Page:The American Cyclopædia (1879) Volume VII.djvu/600

 588 GALVANISM negative and attracted by the zinc, and the other positive and attracted by the copper plate. Let us suppose the liquid to be hydro- chloric acid; the electrical condition and chemical action which take place may be rep- resented as follows : Copper I A plate. I H Cl H + 01 H Zinc plate. The polarization of the molecules of the acid becomes intensified by the presence of the two oppositely electrified metals; and conversely, the two metals have the difference in their electrical states intensified by the action of the acid molecules. When the copper and zinc plates are connected by a wire out of the fluid, their polarity becomes so intensified that the constituents of the molecules of hydrochloric acid are drawn asunder, the negative chlorine being attracted by the positive zinc, and the positive hydrogen by the negative copper plate. Union takes place between the chlorine and zinc, forming chloride of zinc, which dissolves in the water ; but the hydrogen which is at- tracted by the copper plate does not unite with it, but rises in bubbles along its surface to the top of the liquid. This evolution of the hydrogen at the copper plate, and not at the point where the chlorine leaves the acid and unites with the zinc, can only t>e explained by supposing that when this action takes place there is at the same time an interchange of constituents in the intervening chain of mole- cules lying between the two plates, such as is represented in the diagram above. When the exciting liquid is dilute sulphuric acid, it was formerly supposed that water was the electro- lyte or body decomposed, its oxygen uniting with the zinc to form oxide of zinc, which subsequently united with sulphuric acid to form sulphate of zinc, while the hydrogen of the water escaped at the surface of the copper plate ; thus : plate. A I H, H 2 + I- + O H 2 O H 2 O H 2 O Zinc plate. IZi ( But it is now believed that it is the sulphuric acid, now written H 2 S0 4, which is the subject of decomposition, the action being as follows : Copper plate. >er| A I I I Zinc e. I H 2 S0 4 H 2 S0 4 H 2 80 4 H 2 SO 4 I plate. -|+ - + -+I- + - | + Modern theory regards the zinc as combining directly with the body SO 4 or oxysulphion, without preliminary oxidation. In electrolysis where the two plates form the electrodes of a battery, and are composed of metals neither of which combines with the liquid, the case is dif- ferent, and both water and sulphuric acid are decomposed, as will be subsequently shown. Voltaic Batteries. A battery consists of a com- bination of two or more couples for the pur- pose of increasing the electromotive force, each Fio. 2. Voltaic Pile. additional couple adding its force to that of the battery. This idea presented itself to Volta when he invented the pile which bears his name. It may be constructed by laying upon a bottom piece of wood a disk of copper, and upon this a disk of cloth moistened with di- lute acid or a solution of some salt, and upon this a disk of zinc, and repeating this order to an indefinite extent, one end of the battery ter- minating in a copper and the other in a zinc disk; the order of the elements being copper,' fluid, zinc, copper, fluid, zinc, in one direction, and zinc, fluid, copper, zinc, fluid, copper, in the other; so that it is immaterial whether the pile is commenced with copper or with zinc, the positive current, when the terminal plates are connected by a wire, always flowing from the copper to the zinc through the wire, and from the zinc to the copper through the layer of cloth. The cop- per and zinc plates should be soldered to- gether around the edges, to prevent the acid or exciting liquid from getting between them except where they are separated by the cloth, because the chemical action which would en- sue would tend to urge a current in the op- posite direction. Soon after constructing the pile Volta made a different arrangement, which he called the couronne de tasses, or crown of cups. This was formed of a number of cups, each containing dilute acid or a solution of salt, and a plate of copper and one of zinc, a short distance apart; the copper in one cup being connected by a wire with the zinc plate in the next, instead of lying directly upon or against it. This mode of connection answered the same purpose, allowing electrical polarity to be induced with equal facility when the con- necting wires were of sufficient sectional area. When the number of cups, which was indefi- nite, was completed, the circuit was closed, as was the pile, by connecting the terminal plates with a wire. The cups were joined together in the following manner : Commencing at the positive pole or electrode of the battery, there would be a plate of copper in the first cup, bearing a binding screw at its top, by which it could be connected with a wire ; then opposite it, in the same cup, a plate of zinc, connected by a wire or strip of copper with the copper Elate in the next cup, and so on ; the last plate i the last cup being zinc, connected with a