Page:The New International Encyclopædia 1st ed. v. 20.djvu/258

* VOLTAIC CELL. 210 VOLTAIC CELL. in density. The copper electrode is placed at the bottom in saturated copper sulphate, while the zinc is suspended near the top in a weak solu- tion of zinc sulphate. The zinc should never be placed in the copper sulphate solution. The satu- rated copper sulphate is more dense than the dilute zinc salt; it therefore remains at the bottom, except as it slowly diffuses upward. The Buksen Cell, devised by Bunsen in 1841, consists of a glass jar containing dilute sul- phuric acid, and a hollow cylinder of amalga- It is known as a plunge battery, and is con- venient for experimental work. The Edison-Lalakde Cell is an example of the use of a solid depolarizer, namely, l>lack oxide of copper. It was originally due to Lahuide. The electrolyte is a solution of caustic soda, one part of the alkali to three of water by weight. When the cell is working, the zinc forms with the alkali sodium zincate, which is soluble, while hy- drogen is released at the cathode. Copper oxide readily gives up its oxygen to a reducing agent Fig. 4. BaNSE.v cell. t==r=. mated zinc immersed in it (Fig. 4). Within the zinc cylinder is a porous pot containing strong nitric "acid (HXO,) and a prism of dense carbon. The hydrogen ions from the sulphuric acid are intercepted on their way to the positive elec- trode by oxygen from the nitric acid, and are oxidized with the production of water. The nitric acid molecule is broken up and yields a brownish- red gas which is very corrosive and irritating. Tub Chromic Acid Cell usually consists of a plate of zinc between two of carbon dipping into a vessel containing dilute sulphuric acid, to which is added either chromic acid or the bichromate of potassium or sodium. The sodium salt is much to be preferred to the potassium salt. When the bichromates are used an additional quantity of sulphuric acid is needed to liberate chromic acid, which is the real depolar- izer. Fig. 5 illustrates one form of this cell, which is very convenient, liut is open to the objec- tion that since the car- bon plates are usually left standing in the solution, the liquid soon works up- ward and attacks the connections at the top. to a sliding brass rod, lifted out of the liquid in use. The hydrogen coming from the ionized acid is oxidized to ■water by the chromic acid, and polarization is prevented. Fig. G illustrates a form of chromic acid bat- tery, where the several cells composing it have their carbons and zincs suspended from a frame. (iUKM^T CELL. The zinc is attached A, 80 that it may be when the cell is not Fia. 6. PLUXGE BATTERY. like hydrogen. In this cell the copper oxide is employed as a compressed plate, supported in a light copper framework, which is attached to the cover of the jar and forms the positive electrode. In the larger sizes two zinc plates are ised, one on each side of the copper oxide slab. The solution is covered with a layer of hea-y paraffin oil to prevent access of the carbon dioxide of the air, which would convert some of the caustic soda into a carbonate and shorten the useful life of the cell. The serious disadvantages of this cell are high first cost and low electromotive force. When freshly set up the cell may have as high an electromotive force as one volt, but the working value is only about three-quarters of a volt. All of the preceding cells, except the simple one of Volta, are adapted to work on a closed circuit. They do not polarize very seriously and are aide to furnish a fairly constant current for a long period. The LeclanchiS Cell stands in quite a differ- ent class. It is unusually free from local action, but it polarizes easily and is therefore employed for domestic purposes re- quiring intermittent cur- rents for short periods only. For such service it is good for months and even years, without rciilenishment of any- thing except water. The Ledanehe cell consists of a glass jar containing a saturated solution of amnionic chloride (sal annnoniac) in which is immersed a rolled zinc rod (Fig. 7). The porous pot contains n bar of carbon tightly packed in a mixture of manganese y FlO. 7. LECLANCHK CELL.