42 Dr. E. L. Nichols on the Electrical Resistance and 



Table T. The following table affords a further comparison of 

 the six formulae. 



In the columns a to /are given the temperatures, calcu- 

 lated by the several formulae, at which the resistance of the 

 wire, compared with its resistance at 0°, is given in the 

 column marked " ?\" 



Table II. 



Length, 



r. 



Siemens. 



Benoit. 

 d. 



Mat- 



thiessen. 



Platinum 

 thermo- 

 meter. 



a. 



b. 



c. 



o 

 

 420 

 1108 

 1950 

 3170 



1-0000 

 1-0032 

 1-0082 

 10146 

 1-0280 



1-000 



2-000 

 3-000 

 4000 

 5 000 

 6-000 

 7-000 

 8000 



o 





 325 

 692 



1086 

 1464 



1828 

 2170 

 2470 



o 







402 



812 

 1244 

 1682 

 2072 

 2387 

 2692 



o 

 

 378 

 708 

 1000 

 1272 

 1512 

 1766 

 1978 





 342 

 720 

 1170 

 1638 

 2158 

 2800 



o 







375 



917 



1623 



3100 



A glance at the curves and at this table suffices to show 

 how ill-deserved is the confidence generally felt in these for- 

 mulae. The discrepancies involve differences of hundreds of 

 degrees. 



IV. The methods employed by Dr. Siemens in the measure- 

 ments represented by curves b and c were identical ; but the 

 platinum used contained slight impurities. To these impu- 

 rities the disparity was due. Dr. Siemens found that such 

 foreign substances as usually occur in commercial platinum 

 affected both the resistance of the cold metal and the law of 

 the change of resistance with the temperature. 



Benoit's formula (r/) depends for its accuracy upon the 

 determination of the boiling-points of mercury, sulphur, cad- 

 mium, and zinc, for which temperatures lie adopted the values 

 given by Deville and Troost*. M. Ed. Becquerel opposed 

 those values at the time of their publication; and later researches 

 have confirmed him, at least so far as cadmium and zinc are 

 concerned, in thinking them to be entirely too high. 



In the following table the results obtained by Deville and 

 Troost are compared with the more probable values given by 

 other physicists: — 



* Deville and Troost, Annates de Chimie, (3) vol. lviii. 



