Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/103

Rh ELBCTUOMOTIVE POSCB.] ELECTRICITY 93 ie-flui 1 plates usually consisted of zinc and copper, and the tteries. exciting fluid was in general sulphuric acid. Various improvements were made by Cruickshank, Wollaston, Hare, and others, in the way of rendering the battery more com pact, and of reducing its internal resistance by enlarging the plates. Hare carried the last-mentioned improvement to great lengths ; by winding up together in a spiral form sheets of copper aud zinc, insulated from each other by pieces of wood, plates of 40 ur more square feet surface were obtained. In this way the internal resistance was very much reduced, and powerful heating effects could be obtained. When small internal resistance is no object, the cells of the battery may be filled with sand or sawdust, moistened with the dilute acid. In this form the battery is more portable. There are two capital defects to which all one-fluid batteries are more or less subject. In the first place, whether there iriza- * s or s not ex ^ erna ^ metallic connection between the plates, tx a certain amount of chemical action goes on at the surface of the zinc, which consumes the metal without aiding in the production of the current. To this is given the name of local action. It is supposed to arise from inequalities in the zinc, on account of which one portion of the metal is electropositive to a neighbouring portion ; hence local currents arise causing an evolution of hydrogen at some places and solution of the zinc at others. In the second place, when the battery is in action, there is always an evolution of hydrogen at the copper or electro negative plate of the cell, a certain amount of which adheres to the plate and causes a strong electromotive force of polarization. The first of these evils is re- medie.l to a great extent by amalgamating the zinc plate. We thus reduce the surface metal to a fluid condi tion everywhere, and thereby eliminate differences of hard ness and softness, crystalline structure, and so on. The oldest method was to use zinc amalgam for the electro negative metal ; but it is now universally the custom to amalgamate the surface of the zinc plates simply, which may be done by rubbing them with mercury under dilute sulphuric acid. N&quot;o such effective cure has been found for the hydrogen polarization. Smee introduced the plan of vising instead of the copper plates thin loaves of platinum or silver foil, which are platinized by the process already described (p. 87). This, in accordance with what we have already seen, diminishes the polarization. 1 A similar result is obtained by using for the electronegative plate cast iron or graphite ; the action of the latter is much improved by steeping it in nitric acid. This last fact introduces us to a new principle for im proving the action of batteries, viz., the use of an oxidizing agent to get rid of the hydrogen polarization. When the plates of a Smee s battery have been exposed to the air for some time, it is always found that the current is much more energetic than usual just after the first immersion. The improvement is of course only temporary, for the stock of oxygen is soon exhausted, and on raising the plates and dipping them again immediately, the phenomenon does not appear. Davy found that dilute nitric acid acted better than dilute sulphuric acid as an exciting fluid, and the cause of this is the action of the HNO 3 on the hydro gen evolved at the copper plate. Good instances of this kind of action are furnished by the bichromate battery of Biinsen and the Le clanche cell, which occupy a sort of middle position between one and two fluid batteries. The bichromate cell consists of an amalgamated zinc plate, usu ally suspended between two parallel carbon plates, so that it can be raised or depressed at pleasim-. The bichromate solution is made, 1 Fleeining Jenkin gives 47 volt as the available electromotive force of Sniee g cell. The electromotive force when the circuit is broken IB inucli greater. ge above, p. 90. according to Bunsen, by mixing 605 parts of water with 61 8 of potassium bichromate and 116 of sulphuric acid. The electro motive force of the bichromate cell is very great to start with (more than twice that of a Daniell s cell), but it falls very quickly when the external resistance is small. The cell recovers pretty quickly how ever, and may be used with advantage where powerful currents of short duration are often wanted. In the cell of Leclanche the electronegative metal is replaced by a porous vessel filled with carbon and pounded peroxide of manganese. The exciting liquid used is chloride of ammonium. Chloride of zinc is formed at the zinc plate, and ammonia and hydrogen appear at the negative plate ; the latter reduces the MnO,, so that H,0 and Mn a O s are formed, while the ammonia is partly dissolved and partly escapes. This element is tolerably constant if it be not used to produce very strong currents, but its great merit consists in being very permawnL; for it will keep in condition for months with very little attention, furnishing a current now and then when wanted ; hence its exten sive use in working electric bells, railway signals, and so on. It cannot be said that any of the batteries we have Two- described, or in fact any battery on the one-fluid system, fluid satisfies to any great extent the requirements of a constant * )atter &quot; electromotor, which are to give the same electromotive force whatever the external resistance, and to preserve that electromotive force unaltered for a considerable time. The best solution of the problem to construct a constant battery is the two-fluid principle invented by Daniell ; and on the whole, the best application of that principle is the cell originally given by him. This consists essentially of a plate of copper immersed in a saturated solution of copper sulphate, and a plate of zinc immersed in dilute sulphuric acid or zinc sulphate, the copper solution being sepa rated from the other by some kind of diaphragm, usually a porous vessel of unglazed earthenware. The chemical action consists of the solution of the zinc plate to form zinc sulphate, the formation of zinc sulphate at the dia phragm, and the deposition of copper at the copper plate ; thus : Zn S0 4 Zn SO^Ti | SO^Cu StJjCu&quot; Cu gives ZnS0 4 ~ZnS0 4 Zn | S0 4 CuS0 4 CuCu. The evolution of hydrogen and the polarization arising therefrom are thus avoided. A very common arrangement of this cell is a porous vessel, con- Da- taiuing the copper plate and the sulphate of copper, with a small niell s store of large crystals to keep the solution saturated. This vessel element, is dipped into another nearly rilled with a semi-saturated solution of zinc sulphate, in which is placed the zinc plate. With a little care to keep the cell clean by occasionally removing some of the zinc solution and diluting to prevent incrustation, to feed the copper solution, so that it may not get weak and deposit hydrogen instead of copper on the copper plate, to keep down the level of the copper solution, which is constantly rising by osmose (see art. ELECTROLYSIS), and a few other obvious precautions, a bat tery of Daniell s cells will furnish a very nearly constant cur rent, and keep in order for a long time. It is necessary to keep the current going, otherwise the solutions diffuse through the porous vessel, the result of which is a deposit of copper on the zinc, and other mischiefs, which stop the action of the cell altogether. Daniell s element lias been constructed in a great variety of forms, to suit various purposes. The sawdust Daniell, invented by Sir Wm. Thomson 1 (1858), is very convenient when portability is desired. In this form the copper plate, soldered to a gutta- percha covered wire, is placed at the bottom of a glass vessel and covered with crystals of copper sulphate ; over these wet sawdust is sprinkled, and then more sawdust, moistened with a solution of sulphate of zinc, upon which is laid the zinc plate. The cell of Minotto is very similar to this. In these batteries the sawdust takes the place of the porous diaphragm, and retards the interdi (fusion of the fluids. In another class of batteries, of which the element of Mcidinger may be taken as the type, the diaphragm is dispensed with altogether, and the action of gravity alone retards the diffusion. In ileidinger s cell the zinc is formed into a ring, which fits the upper part of a glass beaker tilled with zinc sulphate. At the bottom of this beaker is placed a smaller beaker, in which stands a ring of copper, with a properly insulated connecting wire. The mouth of the beaker is closed by a lid, with a hole in the centre, through which passes the long tapering neck of a glass balloon, which is filled with
 * Jenkin, Electricity and Magnctirm. p. 224.