64 THE ELECTRIC ARC 



Thus, according to Malcolm and Simon, 1 who worked with 

 small currents, the product El is not a linear function of 

 7, as it is in the preceding equations. They found the 

 potential difference for an arc between copper terminals to 

 vary from 80 volts for 0.6 ampere to 58 volts for i.i 

 amperes, the length in both cases being 4 mm. 



Hagenbach and Veillon 2 found it difficult to make any 

 exact measurements of the copper arc, but, as far as they 

 could, they determined its voltage both in air and in a 

 vacuum, and in no case did the curve follow the Ayrton 

 formula. 



There are, however, two facts upon which there is 

 agreement; first, that the potential difference with arcs 

 between metals is in general smaller than that between 

 carbons, and second that the rate of increase with in- 

 creasing length of arc is greater than with carbon arcs. 

 From this it would follow that the sum of the anode and 

 cathode drops in the metal arcs is less than in the carbon 

 arc, and that the electric force is greater. 



The difficulty of making accurate measurements of the 

 metal arcs is partly due to the fact that there are two 

 or more forms of these arcs, and that under some con- 

 ditions there is a constant fluctuation from one form to 

 the other. Thus it has been shown by Cady and Arnold 3 

 that there are two distinct forms of the arc between iron 

 terminals corresponding somewhat to the quiet and hissing 

 forms of the carbon arc. The first stage occurs with small 

 currents, usually less than 2 amperes. The voltage is in 

 the neighborhood of 75 volts. The gas near the anode is 



1 Phys. ZS., 8, 477! 1907- 



2 Phys. ZS., n, 833; 1910. 



3 Amer. Jour. Sc., (4), 24, 383; 1907. 



