ARC IN AIR BETWEEN OTHER SUBSTANCES 65 



non-luminous and the arc is quiet. In passing to the 

 second stage there is a sudden increase in current and 

 decrease in potential difference, the gas near the anode 

 becoming luminous and the arc becoming noisy. 



Though there is an apparent similarity between the 

 second stage of this arc and the hissing arc with carbon 

 terminals, the causes of the two phenomena are not the 

 same. The peculiarities of the second stage with the iron 

 arc are apparently due to vaporization of the anode. 

 The anode loses weight much more rapidly and the spec- 

 trum of the anode is very plain, while with smaller cur- 

 rents it was scarcely visible. Moreover the effect was 

 shown to exist in nitrogen as well as in air, so that it can 

 not be explained as being due to the presence of oxygen. 



It is claimed by Hagenbach and Veillon that there are 

 six distinct forms of the copper arc, that there are three 

 different anode drops which may occur, and that with 

 each of these it is possible to have two different cathode 

 drops. The same phenomenon has been found to a 

 limited extent with other metals, although in general the 

 first stage is absent with metals having a low melting 

 point. 1 



Two attempts have been made to find the anode and 

 cathode drops with the arc between metals in air, but it is 

 very difficult to make accurate measurements. The voltage 

 of such arcs under ordinary conditions is very variable and 

 the introduction of a carbon pencil causes it to fluctuate 

 through even greater ranges. Moreover, with the intro- 



1 See also the following: 



Cady and Vinal, Amer. Journ. Sc., (4), 28, 89; 1909. 

 Fabry and Buisson, Journ. d. Phys., (4), 9, 929; 1910. 

 Hagenbach, Physik ZS., 12, 1015; 1911. 



