130 PROFESSOR TAIT ON A FIRST APPROXIMATION 
of what is now effectively the hot junction becomes ¢ + 6¢, then the two laws of 
thermodynamics give, respectively, 
SE = J(8 + (~, —<,) 82), 
and 

IT «4-6, 
0= oF a) yeas On. 
Here E is the electro-motive force, II the Peltier effect at a junction at tem- 
perature 7, and ~,, ~,, are the specific heats of electricity in the two metals. 
Hence 
SE=J (sili; ) = J; 80. 
Introducing the hypothesis, obtained from considerations of Dissipation of 
Energy (Proc. Dec. 19, 1870), that 
oH =ht, o,=hit, 
we have 
Il dE 
Jy =q =a iG ky) (Tas ist; t), 
where T,, is the well-known ‘ neutral point.’ 
Also 
E=(h,—h;) a ea 
since it vanishes for t=¢,, the temperature of the cold junction. Now, if the 
neutral point be between such limits as 0° C. and 300° C., the exact determina- 
tion of it is an easy matter ; and this exact knowledge of it greatly facilitates 
a 
dt? 
a tangent to the plotted curve. For if one junction be at 7, the other at T,,, we 
have 
the determination of which cannot be very accurately found by drawing 
E,=4(hka— hp) (Tas —2).” 
E, and T,,—¢ are easily measured on the experimental curve, and thus £,—4, is 
found. The following values have thus been (roughly) calculated from observa- 
tions. Where the neutral point was not reached, it is put in brackets. The 
2 : 
unit for k,—%, is 3 or 4 per cent. less than ys of the electro-motive force of a 
good Grove’s cell. 


ay Keg —kes ui Ia —ke 
Fe — Cu (bad) 265 C. —0°00147 || Fe—Al (387) C. —0°00105 
5, — Cu (good) 260 — 00145 | ,,—Arg. (1357) — 00045 
Cod 159 — -00209 || Cu (bad)—Cad | —(23) — -00081 
En 199 — 00189 || ,, avi 46) — -00048 
< She 235 — 00151 | , —Ag | —(687) — -00006 
eer (357) — 00112 || ,,(good)—Pb | —(213) + 00016 
» — Brass (318) — ‘00127 | Pbh—Cd — (74) — ‘00096 
» —Pt (519) — ‘00063 » —Pd — (188) + -00080 
Ses (416) = 500094 | 6 = 7a — (78) — -00060 
— ‘00026 


el (1908) — -00029 || ,, —Ag — (262) 

