3S6 



Cady and Arnold — Electric Arc. 



to avoid touching the anode. Outside the mantle is a thin 

 second mantle or shell, springing from the cathode and prob- 

 ably due to oxidation of the negative globule. 



The arc starts from an intensely bright spot on the cathode, 

 but the anode end is very dim and diffuse. As the current is 

 increased by diminishing the external resistance, the various 

 parts of the arc brighten in intensity, and the terminal voltage 

 decreases in the usual manner. Fig. 5 (p. 403) shows a group 



Fig. 1. The Iron Arc. The positive terminal is below in each case. 

 Right : 1st stage, 1 = 4-5 mm , 1 = 1-3 amp., E= 74 - 5 volt. 

 Left : 2d stage, 1 = 4-5 mm , 1 = 13 amp., E = 61 volt. 



for various lengths of arc. 

 Here 



of characteristic curves for iron 



The curve for 8 mm may be taken as a typical case, 

 the part MA is on the first stage, with large external resist- 

 ance. In this, as in the other diagrams, full lines represent 

 those regions where the discharge is stable, broken lines those 

 where no steady discharge can be maintained. 



When the current reaches a value a little over one ampere, 

 a very small decrease in external resistance causes an abrupt 

 change to take place. The potential difference drops about 

 12 volts, the current increases slightly, a distinct hissing 

 sound is heard, and the blue-white light of the cathode extends 

 nearly uniformly to the anode, where a small bright spot 



