STANDARD CE LS. A standard cell is employed as a standard of electromotive force. It is not intended to furnish any current, but only electric pressure. The first standard cell was invented by the late Latimer Clark in 1S73. In 1S94 it 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 motive force Nvas 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 pate of mercurous sulphate mixed with zinc sulphate. All the must he neutral and specially prepared in accordance with standard specifications. The form specified in the legal adoption in the United states i, the modified H form (Fig. 11). Instead of metallic zinc, the negative electrode consists of a mercury-zinc amalgam containing 10 per cent. of zinc. This is covered to the depth of one centimeter with zinc sulphate crystals. The positive electrode is pure mercury, and on this is placed the mercurous .illphate 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 shellac just before the stopper is inserted in the tube. '1 he ',Lithim 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 Char lottAinlmrg to express the electromotive force of the cell at any temperature: E, = E-0.00119 (t-15 )-0.00000 Thu, near 15° C. a change of temperature of one C. changes the electromotive force about 0.0012 volt. This formula is good between 10° and 25° or 30' C.
To secure portabil ity and a smaller temperature c m e Al (Mutt, Carhart has modified the Clark cell by adopting the form shown in Fig. l2, and by employing a solution of zinc sul phate of density 139, wbieh corresponds to saturation at 0' C.
ineuease in the density of the zinc sul phate lowers the electromotive force of the cell. Ilenee when the temperature of a normal Clark cell rises. more of the zinc sulphate crystal. dissolve. and the increase of density diminishes the electromotive force. This source of variation is removed by having no crystals in the cell. In the Carhart form the zinc is cast 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 sulphate 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 necessarily 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 0.00050 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 metallic cadmium. The crystals of cadmium sulphate should cover the mercurous sulphate paste as well as the cadmium amalgam. This cell, as made by Weston, con tains a solution of cadmium sulphate saturated at 4° C., hut no crystals of the salt.
The electromotive force of the cell with crys tals at any temperature is expressed by the fol lowing formula : (t-20)-0.00000065 The cadmium cell has several advantages 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.