VOLTAIC CELL. 212 VOLTAIC CELL. motive force «as assumed to be 1.434 volts at 15° C. It consists of zinc in a saturated solution of zinc sulphate, and mercury in contact with a paste of uiercurous sulphate mixed with zinc sulphate. All the materials must be neutral and specially prepared in accordance with standard specifications. The form specified in tlic legal adoption in the United States is the mwlified H- form (Fig. 11). Instead of metallic zinc, the ZnAmoIi ZnSOl, Solution Znsq so, ■hJAe of PureHg- Fig. 11. UNITED STATES FORM OF STANDAKD CELL. negative electrode consists of a mercury-zinc amalgam containing 10 per cent, of zinc. This is covered to the deptli of one centimeter with zinc sulphate crj'stals. The positive electrode is pure mercury, and on this is placed the mercurous sulpliate paste ; the paste in turn is covered with crystals of the zinc salt. The cell is finally filled with a saturated solution of zinc sulphate, and the vessel is closed by brushing over the upper edge of the ground glass stopper with sheUac just before the stopper is inserted in the tube. 'Ihe platinum wires sealed into the two limbs of the cell make connection with the zinc amal- gam and the mercury respectively. The change of electromotive force with tem- perature is large in the Clark cell. The follow- ing is the formula of the Reichsanstalt in Cliar- lottenburg to express the electromotive force of the cell at any temperature : E, = E,5— 0.00119 (I— 15 )— 0.000007 ((—15)=. Thus near 15° C. a change of temperature of one degree C. changes the electromotive force I about 0.0012 volt. I This formula is cood I between 10° and' 25° or 30° C. To secure portabil- ity and a smaller temperature c o e ffi - cient, Carhart has modified the Clark cell by adopting the form shown in Fig. 12, and by employing a solution of zinc sul- pliate of density 1.3!', which corresponds to saturation at 0° C. .'
incvease in the density of the zinc sul-
phate solution lowers the electromotive force of the cell. Hence when the teiii])erature of a normal Clark cell rises, more of the zinc sulphate crystals dissolve, and the increase of density diminishes the electromotive force. This .source CastZn Seal Cork— t';^ Soltrtion-J Zn SO^ 5 Asbestos Pt wire Flo. 12. CAiUIAHT-<;LAKK STANDARD CELL. CadmJitmAmalgvn Fig. 13. WE8TON standard CELL. of variation is removed by having no crystals in the cell. In the Carliart form the zinc is east with a perforated foot and is separated from the paste by a diaphragm of good filter paper. The contents of the cell are in this way kept from mixing during transportation. The same form may be used for a normal Clark, but crystals of the zinc sulpliate must then be added to the paste and must cover the exposed zinc. If the stem of the zinc electrode is cov- ered with some appropriate insulating medium, the exposed part of the zinc will necessarilj- be in contact with the sat- urated solution of the zinc salt. The electro- motive force of the Car- hart-Clark cell is 1.440 at 15° C, and the fall of electromotive force per degree C. rise of tempera- ture is 0.00056 volt. The only other stand- ard cell now employed to any extent is the cad- mium cell invented by Weston. In it cadmium and cadmium sulphate take the place of the zinc and zinc sulphate of the Clark cell. The negative electrode of the Reichsan- stalt form (Fig. 13) is cadmium amalgam, con- taining about 12.5 per cent, of metallie cadmium. The crystals of cadmium sulphate should cover the mercurous sulphate paste as well as the cadmium amalgam. This cell, as made liy Weston, con- tains a solution of cadmium sulph.ate saturated at 4° C, but no crystals of the salt. The electromotive force of the cell with crys- tals at any temperature is expressed bj' the fol- lowing formula : E, = Ej„— 0,000038 (f— 20)— 0.00000065 (/— 20)^ The cadmium cell has several advant.ages over the Clark, the chief one being its very low tem- perature coefficient. For many purposes the change of electromotive force with temperature may be neglected. The legal value of the electromotive force of the Clark cell is 1.434 volts at 15° C, Recent determinations have shown that the probable value is nearer 1.433 volts. The older deter- minations were made by means of the silver volt- ameter, but the electrochemical equivalent of silver is not known with any greater degree of accurac,v than the electromotive force of the Clark cell. A number of very careful deter- minations have been made of the ratio between the electromotive forces of the Clark cell at 15° C. and the cadmium cell at 20° C. The mean of the.se determinations, made at the German Reichs- anstalt, due weight being given to each deter- mination, is as follows: Clark at 15° : cadmium at 20°= 1.40070. Also Clark at 0° : cadmium at 20°= 1,42280. These two relations give Clark 0°— Clark 15° = 0.01041 volt. Hence, if the electromotive force of the Clark cell at 15° is 1.4333, that of the cadmium cell at 20° is 1.0180 volts. If the Clark be put at 1.433, the cadmium will be 1.0187. The electromotive force of the cadmium cell as made by Weston is 1.0102, at 20° C. From the point of view of energy a voltaic cell is a device for the direct conversion of potential
