144 



PROFESSOR W. THOMSON ON THE 



measure of the amount of the Peltier effect for a given strength of current be- 

 tween various pairs of metals. 



129. " With reference to laws of electro-motive force in various cases, I have 

 commenced by determining the order of several specimens of metals in the thermo- 

 electric series, and have ascertained some very curious facts regarding varieties 

 in this series, which exist at different temperatures. In this I have only followed 

 Becquerel's remarkable discovery, from which I had been led to the reasoning 

 and experimental. investigation regarding copper and iron described above. My 

 way of experimenting has been, to raise the temperature first of one junction as 

 far as the circumstances admit, keeping the other cold, and then to raise the 

 temperature of the other gradually, and watch the indications of a galvanometer 

 during the whole process. When an inversion of the current is noticed, the 

 changing temperature is brought back till the galvanometer shows no current ; 

 and then (by a process quite analogous to that followed by Mr Joule, and Dr 

 Lyon Playfair, in ascertaining the temperature at which water is of maximum 

 density), the temperatures of the two junctions are approximated, the galvano- 

 meter always being kept as near zero as possible. When the difference between 

 any two temperatures on each side of the neutral point which give no current is 

 not very great, their arithmetical mean will be the neutral temperature. A regular 

 deviation of the mean temperature from the true neutral temperature is to be 

 looked for with wide ranges, and a determination of it would show the law accord- 

 ing to which the difference of the specific heat of electricity in the two metals 

 varies with the temperatures ; but I have not even as yet ascertained with certainty 

 the existence of such a deviation in any particular case. The following is a summary 

 of the principal results I have already obtained in this department of the subject. 



130. " The metals tried being — three platinum wires (P l the thickest, P 2 the 

 thinnest, and P 3 of intermediate thickness), brass wires (B), a lead wire (I/), slips of 

 sheet-lead (L), copper wires (C), and iron- wire (I) ; I find that the specimens experi- 

 mented on stand thermo-electrically, at different temperatures, in the orders shown 

 in the following table, and explained in the heading by reference to bismuth and 

 antimony, or to the terms " negative" and " positive," as often used : — 



Tempera- 

 ture 

 Centigrade. 



t( 



Bismuth 

 Negative." 





















Antimony 

 " Positive." 



-20 



. . . P 3 . 



. . C . . . 



. P 2 . . 



' 





*1 • 









I . 













• P 3 • 



. . V . . . 



. P 2 . . 



. C 



. . 



Pi • 









I . 





37 

 64 







• P 3 • 



• P 3 • 



. . . . b . 



{L'P 2 }. . 



• P 



c 



Pi • 



{CPJ 









I . 

 I . 















130 







• P 3 • 





. P 2 . . 







• {BPJ 



L . . 



C 



• 



I . 





140 







• P 3 • 





P, . . 







• Pi- 



.{BL} 



h 



c 



. I 



{CI} 





280 



Po • . 









300 







3 

 . P 3 . 





. p 2 . . 







x i • 

 Pi • 







b 



. . c . . 



