Electromotive Force of different Forms of the Clark Cell. 137 



is of sufficient size to just admit the passage of the two capillaries 

 from opposite sides. The capillaries are then held in their proper 

 relative positions while the seal is completed, and the free ends of the 

 wires are coiled down in their respective mercury cups. The lower 

 end of the tube is then drawn out slightly to facilitate the final 

 sealing off. 



The materials are filled in after the usual method, but in the 

 reverse of the usual order. Moist crystals of zinc sulphate are 

 packed round the zinc till it is covered to a depth of about a centi- 

 metre. After inserting about half the mercurous paste, the flattened 

 and amalgamated platinum wire is coiled down into a suitable posi- 

 tion, and more paste is added. The end of the cell close to the point 

 where it is to be finally sealed, is preferably filled with moist crys- 

 tals instead of paste. The object of this is to avoid leaving any of 

 the paste close to the seal, where it might suffer decomposition from 

 the heat in sealing off. Any excess of solution is dried off with filter 

 paper, and the narrow neck is then sealed off with a fine flame. 



On the whole, we prefer this portable form of cell to any of the 

 other forms we have tried. The shape of the cell and its small size 

 make it very convenient. The seal is comparatively easy to make, 

 and the narrow glass neck separating the cell from its connexions is 

 also an advantage, as it diminishes the risk of any error of tempera- 

 ture arising from conduction along the tube. 



We have not found that there is any advantage in using mercury 

 as compared with amalgamated platinum. The cell has a higher 

 internal resistance, and gives a smaller current on short circuit, but 

 the recovery appears to be equally rapid and complete. We cannot 

 find any systematic difference in the electromotive force at any 

 temperature. If anything, the amalgamated platinum has the advan- 

 tage over the mercury, as the purity of the mercury is then compara- 

 tively unimportant, and redistillation is unnecessary. There does 

 not appear to be any advantage gained, in our experience, by using a 

 strip of platinum foil in place of the fine flattened wire. 



§ 18. Tests of Hermetically Sealed Cells. 



We have similarly tested several saturated cells of the H-form, 

 and various other patterns above described as hermetically sealed 

 cells. We find that they all show the same temperature change of 

 E.M.F. as the B.O.T. crystal cells. The agreement in nearly every 

 case is within one or two-hundredths of a millivolt even at 0° and 

 30° C, the limits of the range. 



The largest divergence was found in the case of a portable cell of 

 type (3), with an amalgamated platinum wire in place of mercury. 

 In this cell, both at 0° and 30° C, the difference from 15° C. 



