Davy supposed that this effect was due to the difference in action of the two liquids upon the platinum; but if two cups, each containing a solution of the same salt, as nitre, are placed one on either side of the first named cup, and connected with them by bundles of asbestus, and the platinum slips connected with the galvanometer are placed in the solution of nitre, a current will be produced flowing in the same direction as in the first instance, but it will be weaker, because of the increased resistance offered by the additional liquid. The action of one acid upon another will also generate galvanic currents. Let strong nitric acid be placed in one branch of a U tube, and strong sulphuric acid be carefully introduced into the other so as not to mix (for which purpose a dividing membrane may be used), and platinum strips connected with the wires of a galvanometer be placed one in either branch; a current will thus be generated, passing from the sulphuric acid through the galvanometer to the nitric acid. Fig. 10 represents a galvanic couple composed of two liquids and one metal, devised by Becquerel, and called an oxygen circuit.

A bottle, d, contains nitric acid, and into its mouth is inserted a tube containing a solution of caustic potash, and having a cork in the top through which passes a wire. The bottom of the tube is stopped by a piece of linen cloth, which is covered with clay, and this with cotton wool, to prevent the clay from mixing with the liquid. The wire connects two plates of platinum, a and p and the connection may be made through the coil of a galvanometer if it is desired to measure the strength of the current. The two liquids meet each other in the clay, and a current of considerable strength is generated, which passes through the wire from the acid to the potash solution, and through the clay from the potash solution to the acid; the latter answering to the copper, and the potash solution to the zinc plate of an ordinary couple. The water in the potash solution is decomposed, its oxygen escaping in bubbles, and its hydrogen going to the nitric acid, which it reduces to nitrous acid. The current which is generated is of constant strength, and the plates do not become polarized. The power is increased by making the plate in the potash solution of amalgamated zinc, which being attacked by the nascent oxygen produces polarization in the direction of the current.

A simple couple of this kind is sufficient to effect the electrolysis of water, and several couples form a powerful battery.-Dry Piles. A modification of the voltaic pile was made by Deluc in 1800, and improved by Zamboni in 1812, which is remarkable for generating electricity of very high tension, approaching in character that of the frictional machine. The dry pile of Zamboni is made by covering a sheet of porous paper on one side with tin foil, and on the other with a paste made of powdered peroxide of manganese mixed with moistened gelatine, starch, or British gum. When the paste is dry the paper is cut into circular disks about an inch in diameter. These are placed one above the other in a glass tube, the tin-foil faces all turned one way, and the peroxide of manganese ones the other, so that one end terminates in tin foil and the other in manganese. Several hundred or thousand of these couples may be formed into a pile. They should be sufficiently pressed together to effect good contact, each end being fastened by a metallic cap, and one end provided with a knob, the other standing upon a metallic plate. The peroxide of manganese end shows positive, and the tin end negative electricity.

Within the pile, therefore, tin corresponds to zinc, and peroxide of manganese to copper, in the ordinary pile or battery. It must not be supposed that the pile will act if it is perfectly dry; it requires, and under ordinary circumstances possesses, a certain degree of moisture. It does not produce a continuous current, but after being discharged requires a certain time for the electricity to acquire sufficient tension to pass through the paper from the tin to the peroxide of manganese. A pile of several thousand disks causes strong divergence of the leaves of the electroscope and will decompose water. Its principal use is in the construction of Bohnenberger's electroscope, represented in fig. 11. Two piles stand with their opposite poles upon a metal plate, and from the top of a bell glass which covers the piles a strip of gold leaf is suspended from a conductor which passes through the top and terminates in a knob. The gold leaf hangs between the two knobs of the piles, and the instrument is so delicate that whenever a body only slightly electrified is brought within a few feet of it, the gold leaf will move toward one or the other of the piles.

A pith ball suspended by a silk thread between them will oscillate as long as the chemical action of the pile continues, which may be for two or three years. - Galvanometers. It has been stated that if a magnetic needle is brought near a wire through which a galvanic current is passing, it will be deflected; the direction depending upon the relative position of the wire to the needle, and upon the direction of the current. Upon this peculiar action depends the construction of an instrument which is used for measuring the strength of a galvanic current, called a galvanometer. If the wire is held above the magnetic needle, and parallel to it, and a current is passed in the direction of its north end, that end will be deflected to the left, as represented in fig. 12, when the observer is looking downward and to the north. If the wire is held under the needle, and the current passed in the same direction, the north end will be deflected to the right; but if the current is passed from north to south, the needle will be deflected in the same direction as when the current passed above it from south to north.

If, therefore, the wire is turned upon itself, as represented in fig. 13, two forces will act upon the needle, tending to deflect it in the same direction; and if the wire is formed into a flat coil, the deflecting force exerted upon the needle will be multiplied nearly as many times as the wire passes backward and forward. Schweiger's multiplier, constructed in this manner, is shown in tig. 14. The sensitiveness of the instrument is increased by using what is called an astatic needle, which is constructed by placing two magnetic needles upon the same axis, but with their north and south ends in opposite directions, and suspending them horizontally by a delicate fibre of silk. If their axes are perfectly parallel, and they have precisely the same magnetic force, they will form a system which is astatic; that is, they will when acted upon only by the earth's magnetism point indifferently in any directipn. It is however impossible to place them perfectly parallel, and it therefore follows that when they have equal magnetic force they will only come to rest when at right angles to the plane of the magnetic meridian.