GALVANISM. 



353 



In explanation of these tables, it may be observed, 

 that in all those cases where the fluid menstrua 

 afford oxygen, those metals which have the strongest 

 attraction for oxygen are those which form the 

 positive pole. But when the fluid menstrua afford 

 sulphur to the. metals, the metal, which, under the 

 existing circumstances, has the strongest attraction 

 for sulphur, determines the positive pole. Thus, in 

 a series of copper and iron plates, introduced into a 

 porcelain trough, the cells of which are filled with 

 water or with acid solutions, the iron is positive and 

 the copper negative ; but when the cells are filled 

 with a solution of sulphuret of potash, the copper is 

 positive and the iron negative. When one metal 

 only is concerned, the surface opposite the acid is 

 negative, and that in contact with the solution of the 

 alkali and sulphur, or of its alkali, is positive. 

 Simple galvanic circles are possessed of but feeble 

 powers ; yet these are often sufficiently obvious, as 

 in the instance above alluded to, of a slip of zinc 

 laid upon the tongue and a piece of silver under it. 

 In this case, we have an example of the arrange- 

 ment of two perfect conductors (the metals) with one 

 imperfect one (the tongue, or rather the fluids which 

 it contains). A piece of zinc, immersed in water 

 which is freely exposed to the atmosphere, oxydizes 

 very slowly ; but when placed in the same situation, 

 in contact with a piece of silver, its oxidation is 

 much more rapid. By immersing iron and silver 

 (also in contact with each other) in dilute muriatic 

 acid, the action of the acid upon the iron is consider- 

 ably increased ; and hydrogen gas is evolved from the 

 water, not only where it is in contact with the iron, 

 but where it touches the silver. These facts explain 

 why, in the sheathing of ships, it is necessary to use 

 bolts of the same metal which forms the plates ; for 

 if two different metals be employed, they both 

 oxidate very speedily, in consequence of their forming, 

 with the water of the ocean, a simple galvanic 

 circle. 



Compound galvanic circles or galvanic batteries, 

 are formed by multiplying those arrangements which 

 compose simple circles. Thus, if plates of zinc and 

 of silver, and pieces of woollen cloth of the same 

 size as the plates, and moistened with water, be piled 

 upon each other in the order of zinc, silver, cloth ; 

 zinc, silver, cloth ; and so on, for twenty or more 

 repetitions, we have the voltaic pile, the description 

 of which was given above. The power of such a 

 combination is sufficient to give a smart shock, as 

 may be felt by grasping in the hands, previously 

 moistened, the wires connecting the upper anc 

 lower extremities of the pile. The shock may be 

 renewed at pleasure, until after a few hours, the 

 :u\ ity of the pile begins to abate, and finally 

 ceases altogether. 



But the galvanic apparatus, by far the most con- 

 venient, and generally used, was invented by Mr 

 ruickshank the galvanic trough, as it is named ; 

 and which consists of a long and narrow trough, 

 nade of -baked wood. Grooves are cut in the trough, 

 ipposite to, and at the distance of a half and three 

 quarters of an inch from each other ; and into these 

 ire let down, and secured by a cement, square plates 

 of zinc and copper, previously united together by 

 soldering. The space, therefore, between each pair 

 of plates, forms a cell for the purpose of containing 

 the liquid by which the combination is to be made 

 active. The plates may be from three to six or 

 eight inches square ; and care is to be taken in their 

 arrangement in the trough, that the order in which 

 they are inserted be not in any instance re- 

 versed, but that the copper side of each double plate 

 be always towards one hand, and the zinc side to- 

 wards the other. The galvanic trough, thus con- 

 structed, is more easily put in action than the pile, 

 and more easily kept clean ; and, besides, it can be 

 continued longer in action, as it contains more liquid, 

 owing to which cause it is also more energetic. For 

 ordinary experiments, a trough containing fifty pairs 

 of plates four inches square is sufficient. In those 

 cases where a greater power is wanted, it may be 

 commanded by uniting the power of several such 

 troughs through the union of the zinc end of one 

 trough with the copper end of another, by a metallic 

 slip or wire. The battery of the royal institution, 

 with which Sir Humphrey Davy made his great dis- 

 coveries, is composed of 2000 pairs of plates, each 

 plate having thirty-two square inches of surface. 

 The following cut represents an unproved form oi 



the voltaic battery, the hint for which was derived 

 from the couronne de tosses : it consists in keeping 

 the plates detached, instead of soldering them toge- 

 ther. They are connected at the upper edge by a 

 metallic arc, and are introduced into a trough di- 

 vided into cells by partitions of glass (or sometimes 

 into troughs wholly made of earthen ware), in such a 

 manner that one plate is on one side of the partition, 

 the other on the other. This arrangement has the 

 advantage, that, both surfaces of each plate being 

 acted on, a greater power is obtained. 



Doctor W'ollaston has heightened the improve- 

 ment by placing in each cell one plate of the one 

 metal, as the zinc, and two of the other, the copper, 

 so that each surface of the zinc may be opposed to a 

 surface of copper. The plates of copper are con- 

 nected by metallic arcs, both at the top and bottom ; 

 and between them, supported by pieces of wood, is the 

 plate of zinc, distant nn eighth or a fourth of an inch 

 from the copper on each side. The communication be- 

 tween these triple plates is established by arcs of lead 

 or other metal, connecting each central zinc plate with 

 the copper of the adjoining cell. This arrangement is 



