ARC IN AIR BETWEEN CARBON ELECTRODES 41 



been due to the greater volume of the hot gas, not to its 

 higher temperature. 



We have somewhat more definite knowledge concerning 

 the temperature of the carbon terminals of the arc, and 

 this is what is commonly meant when reference is made to 

 the temperature of the arc. Yet all attempts to find this 

 temperature make use of some assumption which cannot 

 be strictly verified. With one exception all experimenters 

 have made use of observations on the radiation from the 

 carbons and have computed from these the temperature by 

 means of some formula. The majority of these formulae 

 have been empirical and while they have been found to 

 hold for lower temperatures, there is no proof that they 

 also hold for higher ones. This is certainly true for those 

 used in the earlier work. Thus Becquerel 1 in 1833 assumed 

 that L = a (e b(j i), where L is the intensity of the red 

 light, the absolute temperature of the surrounding body, 

 T the temperature of the luminous body, and a and b con- 

 stants depending on the nature of the body. From this 

 he computed the temperature of the arc to be 2070 C. 



Rosetti, 2 using a somewhat more complicated formula 

 found the temperature of the anode to lie between 2400 

 and 2900 C., and that of the cathode between 2138 and 

 2530 C. Other formulae were given by Le Chatelier, 3 

 Wilson and Gray, 4 Dewar, 5 and Petavel. 6 Their results 

 ranged from the value given above by Becquerel to 

 6000 C., the value given by Dewar. This last was com- 



1 Ann. de. Chem. et Phys., (3), 68, 49; 1833. 



2 Nuovo Cim., (3), 6, 202; 1879, and 7, 138 and 185; 1880. 



3 Journ. de Phys., (3), i, 185; 1892. 



4 Proc. Roy. Soc., 58, 24; 1895. 



5 Proc. Roy. Soc., 30, 85; 1880. 



Phil. Trans., 191 A., 515; 1898. 



