THE MERCURY ARC 93 



energy is given to it. If we had any definite knowledge 

 concerning the relation between the temperature of a 

 luminous vapor and the rate at which it radiates heat, 

 it would be a simple matter to make this computation, but 

 while we know something about the rate at which a non- 

 luminous vapor radiates heat, there is a small portion of 

 the gas which is luminous and this radiates energy at a 

 very much greater rate. Arons did indeed make some 

 assumption concerning this and arrived at the conclusion 

 that the temperature of the vapor must be 4000 C. or 

 5000 C. This certainly is much too high, so that it would 

 appear that theoretical considerations help us but" little 

 with this problem. 



For the same reason we can not accept the measurements 

 of this temperature made by Fery, 1 who compared the 

 intensity of the light in different parts of the spectrum of 

 this arc with corresponding parts of the spectrum of the 

 carbon arc and concluded that the temperature of the 

 mercury vapor was 3500 C. This temperature may 

 possibly be the correct temperature for the luminous part 

 of the vapor, but can hardly be correct for the non-luminous 

 part. 



It indeed seems more probable that the measurements 

 made with a thermojunction are not far from the correct 

 value, at least for pressures of several millimeters. How- 

 ever, as the density of the vapor becomes less the error will 

 become greater and there is no means known for deter- 

 mining the exact amount of the error. 



Characteristic Curves for the Mercury Arc. A knowl- 

 edge of the relation between the current and the voltage is 

 also important in forming a theory of the arc, and it is also 

 1 Soc. Franc. Phys. Bull., No. 3, 305; 1907. 



