Sosman — Platinum-Rhodium Thermoelement. 7 



Diopside, m. p. 1391-2° 



Nickel, m. p. (in NJ 1452-3° 



Cobalt, m. p. (in NJ 1489-8° 



Palladium, ni. p. 1549-2° 



Platinum, m. p. 1755° 



4. Interpolation. 



For interpolation between these points we may use either an 

 empirical equation or series of equations, or we may plot the 

 temperatures and microvolts and draw a smooth curve through 

 the points. The results of one method have no better claim 

 to accuracy than the results of the other, for an empirical 

 equation is essentially nothing but an imaginary curved ruler. 

 A plotted curve on a scale large enough to get the requisite 

 accuracy of reading would, however, take a sheet at least 30 



Fig. 2. 



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too' 200' 300' WO' 500' 600' 700' 800' 300' 1000' IIOO" 1200" 1300' 1400" 1500" 1600" 1700" 175S' 



Temperature. 



Deviation of typical thermoelements from standard curve. 



feet square. But if instead of plotting microvolts directly 

 against degrees, we plot the deviation from the straiglit line, 

 <?:=10 t, the sheet required is reduced to about 3 feet square. 

 If further, we plot the deviations of each element from an 

 arhitrary standard curve^ instead of the deviations from a 

 straight line, the usual bO""^ X 40''™ sheet is ample. 



The iigures of Table II represent such a curve, which lies 

 very close to the actual curve for the standard thermoelement 

 E used in the work on the nitrogen thermometer. It is made 

 up of several parabolas connected by transition curves. The 

 deviations of various other elements, in use in the laboratory, 

 from this standard are plotted in fig. 2. These curves are 

 obtained by plotting the differences between the reading of 

 the element and that of the assumed standard at each calibra- 

 tion point. 



An example will serve to make clear the method of convert- 

 ing microvolts into degrees with this table and curve. It is 



