378 



Mr. C Burns on Thermoelectrics of 



An inspection of this tabic shows that above 1500° cooling 

 is too rapid for the interpolation method. Hence, in Table II., 

 the burner was left in place, and the furnace heated to the 

 maximum intensity (above 1700°). Changes of temperature 

 were now so gradual that observations could be taken every 

 minute, two for iridioplatinum including one for rhodio- 

 platinum. The means of the two former were considered 

 identical, as to time, with the latter. 



Table II. — Thermoelectrics of iridioplatinum and rhodio- 

 platinum. Flame on. 



Time. 



t. 



Pt-Ir. 



e. 



Pt-Kh. 

 e' . 



Pt-Ir. 



^20- 



Pt-Kh. 



v 20 . 



10 3 xe' 20 / e 2o- 



12 



321 



22930 



17885 



23020 



17960 



779 



15 



321 



23335 



18168 



23430 



18240 



779 



18 



322 



23450 



18248 



23540 



18320 



778 



21 



322 



23637 



18399 



23730 



18470 



778* 



24 



323 



23297 



18059 



23390 



18130 



775* 



27 



32-3 



23260 



18059 



23360 



18130 



776* 



30 



32-4 



23421 



18168 



23520 



18240 



776* 



* Platinum crucible melts, § 3. 



5. Digest. — The results of these tables have been put in 

 the chart, thermoelectric powers (microvolts) of iridioplati- 

 num as abscissae, and of rhodioplatinum (microvolts) as ordi- 

 nates. The cold junctions are at 20° C. The consecutive 

 regions of temperature of the hot junctions are marked on 

 the curve. 



It will be seen that within 1200° the ratio of electromotive 

 forces is strikingly constant, viz. 0' 7 7 24 4- 0*0016, between 

 340° and 1220°. Moreover the probable error is due to 

 difficulties of observation, and not to any apparent march of 

 the data. Platinum-iridium, 20 per cent., is thermoelectri- 

 cally stronger than platinum-rhodium, 10 per cent., in the 

 ratio of 100 to 76, from which, however, it does not follow 

 that a 10 per cent, alloy of the former metal would be weaker 

 than the latter. Thermoelectric power (ccet. par.) usually 

 increases at a retarded rate with the quantity of the foreign 

 metal added to platinum. 



Between 1200° and 1700° the rhodioplatinum couple gains 

 thermoelectrically on the iridioplatinum couple, so that the 

 relation is no longer linear ; but the change of curvature is 

 very gradual, and uniformly continuous. Even above 1 700° 

 the ratio of thermoelectric powers is (Table II.) 



•7774 + -0016, 



or only about 2 per cent, in excess of its value below 1200°. 



