240 POTENTIAL DIFFERENCE. chap.
Seat of the Electromotive Force in a Daniell Ele- ment. — From what has been said it is easy to form a conception of the mode of action of a Daniell element, Cu I CUSO4 I ZnS04 I Zn. In this there occur four potential differences, namely —
Zn I Cvi,
Cu I CUSO4, CUSO4 I ZnS04, ZnS04 1 Zn.
Of the first of these electromotive forces we may assume that its value can be measured by the Peltier effect, accord- ing to Edlund {ll)y and it is therefore a few thousandths of a volt (0 006 volt). The potential difference between the liquids is, according to Planck (see p. 228), of the same order of magnitude. In the case cited, when the concentra- tions are the same, it is almost zero, because copper and zinc sulphates have nearly the same transport numbers. When the concentrations are different, the difference of potential may rise to a few millivolts. When the two solutions are of about the same concentration, therefore, the electromotive forces referred to cannot contribute much to the total electro- motive force, which reaches the value of 1*114 volts. There remain the other two potential differences between metals and solutions. The values of tt in the table on p. 237 have been corrected for these two small electromotive forces.
If the solution pressure of copper is Pi, the osmotic pressure of the copper ions in the copper sulphate solution pi, and if the corresponding values for zinc are Pa and p^y the total electromotive force is given by —
��2 ^{'^jr"^~i)
1-99 xlO*/, Pi , pi\ = 2 n.^^^P.-^V-
In this case E has a high negative value, since P«, the
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