ARC IN AIR BETWEEN CARBON ELECTRODES 43 



__ 2_ 



/= d\~ 5 e Ar , where J is the amount of energy of any 

 given wave length, X the wave length, and GI and c% con- 

 stants; and Planck's modification of Wien's law, i.e., 



X~ 5 

 JCi - . This last has been found to hold through 



a more extended range of temperature than that of Wien's, 

 but for wave lengths in the visible spectrum Wien's law 

 applies with sufficient precision. 



Although these equations have been proven only for 

 ideal black bodies, carbon comes sufficiently near to being 

 such a body to enable us to make use of them in getting 

 an approximate value for the temperature of the arc. 

 The first of the equations was used by Fery 1 and the value 

 which he gave to the temperature was 3490 C. 



Lummer and Pringsheim used the second formula. 

 They were led to believe that the radiations from the car- 

 bon of the arc were intermediate between those from 

 platinum and those from a black body. From this they 

 computed the temperature of the arc to lie between 3750 

 and 4200 degrees absolute temperature. Very 2 applying 

 this same method to data given by Abey and Testing 3 

 gave the temperature of the arc as between 3600 and 4000 

 degrees absolute. 



Wanner, using the third formula, found the temperature 

 to vary from 3700 to 3900 degrees absolute, depending 

 on the kind of carbon used. Fery using the same method 

 gave a much higher value. Waidner and Burgess 4 using 



1 C. R., 134, 977 and 1201; 1902. 



2 Astrophys. Journ., 10, 209; 1899. 



3 Proc. Roy. Soc., 35, 334; 1883. 



4 Phys. Rev., 19, 241; 1904. Bull, of Bureau of Standards, 1, 113; 1904. 



