VOLTAIC CELL. 211 VOLTAIC CELL. dioxide and graiiuliitcd carbon. When the cir- cuit is closed, the sal uiiuiioniac acts on the zinc, loniiiuj,' zinc chloride and liberating am- monia and hydrogen. The hydrogen is slovvdy oxidized by the nuuiganese dioxide. On a closed circuit the hydrogen is liberated faster than it is oxidized, and the cell polarizes. If 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 rod at this point. The jar is thus practically closed for the purpose of preventing evajjoration. 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 doubled that an additional source of o.v,gen is the gas ab- sorbed by the carbon and dissolved in the electro- l3te. With readj' access of air and frequent in- tervals of rest, it is quite possible that nearly enough ox3'gen may be supplied from the air without any other depolarizer. It is for this reason that the carlxm cell, which is the Le- clanehe with the porous pot and the manganese dioxide omitted, is quite eiTective for easy and intermittent service. Det Cells have come into extensive use in re- cent years for the purpose of securing portabil- ity, which is oliviously lacking in a ccdl with n liquid electrolyte. The name is a misnomer, since a so-called dry cell is practically a Le- clanche cell in which the electrolyte is in the form of a soft paste. The chief difficulty in de- vising a dry cell is to find an absorbing medium for the paste sufficiently hygroscopic to keep it wet. Dry cells deteriorate by local action arid by loss of water. If worked hard they are liable to burst by gas pressure. The Helle.sen Cell is shown in section in Fig. 8. The cylindrical containing vessel, Z, is the zinc electrode. It is covered with a eani- board case, A, and the space between the two is filled with sawdust. S. A thin layer of bituminous conjpound, E. in the bottom of Z insulates the carbon rod C from the zinc. Around the carbon is placed a depolarizing paste, F. It is held in position by a canvas bag. Outside this is the white paste G, con- taining the electrolyte. The cell is finished off with a bituminous seal, H, through which passes a small vent tube, V. The black paste contains manganese dioxide, and the white paste am- monic chloride. Both are reduced to a pasty con- dition by the addition of lime and ferric oxide or zinc oxide. The OB.cn Cell is substantially the same as the Hellesen except that the electrolytic paste consists of about 85 per cent, plaster of Paris and 15 per cent, flour, moistened with a solution of sal ammoniac. These are good types of dry cells and they illustrate a whole class. Figures !) and 10 exhibit graphically tests of a Leclanchf and an Obaeli dry cell respectively. Each cell was on a closed circuit through an ex- ternal resistance of five ohms for one hour; the recovery from polarization was traced for the following hour, the data for the latter being plot- ted backward toward the starting point. The polarization of the dry cell was somewhat less than that of the LeclanchCs but after the first five minutes the current through the Leelanch6 was more constant in value than that through the dry cell. Standaki) Cells. A standard cell is employed as a standard of electromotive force. It is not 3.0 2.0 1.0 S X o I.S 1.0 05 « t- _J o > ..J OV^R- r ^^ ^^ —_ — -^ -*i^ V r p-
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6 Fig. 9. DIAGRAM BIIOWIXG TEST OF A LECLANCHE CELL. intended to furnish any current, but only electric pressure. The first standard cell was invented by the late Latimer Clark in 1873. In 1894 it ,60 10 1.4- 1.3
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H HI I I.TMI^N CELL. MINUTES. 40 30 20 10 ?ENT (AMP.) _ib KIT RESISTANCE fOHMS) D 10 20 30 40 50 50 MINUTES. Fig. 10. DIAGRAM SHOWING TEST OP AS OBACH CELL. was made a legal standard in Great Britain and the United States; and, in accordance with the best determinations up to that time, its electro-
