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equally to the Clark cell. The reason for some of the directions will appear from the theoretical discussion following.

The Cadmium cell is usually constructed in an H form, as shown in Fig. 51. The containing vessel is of glass, and must of course be thoroughly cleaned and dried before use. The negative electrode consists of an amalgam containing 12 parts by weight of cadmium to 87 of mercury. The two metals combine on warming, and the amalgam may be preserved from

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oxidation by a layer of paraffin. The amalgam, which is solid at ordinary temperature, may be poured into one limb of the H, or better, cast on a platinum wire as shown in the figure. The wire is first sealed through the end of a capillary glass tube (about 1 mm. internal diameter) with the aid of “Schmeltzglas," and then, some amalgam being melted in a glass tube, the wire is placed in it, and on cooling, the amalgam forms an adherent cap round the wire. Connection is made by means of a drop of mercury in the capillary. This method of

construction ensures good insulation of the cell when not in use. The positive pole may consist of liquid mercury (scrupulously purified), but is better made in a manner similar to the negative. A platinum wire is sealed in a glass tube, a piece of platinum foil about 1 cm. square welded to this, and the foil, after cleaning with aqua regia, is amalgamated by electrolysis in a solution of mercurous nitrate. Mercury obtained in this way can be relied on for purity, and as only a small amount adhering to the platinum is used, the cell is less liable to be damaged when carried about.

The electrolyte is made by mixing equal weights of cadmium sulphate crystals (free from zinc and iron) and water, and rubbing them well together. The arm of the H-tube containing the negative electrode is filled with this, the undissolved crystals being allowed to settle round the electrode so that it is completely covered with them. The other arm is similarly filled with a paste composed of cadmium sulphate solution and mercurous sulphate well rubbed together to the consistency of a thick cream. The mercurous sulphate must be very carefully freed from mercuric salts: if it shows any appreciable quantity of these it should be rejected, as satisfactorily pure mercurous sulphate is now to be had commercially. The paste settles down round the amalgamated platinum electrode, and eventually hardens so that the cell can be turned upside down without displacing it.

The cell must be sealed off so as to protect it completely against evaporation. The method shown in the figure is satisfactory. A layer of paraffin wax is poured on to the electrolyte, taking care, however, to leave an air-bubble, else there is risk of the glass cracking in the summer. A cork disc is then pushed down over the electrode tubes, to the level of the paraffin, and the top of the cell very carefully sealed with sealing-wax.

The cell may conveniently be packed with cotton wool inside a metal case. The electromotive force only varies 30000 part per degree, but if the two electrodes are not at the same temperature, an error considerably greater than this may be introduced, as a thermoelement will be formed, the electromotive

force of which will be added to or subtracted from that of the chemical cell. The above arrangement avoids this error.

The electromotive force of cadmium cells made up as above is given by the formula

E=10186-0'000038 (t-20°)-0'00000065 (t −20°)2 volts (see table, p. 260).

The earliest attempt at using cadmium for standard cells was by Czapski in 1884, who studied the combination

Cd CdCl2: Hg2Cl2 : Hg

Such cells are open to objection with regard to condition (v.) above, and did not appear in practice to be satisfactorily constant; they have not come into use as standards.

The sulphate cell was introduced about 1890 by Weston, and is still manufactured by the European Weston electrical instrument company of Berlin. The so-called Weston cell, however, differs in one respect from the pattern described above. It contains no crystals of CdSO4, but is made up with a solution saturated at 4°. The solubility of the salt is a minimum about that temperature, so that when raised to higher temperatures the electrolyte of a Weston cell is slightly unsaturated. Its electromotive force at 4° necessarily coincides with that of the standard cell, viz.

1'0190 volts

and as the Weston cell is found to have no measurable temperature coefficient, it may be taken at that value for any ordinary atmospheric temperature.

The cell with crystals, which we have described as the standard type, is that of the Berlin "Reichsanstalt," and has been studied principally by Jaeger, to whose work we are indebted for the table of electromotive force on p. 260.

The behaviour of cadmium amalgams is somewhat unusual, and the construction of cadmium cells is dependent chiefly on their peculiarities.

The electro-affinity of pure cadmium is about —0*143 volt. Addition of mercury raises this value appreciably, so that one cannot, as with most metals, amalgamate the surface without

seriously altering its nature. Addition of 85 per cent. of mercury raises the potential by about o'05 volt, but at this point some definite compound (probably Cd,Hg) is formed, the electro-affinity of which determines that of the amalgam until 95 per cent. of mercury is reached, when the potential again rises, approaching that of pure mercury. These changes are shown in Fig. 52. The range of constant electro-affinity

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(5 to about 15 per cent. Cd) is clearly the most favourable to choose for making standard cells. Amalgams of less than 10 per cent. are liquid at ordinary temperature, and therefore less convenient than those containing a larger percentage of cadmium. Weston originally used 14'3 per cent. (th) Cd, which lies just within the range of constancy: but cells constructed with this material were found to show an erratic

behaviour, not always giving the same E.M.F. at the same temperature. These irregularities were eventually traced by Cohen1 to a transition which the 14'3 per cent. amalgam undergoes at 23°. If the cell containing it be heated above that temperature and then cooled, the amalgam remains at first in the condition which is stable above 23°, and therefore unstable below it, but slowly changes to the other form, with concurrent change in electromotive force. Such a cell is, of course, entirely unsuitable as a standard. Fortunately it was shown by Jaeger and Lindeck 2 that amalgams with a smaller content of cadmium do not show this transition point—at least, within the range of temperature practically needed. Amalgams of from 10 to 13 per cent. can be used with perfect confidence from the freezing-point to at least 30°.

The physico-chemical behaviour of cadmium sulphate is also of importance for the cell. When the irregularities in electromotive force were first observed they were put down to a probable transition point of the salt (analogous to that of ZnSO, (p. 220)). This was found not to be the case, however. The saturated solution yields crystals of composition CdSO. H2O at all temperatures up to 74°, when CdSO4.H2O separates instead; there is no transition below that temperature, therefore. The salt is very soluble; the solubility passes through a minimum at about o° to 4°, being 75.5 parts by weight of CdSO, to 100 of water, corresponding to the formula CdSO,.15 3HO. It increases very little in solubility over the ordinary range of temperature: this is partly the reason. why cadmium cells vary so little in E.M.F. with change of temperature; and it has the further advantage that if the temperature be raised the extra salt needed to maintain saturation is soon dissolved, and there is little tendency for the E.M.F. to lag behind the temperature on this account.

Before the cadmium cell had established itself as a trustworthy standard, the cell designed by Latimer Clark in 1872 Zn: ZnSO, (saturated): Hg,SO: Hg

Zeitschr. phys. Chem., 34. 621 (1901).

2 Ann. d. Phys., (4) 5. 1 (1901), or Zeitschr. phys. Chem., 37. 641 (1901).

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