ALTERNATING-CURRENT ARCS 



III 



rapidly changing. Thus, if the different values of the 

 current as shown in Fig. 31 are taken as abscissae and the 

 corresponding values of the potential difference as ordinates, 

 one would have the curve given in Fig. 33, which is a 

 dynamical characteristic curve. 



This gives in a different way some of the things shown 

 by Fig. 30 and also calls attention to the fact that the 

 relation between potential difference and current is not the 

 same when the current is increasing as when it is decreasing. 

 Thus the part of the curve at the left indicates the 

 relation existing when the current is changing from the 

 greatest negative current to the maximum positive. The 

 curve on the right indicates the relation 

 when the current is changing from posi- 

 tive to negative. 



When the current is increasing, the 

 carbons are not so hot as they are when 

 it has been large and is becoming small. 

 With the cooler electrodes the potential 

 difference is greater than when the 

 electrodes are hotter, although the cur- 

 rent is the same. This peculiarity of 

 the curves is called by Simon "arc-light 

 hysteresis." 



These curves are quite different from 

 those shown in Figs. 3-7. There the 

 potential difference approached infinity 

 as the current approached zero. Here 

 the two approach zero together. The 

 reason is that the part of the curve 

 near the origin does not strictly 



FIG. 34. 



refer to an arc. It refers rather to the simpler form of 



