82 MEASUREMENT OF HIGH TEMPERATURES. [bull. 54. 
trically combined with platinum, and if u\ and w 2 be the resistances of 
these wires, then 
to i 
ei+e 2 — 
^2 
where e is the equivalent electro-motive force. Hence if r=iv x \ w 2 , 
e=j^{ (I , -«)(ai+fl2*')+(f t 7-'fK 6 i+** r ) | 
which may be abbreviated 
e =*{A(T-*) + B(Z*-*»)}. 
From this it follows that 
If in this equation w 2 =oo, which supposes this metal to vanish from 
the alloy, and if additionally «! and b x be made equal to zero, we arrives 
at an expression of T in terms of -§-. This result shows that howeverr 
near we may approach the limit couple platinum-platinum, any thermal 
datum derived by thermo-electric means will none the less be depend- 
ent on the properties of the metal combined in multiple arc with pla- 
tinum. 
In the case, however, of metals alloyed, the results are quite different:: 
for here the thermo-electrics of the alloy bear no intelligible or genera] 
relation to the ingredient metals of the alloy, so far as our present knowl 
edge goes. Indeed it is not infrequent to find the admixture of an 
electro-negative ingredient produce a distinctly electro-positive result 
And hence it follows that constants referring to the final or infinitely 
dilute alloy have a special and unique significance. In the final alloy 
we have one metal combined by fusion with another in such a way 
as to produce absolutely no variation of molecular arrangement. If 
therefore, data for the limit couple be investigated, they are those from 
which a clue respecting the dependence of the thermo-electrics of tin 
compound upon those of its constituents may most probably be obtained 
Table 7 shows the sensitiveness of couples to be frequently such thai 
the final element platinum-platinum may be approached very near, and il 
is for this purpose largely that the table was dra wu up. With the objecl 
of basing the discussion on electric data exclusively, the constant 0' 
may be taken as a symbol of the composition of the alloy-component oj 
the thermo-couples of a given series ; and hence the curves or loci hen 
in question are obtained by representing any fixed datum (for instance 
the value of the boilingpoint of zinc which obtains for the special couple 
under consideration) as a function of a. We have attempted this witL 
(730) 
