Cady and Arnold — Electric Arc. 



403 



pointed out. The cause of this overlapping 1 can now easily be 

 understood. Beginning with point B, if the current is 

 decreased, the change back to the first stage does not take 

 place at once along the line BA, because the arc on the second 

 stage is very contracted at the anode, so that the high tem- 

 perature of the positive base of the arc is maintained until a 

 smaller value of current, represented at G, has been reached. 

 Here the expenditure of energy at the anode is so small that 

 vaporization is no longer possible, and the first stage enters in. 



VOLT 



Z 3 AMP. 



Fig. 5. Characteristic curves, iron arc. 



For an analogous reason the change from first to second stage 

 occurs along AB, not DC. 



It will be noticed that the change AB occurs with a larger 

 current for long than for short arcs, and that the change in 

 voltage is also greater for long arcs. The probable explana- 

 tion is that with a long arc on the first stage the anode is com- 

 paratively cool, so that tlie current has to increase farther 

 before evaporation sets in. If the rate of vaporization increases 

 with the current, the change in current, and therefore also 

 in voltage, must be greater the longer the arc. This is the 

 opposite to what occurs at the hissing point of the carbon arc* 



If the arc burns with the electrodes almost in contact, the 

 * Mrs. Ayrton, The Electric Arc, p. 288. 



