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V. Thermo-electric Powers of Metals and Alloys between 

 the Temperatures of the Boiling-Point of Water and the 

 Boiling-Point of Liquid Air. By James Dewar, LL.D., 

 F.R.S., Professor of Chemistry in the Royal Institution, Sfc, 

 and J. A. Fleming, M.A., D.Sc, F.R.S., Professor of 

 Electrical Engineering in University College, London, fyc* 



[Plates III. & IV.] 



1. TF one junction of a thermo-electric couple is at a 

 A temperature t°, and if the two junctions of the couple 

 differ in temperature by a very small interval dt, and if the 

 electromotive force of the couple under these conditions 

 is <£E, then the thermo-electric power of the couple at the 

 temperature t°, denoted by P, is defined by the relation 

 dE 

 dt' 

 The well-known thermo-electric diagram of Professor Tait 

 embodies the results of his own investigations on the thermo- 

 electric powers of metals within the limits of the temperatures 

 which can be reached by mercury thermometers and certain 

 temperatures above this range. Starting from the facts 

 first made known by Lord Kelvin in his classical thermo- 

 dynamical researches, Professor Tait showed that on certain 

 assumptions the curves representing thermo-electric power 

 plotted as a function of temperature should be straight lines, 

 and consequently that the lines of thermo-electromotive 

 force obtained by plotting as ordinates the electromotive 

 force in a thermo-electric circuit of two metals, one junction 

 of which is kept at a constant temperature, whilst the tem- 

 perature of the other junction is varied, should be parabolas 

 with their axes vertical if plotted in terms of the variable 

 temperature as abscissae. Experiment within the above- 

 mentioned range of temperature confirmed these conjectures 

 for a large number of thermo-couples, but led to the remark- 

 able discovery that the thermo-electric lines of certain 

 metals — notably of the magnetic metals, nickel and iron — 

 are, for temperatures above zero Centigrade, broken lines 

 characterized by sudden changes of direction. This thermo- 

 electric diagram of Professor Tait exhibits in a very elegant 

 manner the chief thermo-electric facts, and enables the 

 temperature of inversion to be determined by inspection. 

 2. Having it in our power to carry down the temperature 



* Communicated by the Authors. 



