The Galvanic Trough, introduced by Cruikshank, is a trough into which rectangular plates of copper and zinc, like those of Volta's pile, are fixed, the cells included between each pair being filled with dilute sulphuric acid. The inner surface of the trough is coated with an insulating sulAtance.
Wollaston's couple of this battery is made up of a plate of copper, doubled up so as to include a plate of zinc, from which it is kept apart by strips of wood. Both faces of the zinc are thus equally exposed to chemical and galvanic action, a device by which the quantity of electricity is increased. In such a battery the comiectimr strips of metal are fixed on a wooden rod, which allows of them being lifted or lowered together. When the battery is put in action, the whole is lowered, and the couples arc immersed each in a trough filled with dilute sulphuric acid (1 of the acid to 12 of water). When out of action, the whole is lifted and fixed by binding screws to the supporting pillars. When the number of pairs is small it is consequence whether one large trough or a number of small ones be used.
Smce's Smee's conple, the position of the plates of Wollaston's couple is reversed. It consists of a silver plate, with a zinc plate on either side, kept separated from it by slips of wood, the two zinc plates being fastened by a coupling. There are thus two positive plates to one negative, instead of two negative to one positive, as in Wollaston's couple, and this is found to increase still more the strength of the current produced. The silver plate is platinized—that is, covered over with finely divided platinum—and this is found to lessen the adhesion of the hydrogen bobbles to the plate, thereby greatly improving the constancy of the action. SMCO'S battery has the same arrangement as Wollaston's.
Grove's Gas battery is more intended for instruction than use. Into the two outer necks of a three-necked bottle, two glass tubes are fitted by means of corks through which they pass. Each of these fillies is open below, and a platinum wire enters them hermetically above, to which a long strip of platinum is soldered, extendnig nearly to the bottom of the tube. Little cups containing mercury stand at the upper ends of these wires. The whole apparatus is filled with slightly acid water, and the poles of a galvanic battery are placed in the little cups. Water is thereby decomposed: oxygen forms in the one tube and hydrogen in the other. When the battery wires are removed, no change takes place till metallic is established between the cups, and the oxygen and hydrogen gradually disappear, attended by an electric current which passes from the oxygen to the hydrogen. When several of these are put together-2n a battery, the connection being always oxygen to hydrogen, they can decompose water. The most important fact illustrated. by Grove's battery is, that the oxygen and hydrogen, liberated by galvanic agency, when left to themselves, produce a currrent the opposite t,, that which separated them. When the poles of the decompos ing battery were in the mercury cups, hydrogen is given off at the negative, and oxygen at the positive pole;. and as opposite clectricities attract, it is manifest that the hydro
gen in this action is positive, and the oxygen negative. When the two gases form, by means of the platinum plates, a galvanic pair by themselves, the current must proceed, as in all cases, from the positive tothe negative within the liquid, and the reverse way between the poles; but this is the opposite of the direction of the original current. It is therefore manifest that where oxygen or hydrogen is set free at any point in a gal vanic circuit, they will tend to send a counter-current.. This action is called galvanic polarization. This accounts for the sudden falling off of strength in All galvanic couples where hydrogen is set free at the negative plate. The bubbles of the gas the plate, not only lessen the surface of contact between the plate and the liquid, exert an electromotive force contrary to that of the pair, and this goes on increasing until the action becomes greatly reduced. In all improved forms of the pair, it therefore becomes necessary to adopt some means for preventing the disengagement of hydrogen at the negative plate, and this is done in all constant batteries by employing two fluids instead of one. The best known constant batteries are those of Daniell, Grove, and Bunsen.
containing vessel of the Daniell cell is of copper, which serves likewise as the negative element of the pair. Inside of this is another vessel of porous unglazed earthenware containing a rod of zinc. The space between the copper and the porous cell is filled with a solution of the sulphate of copper, which is kept concentrated by crystals of the salt lying on a projecting shelf, near the surface of the solution, and dilute sulphuric acid is placed with the zinc in the porous cell. When a tangent galvanometer is included in the circuit, the needle keeps steadily at the same point for hours. The rationale of its action is given as follows: the porous cell which keeps the fluids from mingling, does not hinder the passage of the current; when the atoms of hydrogen that would ultimately be freed at the copper reach the porous cell, they displace the copper in the sulphate of copper, and copper instead of hydrogen is thrown on the copper plate. To give a graphic representation of this action, it is neces sary to suppose that the sulphate of copper is CuSO4, the direct combination of the metal (On) with a salt radical called sulphion, and that the dissolution of the zinc arises from the of sulphuric acid; regarded as the sulphionide of hydro 'gen the directly attacking the metal. This view of the composition of oxygen salts, though new in Daniell's time, is now universally admitted. Taking these letters to represent the molecules, and beginning with the copper (Cu) of the outer vessel, and ending with the zinc (Zn) of the rod, we have the arrangement before dis-, charge, Cti,CuSO4,CuSO4,Idli,SO4 Zn; and after it, CuCu .