144 PROFESSOR W. THOMSON ON THE 
measure of the amount of the Petter effect for a given strength of current be- 
tween various pairs of metals. 
129. * With reference to laws of cleckoamniien 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 Jouiz, 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, the thickest, P, the 
thinnest, and P, of intermediate thickness), brass wires (B), a lead wire (L’), 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 :—- 
are kag. Bismuth — Antimony 
Centigrade. “ Negative.” * Positive.” 
—20 | Be c BE SOT GO DAonitab.4e: TeRee eee 
0 P U Ps C Pi Peat Abckcen Ty s2) 20 
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64 Lees ae pt PB. eee Ora IM as cece fy see, 
130 Bs Divina kenay nis! avis {BBE dlag.e. C Tavs 5b ae 
140 UE co eee fe ee Lime OMe, Ne Pie out 1 Le 3 tata 
280 \tembeyncars Soros Isr loaciche tha By 3, £0" sanaey Ce ha eat eae 
300 Ey ere te ates POY a seitckteme a atemte ere ote b I Cc 
