ADAMS : A USEFUL TYPE OF FORMULA 



473 



The result of the application of this equation to the copper- 

 constantan element is shown in table I. The eonstantan wire 

 consisted of selected lengths cut from a spool of No. 30 B. and S. 

 "Ideal" wire made by the Electrical Alloy Co. The points at 

 100°, 217?95 and 305?9 were obtained by immersing the ele- 

 ment (enclosed in a glass tube) in the vapors of boiling water, 

 naphthalene and benzophenone;^ those at the lower temperatures 

 by comparison with an element"^ previously calibrated against a 

 resistance thermometer. As table I shows, the curve when 

 passed thru the three upper points fits the three lower points 

 very satisfactorily; indeed the divergence is no greater than the 

 experimental error. 



TABLE I 



Comparison of Observed e. m. f. (E) of Copper-constantan Element with 

 THAT Calculated from the Equation E = 74.672 t - 13892 (i-g- 0-00261 /■) 



^1 microvolt corresponds to about 0?02. 



The above equation as compared with a series formula con- 

 taining a like number of constants (i.e., a cubic equation) pos- 

 sesses the following advantages, (1) In accord with what might 

 be expected from the general shape of the curve a better agree- 

 ment has been obtained between '^observed" and ''calculated" 

 ^'alues. For instance if a cubic should be substituted for the 

 exponential equation described, the differences in the fourth 

 column of table I would each be greater than 5 microvolts 

 (0?1). (2) Since values of (1 — e~^) may be obtained by inspec- 



^ Proper correction being made for barometric pressure. Cf. Adams & Johns- 

 ton, Am. J. Sci., 33: 538 et seq. 1912. 



* This element belongs to Dr. W. P. White of this Laboratory. For descrip- 

 tion of the comparison of this element with the resistance thermometer, see Phys. 

 Rev. 31, 159. 1910. 



