ARC IN AIR BETWEEN CARBON ELECTRODES 



increase in the current, the potential difference needed 

 for the arc would be less than the E.M.F. of the dynamo, 

 and under such conditions the current would increase 

 still further. This would cause a further decrease in the 

 voltage used by the arc and if the E.M.F. were main- 

 tained, this process would continue until the dynamo was 

 burnt out. 



-B 



CURRENT 



FIG. 8. 



If the current momentarily decreased, a higher potential 

 difference would be needed than the dynamo was giving, 

 and the arc would be extinguished. 



On the other hand if there is to be resistance in the cir- 

 cuit, then the E.M.F. of the dynamo must be greater than 

 the voltage of the arc. In order to determine how much 

 greater the E.M.F. must be, we need to remember that 

 there must be enough resistance in series with the arc, so 

 that any increase in the current will produce a greater in- 

 crease in the drop in potential through the resistance 

 than the decrease in voltage occurring in the arc. This 

 can perhaps be made clearer by referring to Fig. 8. Let 

 the curve PP'P" represent the relation between the cur- 



