Barker — Thermoelectromotive Forces of Potassium, etc. 159 



Art. XV. — The Thermoelectromotive Forces of Potassium 

 and Sodium ivith Platinum and Mercury / by Harold C. 

 Barker. 



Following Seebeck's discovery in 1821, many investigators 

 have measured the thermoelectromotive forces produced when 

 the two junctions of a given metallic conductor with a second 

 metallic conductor are maintained at different temperatures. 

 But comparatively few attempts have been made to determine 

 the values of these thermoelectromotive forces for couples 

 consisting of sodium or potassium combined with other metals, 

 for the obvious reason that the ease with which the alkali 

 metals oxidize renders their manipulation difficult. 



Matthiessen* compared the thermoelectromotive forces of 

 silver-sodium and silver-potassium couples with that of a silver- 

 copper couple, and expressed his results in terms of the latter. 

 The two couples were placed in series with a galvanometer, 

 and the ratio deduced from the observed deflections when the 

 two couples were alternately arranged, by a com mutating 

 device, to assist or oppose each other. The difference of tem- 

 perature between junctions did not exceed 26° in any case 

 given. The sodium and potassium were contained in straight 

 wide thermometer tubes into which the metal was introduced 

 melted, in an atmosphere of hydrogen. Into the ends of the 

 tubes short platinum wires had been fused, and to these the 

 silver wires were soldered. The results cannot be regarded as 

 of great value at this time ; they merely express the ratio 

 between the thermoelectromotive forces of, for example, silver- 

 sodium and silver-copper for a particular small temperature 

 difference between junctions. 



Taitf made some measurements with couples of which one 

 member was sodium or potassium in order that the lines of 

 these metals might be included in his " First Approximation 

 to a Thermoelectric Diagram." I can find no record of the 

 observations on which his results were based, nor any state- 

 ment as to the means of measurement adopted for the particu- 

 lar cases. It may be presumed, however, that the method was 

 similar to that used by him in other cases, and consisted essen- 

 tially in a measurement of the thermoelectric current-strength 

 based upon the observed deflections of a sensitive galvanome- 

 ter. As to the construction of the couples, he states that he 

 had prepared for him "a long quill tube of German glass 

 with platinum wires inserted near the ends ; exhausted it by 



*Pogg. Ann., ciii, p. 412, 1858. 



fProc. Edinb. Soc, viii, pp. 350-362, 1873-1874. 



