Cady and Arnold — Electric Arc. 391 



ingly unstable first stage, but it could not be established with 

 certainty. 



A very unstable region was found at pressures between 10 and 

 30 cm . Here the discharge changed back and forth from glow 

 to arc so rapidly that it looked like both simultaneously. 



The Critical Point due to Vaporization. 



§10. After it had thus been proved that the difference between 

 the first and second stage of the iron arc cannot be attributed 

 to oxidation, the thought suggested itself that perhaps in the 

 first stage we have, even in free air without artificial cooling, 

 an anode that is below the temperature of vaporization. The 

 observation that the positive globule appears in a more agitated 

 condition on the second stage, as well as the glow-like appear- 

 ance of the arc in the neighborhood of the anode on the first 

 stage, tend to strengthen this view. This means, of course, that 

 on the first stage the discharge has the character of an arc at 

 the cathode, but of a glow at the anode. 



Such a possibility was mentioned several years ago by Stark,* 

 and more recently Stark and Cassutof have carried out observa- 

 tions with an arc in which the anode was not even incandescent. 

 The mercury arc with iron anode belongs also to this class of 

 phenomena. What made the case less obvious with the iron 

 arc was, first, the incandescent state of the positive globule on 

 the first stage, and second, that any characteristic of the glow 

 discharge was hardly to be anticipated with ordinary electrodes 

 in free air with a supply voltage as low as 110. 



§11. Several methods of testing .this hypothesis at once 

 suggested themselves. The first was, to test for vaporization 

 by measuring the loss in weight of the positive globule on the 

 two stages. Even to the eye it was apparent that the positive 

 globule grew somewhat more rapidly on the first than on the 

 second stage. 



In order to avoid the growth of the globule from oxidation 

 of the anode or from matter transported across from the 

 cathode, a small globule of magnetic oxide was placed as anode 

 in a cavity in the end of an electric light carbon, while another 

 carbon served as cathode. The arc was allowed to burn long 

 enough for the globule to absorb all the carbon it could, and 

 the globule was taken out and weighed. The loss in weight 

 was then observed after the arc had burned a certain number 

 of minutes on the second stage, then again for the first stage. 



Table I contains a summary of the results : 









Table I. 







Stage 



Current 



Voltage 



Arc Length 



Watts 



Loss per Min. 



1st. 



1-15 



61 



omm 



70-1 



0-82 ra s 



2d. 



1-4 



48 



Otnm 



67-2 



5-75 m s 



*Die Elektr. in Gasen, 1902, p. 154. \ Phys. Zeitschr., v, 264, 1904. 



