C. Barns — Status of High Temperature Research. 333 



the 20^ iridioplatinum-platinum couple, when electromotive 

 forces are expressed in units of 10 microvolts, 



a = 2160, 

 k l — 985, 

 k = 2560, 



values which may be accepted for the time being, though they 

 are by no means the best obtainable. 



Above 500° this curve will then be found in remarkably 

 good accord with the observations, § 4. Below 500° degrees, 

 however, the agreement is less good, and the computed values 

 lie as much as 10° above the observed values. Indeed to fully 

 express the case one would have to use a string which grows 

 successively heavier per unit of length towards the vertex ; i. e. 

 one would have to introduce another parameter conformably 

 with the occurrence of two metals in the thermo-couple. It is 

 difficult to reach this case practically. 



Anticipating §§ 5 and 6, however, I may state that the cate- 

 nary takes a mean course (above 500°) between the extreme 

 curves there given. 



3. Second extreme case. — Waiving the symmetrical method 

 of finding suitable constants in equation (1), one may proceed 

 by a process which is nearly the converse of this, and compute 



. + *.= !<?*■» (2) 



first, and thereafter add the second term 



io pl + Q1 * 



in accordance with the insufficiencies of equation (2). This 

 may be done preferably with a table of Gaussian logarithms, 

 by the aid of two pairs of observed data, when the thermal 

 distance apart of each pair is the same. For in Grauss's tables 

 pairs of values of A and B for two numbers a and b, are tabu- 

 lated so that 



B = log a — log b, 



A = log (a — b) — log b. 



If therefore for four temperatures 0, 0', 0,, 0',, where — 0' = 

 1 — 0' v e be eliminated from equation (2), the values B and 

 A will take the form 



B=Q(6-6'), B^Q^-6'X 



A = log (e - e') -(P+ Qff), A, = log (e, - e'J - (P + Q0\). 



But since B — B^ A = A x . Hence, as is otherwise evident, 



Q = log ((«- O/^-O)/^'^). 



Q being given, B is given, and therefore A can be found from 

 the tables so that P is known. Finally e may be found from 

 the equation (2). 



