TILE CIIROMIC ACID CELL usually consists of a plate of zinc between two of carbon clipping 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, hut 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.
The zinc is attached to a sliding brass rod. A. so that it may be lifted out of the liquid when the cell is not in use The hydrogen cooling from the ionized acid is oxidized to water by the chromic avid, and is prevented.
Fig. U illustrates a form of chromic acid bat tery, %%here the several cells composing it have their carbons and zincs suspended from a frame.
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 used, one on each side of the copper oxide slab. The solution is covered with a layer of heavy 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 able to furnish a fairly constant current for a long period.
THE LECLANCIIIt 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 replenishment of any thing except water. The cell consists of it glass jar containing ft saturated solution of ainnionie chloride (sal annnonincl in which is immersed a rolled zinc rod (Fig. 7). The porous pot midair's a bar of carbon tightly packed in a mixture of manganese dioxide and granulated carbon. When the cir cuit is closed, the sal ammoniac acts on the zinc, forming zinc chloride and liberating am monia and hydrogen. The hydrogen is slowly oxidized by the manganese dioxide. On a closed circuit the hydrogen is liberated faster than it is oxidized, and the cell polarizes. 11 it is al lowed to rest it recovers from polarization. The form shown in the illustration has a porous pot provided with a flange which fits the top of the glass jar. The zinc rod enters the jar through a small hole, which is closed by a piece of rubber tubing surrounding the roil at this point. The jar is thus practically closed for the purpose of preventing evaporation.
Investigation has shown that the manganese dioxide furnishes only about half the oxygen to effect complete depolarization in a cell worked intermittently. It can hardly be doubted that an additional source of oxygen is the gas ab sorbed by the carbon and dissolved in the electro lyte. With ready access of air and frequent in tervals of rest, it is quite possible that nearly enough oxygen may be supplied from the air without any other depolarizer. It is for this reason that the carbon cell, which is the Le dandle with the porous pot and the manganese dioxide omitted, is quite effective for easy and intermittent service.