8 Sosma/i — Platin >///)-7?/ioch'uw Thermoelement. 



desired to tiiul tlie tempeniture correspoiidiiii); to a readiiiji: of 

 8031 microvolts on element Z. It is evident from the table that 

 the temperature is in the neighborhood of 950°. At about this 

 temperature, element Z reads 92 microvolts below the assumed 

 standard ; adding 92 microvolts to 8931 gives 9023 microvolts 

 as the corresponding standard reading, and this by interpola- 

 tion in the table gives 952-3°. The tenths, of course, mean 

 little in absolute vahie ; but temperature differences^ in case 

 measurements are made with similar elements under similar 

 conditions, can often be obtained to tenths of a degree. 



The use of this table and deviation-curve avoids the calcu- 

 lation and recalculation of thermoelement curves and the tabu- 

 lation of their readings. If the calibration of an element 

 changes by a few microvolts, the deviation-curve is merely 

 raised or lowered by a corresponding amount. If the value 

 adopted for one of the calibration points is changed, the cor- 

 responding reading in microvolts of the assumed standard is 

 also changed, and all the deviation-curves take a slightly dif- 

 ferent course in the neighborhood of that point. The table 

 and curves make it possible, furthermore, to estimate tempera- 

 tures (with an accuracy of perhaps 5°) with a new thermo- 

 element, by simply calibrating it at, say, two points such as 

 silver and diopside, and thus locating it among the family of 

 deviation-curves. 



5. Relation of Thermal E.M.F. to Composition. 



In the course of the work on the nitrogen thermometer, the 

 standard 10 per cent elements were compared with elements 

 whose alloy wires contained 1 per cent and 15 per cent rho- 

 dium. The e.m.f. of the 20 per cent alloy, of which the bulb 

 was made, was determined by two methods,* for the purpose 

 of evaluating the differential readings on the nitrogen ther- 

 mometer bulb. To make the series more complete, a 5 per 

 cent alloy was obtained from Heraeus and its readings against 

 pure platinum wei'e compared with the standards. 



A similar series of comparisons was made in 1892 by Hol- 

 born and Wien,t using alloys with 9, 10, 11, 15, 20, 30, 40, 

 and 100 per cent rhodium. This work was done, however, 

 just at the beginning of the careful work of Mylius on the 

 separation of the platinum metals, and the alloys then available 

 were not pure. In the lower percentage alloys, different 

 elements of the same nominal composition gave e.m.f. 's differ- 

 ing by 10 per cent or more, and varying differentl}' with 

 temperature. In the higher percentages, the e.m.f. varies 

 little with the composition, and the results have therefore 



* Day and Sosman, loc. cit., p. 119. 



f L. Holbom and W. Wien, tjber die Messting hoher Temperaturen. Ann. 

 Phys., xlvii, 107-134, 1893. 



