ARC IN AIR BETWEEN CARBON ELECTRODES 13 



amount of energy used in the arc increases. The greater 

 amount of heat thus produced causes the conductivity of 

 the gas between the electrodes to be greater, and the con- 

 ductivity increases so rapidly that the voltage needed 

 actually becomes less with the larger currents. Further 



02 46 8 10 12 14 16 18 20 22 24 26 28 30 



CURRENT IN AMPERES 

 FIG. 5. 



consideration of this will be given in the chapter on the 

 theory of the arc. 



After decreasing to nearly a constant value the poten- 

 tial suddenly drops, as is indicated in the dotted lines in 

 the curve. At this point the arc begins to emit a loud, 

 hissing sound. Such an arc is called a hissing arc. This 

 will be more fully described in a following paragraph. 



A similar set of curves is shown in Fig. 6, where the 

 positive carbon is cored and the negative one is solid. 



