According to the statements made in section 9, with like mean surfaces, similar clay cells and equally dilute sulphuric acid, the resistance to conduction of the zinc and carbon battery is to that of Daniell's, as or as 43 to 78; 1 to 1.8. Stöhrer, of Leipsic, has recently considerably improved the Bunsen battery, and made it more convenient for use. His carbon cylinders are steeped in coal-tar instead of sugar-water, and are then brought to a red heat. They are far more solid and have a much smoother surface, which gives them the advantage of absorbing much less nitric acid, which before rendered the use of this battery particularly unpleasant and expensive. In the first zinc and carbon batteries the copper or zinc ring, which embraced the upper edge of the carbon cylinder, was generally movable. Stöhrer has rendered this fixed. A strip of brass wire is wound about the edge of the carbon cylinder, and a copper ring is screwed in this as firmly as possible. The whole of the upper part is then coated with a solution of shellac. A wire, about one inch long, is fixed to the copper ring, serving as a connexion with the next zinc cylinder. A kind of wire cord, coated with gutta percha, is fastened to the zinc cylinder, and terminates in a binding screw, which can be attached to the copper wire of the following carbon cylinder. § 20. Zinc and iron battery.-It has been proposed by many to use iron instead of platinum or copper in the construction of galvanic batteries. Roberts made a zinc and iron battery in the following manner. A cast-iron vessel, ten inches high and 3.9 inches in diameter, served for holding a mixture of one part concentrated sulphuric acid and three parts of strong nitric acid; in this liquid an earthen cell filled with dilute sulphuric acid was placed, which cell also served for the reception of the zinc cylinder 9.9 inches high and 3.3 inches wide. Five such elements yielded forty cubic inches of detonating gas in a voltametre placed in the circuit. This is certainly quite a considerable effect. (Dingler's Journal, vol. 84, p. 386.) In the same volume of this Journal, p. 385, Schönbein describes a zinc and iron battery which also produced very considerable effects. Roberts proposed a battery of this kind, with one liquid, for blasting rock. (Dingler's Journal, vol. 87, p. 104; Mechanics' Magazine, 1842.) 20 iron plates and 20 zinc plates, each having 7 square inches of surface, are properly connected and so placed in a frame of slats, that they may be immersed in a trough containing a mixture of 1 part sulphuric acid to 10 parts water. Callan constructed a zinc and iron battery, (Dingler's Journal, vol. 109, p. 432; Philos. Mag., July, 1848, p. 49,) of a form similar to that which Grove had originally given to his zinc and platinum battery, viz: rectangular smooth earthen cells, 4 inches long and 41 high. A turkey-cock was instantly killed by the stroke of such a battery, composed of 620 elements; and, on examination, the craw was found burst. Callan says this battery acts fifteen times as strong as one of Wollaston's of the same size, and 1 as strong as an equally large Grove's battery. This estimate seems exceedingly loose; no facts, no measurements are given, from which the constants of this battery can be computed, even approximately; without this knowledge a correct valuation of a galvanic combination cannot be made. Measurements of the zinc and iron battery may be found in the 81st volume of Dingler's Journal, p. 273. Poggendorff found for the electro-motive force of different combinations the following values: The zinc being in dilute sulphuric acid, and the platinum, iron, &c., in concentrated nitric acid. The resistances are tolerably equal in all these combinations. § 21. The iron and iron battery.-That instead of the platinum in Grove's battery, iron can be successfully substituted, is owing, no doubt, to the fact that iron immersed in concentrated nitric acid becomes passive, and in this state acts like a strong electro-negative metal. From this Wöhler and Weber inferred that iron, placed in concentrated nitric acid, might act towards iron in dilute sulphuric acid as platinum does towards zinc. Their expectation was entirely confirmed on trial, and they constructed a very powerful battery in this manner. They found it advantageous to use ordinary tin-plate iron for the metal immersed in the dilute sulphuric acid. Schönbein, also, by his researches on the passivity of this metal, was led to the construction of a battery of passive and active iron. (Dingler's Journal, vol. 84, p. 385.) The most convenient form of the iron battery is perhaps the following: A cast-iron vessel receives the nitric acid and the earthen cell, in which the dilute sulphuric acid is placed with the active iron. The rusting of the part of the iron vessel extending beyond the liquid acts injuriously on the working of the battery. § 22. Callan's zinc and lead battery.-In the Philos. Mag. for 1847, (sec. III, vol. XXXI, p. 81,) Callan describes a new voltaic combination, of which Poggendorff gave an account in volume LXXII of his Ann., page 495. For the platinum of Grove's battery is here substistituted platinised lead, which is immersed in a mixture of four parts concentrated sulphuric acid, two parts nitric acid, and two parts of a saturated solution of nitrate of potash. The zinc is in dilute sulphuric acid, separated of course from the other liquids by an earthen cell. The action of this battery, according to Callan's account, is not inferior to that of Grove's. Poggendorff found that in fact the electro-motive force of this combination was equal to that of Grove's; and that the current from it for many hours indicated the same constancy as that of a zinc and platinum battery. But, on the other hand, he found the addition of saltpetre to the nitric acid no improvement, but the addition of concentrated sulphuric acid has the advantage of protecting the lead from the action of nitric acid, which the pulverulent coating of platina cannot do, and allows, besides, the use of dilute nitric acid. Considered strictly, this combination is a zinc and platinum battery, since the lead serves properly only as a support for the thin film of platinum; therefore zinc and platinum are the terminations of the metallic circuit immersed in the liquid. § 23. The most convenient combination of a given number of voltaic elements for obtaining the greatest effect with a given closing circuit.— Theoretically, this subject has long since been settled, but the investigations are mostly conducted by the aid of the higher calculus, and the whole is presented in such a form, that the practical use of the proposition is indicated rather than fully exhibited; on this account, a somewhat more detailed exposition may here be in place. Generally, the question is stated thus: How should a given metallic surface, which is to be used in constructing voltaic elements, be arranged (that is, how many elements and how large should they be,) in order that a maximum effect shall be obtained with a given closing circuit? This form of the question does not correspond exactly with practical We are not required generally to construct the voltaic battery for a given closing circuit; but the question is, how to combine a disposable number of galvanic elements to obtain a maximum effect. A maximum strength of current may be obtained from a given number of elements, if they be so arranged, that the resistance in the battery is equal to the resistance in the closing arc. I will first explain this proposition, then prove it. A given number of elements can be combined in the most varied manner. For in stance, 24 elements can be arranged in 8 different ways, as rendered 6. 7. ...do......... 3 eight-fold elements. 8. .........do......... 1 twenty-four-fold elements. Which one of these combinations should be selected in a given case, depends upon the resistance to conduction of the circuit. That combination must be taken the resistance of which is nearest to that of the given circuit. Denoting by 1 the resistance of an element, the resistance of the 1st combination is 24. 2d ........do........ 6. 3d 4th ........do........ 1.5 5th combination is 0.666 8th ........do........ 0.046 If the resistance of the given circuit is less than 0.1 of the resistance of an element, the least combination must be selected; but if greater than that of 15 elements, the first must be chosen. If the resistance to be overcome lies between 15 and 4.3, between 4.3 and 2, between 2 and 1.08, &c., the selection must fall upon the 2d, 3d, 4th, &c., combinations respectively. We have yet to prove the foregoing proposition. Considering the different combinations of 24 elements, as represented in Fig. 14, it is easily seen that if the pile be shortened, it becomes broad in the same proportion; that is, if fewer elements be placed one after the other, we can, by using the same number of elements, place more of them beside each other, in the same proportion. Commencing with the second combination, we have here 12 double elements. If we reduce the length of the pile by one-half, or to 6., |