12 



THE ELECTRIC ARC 



Even when the arc has reached a constant condition 

 with a given current it requires several minutes for it to 

 become steady again if the current is changed. This is 

 shown in the curves given in Fig. 4. The positive carbon 

 was 13 mm. in diameter and was cored, the negative n mm. 

 and solid. The length of the arc in the upper curve was 

 4 mm. and in the lower 3 mm. 



60 



55 



50 



040 



50 



45 



40 



o5 Amps. 



10 Amps. 



16 Amps, 



25, Amp; 



10 20 30 40 50 60 70 80 90 100 110 



TIME IN MINUTES 

 FIG. 4. 



The most careful examination of the relation between 

 current and voltage has been made by Mrs. Ayrton, and 

 some of her results are shown in the accompanying figures. 

 Fig. 5 shows a series of curves, called " characteristic" 

 curves, giving this relation for different lengths of arcs 

 with solid carbons. They show clearly the difference be- 

 tween the resistance of the arc and that of a metal. With 

 a metal the potential difference increases as the current 

 increases and is directly proportional to it. Here the volt- 

 age decreases when the current increases. It seems at 

 first as if the greater the cause the less the effect, but it 

 should be noticed that while the voltage decreases the 



