the Coefficient of Expansion of Incandescent Platinum. 41 



TVhen, as is frequently the case, it is more convenient to 

 measure the length of a wire than its resistance, we may em- 

 ploy Matthiessen's formula, 



1= 7 (1 + -00000851 1 + -0000000035 i 2 ) . 



(0 



or we may use the uncorrected scale of the platinum thermo- 

 meter. The latter scale is expressed by the formula 



l=l (l + -000008860- 



.... (/) 



These being almost the only data we possess for the calcu- 

 lation of the temperature of a hot wire, the question of their 

 accuracy is of some importance. The formulae may be best 

 compared by plotting side by side the curves which represent 

 them (fig. 2). 











Fig. 2. 

























2400° 1 













jEfi 





2200° 









mm 



■1 



jBU 





200O c 









M¥i 









1 5 1 1 - 









WSL 



Wa 







g 







r/i 



mm 



m 



B 





P. 



fi 1200° 



1000° 









mm 















2EiB 















P 











6oo° 



400° 































200° 

































In fig. 2, resistance is substituted for length in curves e 

 and /, using for that purpose the measurements given in 



