copper wire be made to touch the lower plate of a condenser whilst the finger touches the upper, on both being withdrawn, the leaves of the electroscope diverge with the positive electricity sent, to it from the copper plate. It can be shown, moreover, that the current is not confined to the connecting wire, for if a magnetic needle be suspended between the plates when they lie north and south, slightly above the surface of the liquid, it will deviate from its usual position when the wires are joined, and in the opposite way to that which it shows when heal above the wire placed in the same direction. The current thus passes within the liquid from the zinc to the copper the opposite way to that in which it runs in the connecting wires, so that it snakes a complete circuit: Hence we may conclude, generally, that in the galvanic pair d current of electricity runs within the liquid from the chemically active to-the chemically passive plate, and without the liquid, from the chemically passive to the chemically active plate, making a complete circuit; and that if the connection be interrupted the pair shows electric polarity, the chemically passive plate being the positive pole, and the chemically active plate the negative pole.. • The theory of the action of the galvanic pair may be thus given. When the two plates are put into the water and sulphuric acid, they assume opposite electric states. There is developed at the surface of the zinc an electric force arising from its affinity for the oxygen of the water, which throws the whole arrangement into a state of polarity. The zinc plate with its wire becomes polarized, showing negative electricity at the extremity furthest from the liquid, and positive electricity at the extremity next the liquid. The copper plate with its wire is polarized in the. opposite way, being positive at its outer end, and negative at its end next the liquid. The com pound molecules of water (1120). consisting of oxygen (0) and hydrogen are likewise polarized, but the polarization takes place in the individual molecules. It appears, moreover, to have reference to their compound nature, and we may imagine them placed in series, with their oxygen or negative pole toward the zinc, and their hydrogen or positive pole toward the copper. When the ends of the wires are brought near each other, we might anticipate that a spark discharge would restore quiescence. This, 'however, is not the case, for the electric tension is SO' low that nothing short of contact can effect a discharge. When the discharge thus takes place, the polarity of the circuit for the instant ceases; the tendency to union of the zinc with the atom of next it is completed by the formation of the oxide of zinc. But in order to accomplish this, th,e hydrogen of the molecule of water next the zinc thus set tree unites with the oxygen of Molecule to re-form'Water,.. and the sane transference and union is continued` ontinued along the Whole Series until the hydrogen of the molecule next the copper is thrown on the copper, where, being unable to unite chemically with it, it is given off as a gas. From the fact that pure water has almost no action on zinc, a more probable hypothesis is held that it is not the water, but the sulphuric acid (142,SO4) that is concerned in the action. We have II, as before; but instead of 0, we have SO4, a compound molecule forming zincie sulphate at once. In either case the zinc is left clean, either by the acid dissolving the oxide, or the water present dissolving the sulphate. After the first discharge, therefore, the whole is as at first, so that a second discharge instantly follows, then a third, and so on. A series of discharges is thus transmitted through the circuit, constituting what is termed a current.