THE ELECTRIC ARC 



this view. The introduction of a carbon pencil into such 

 an arc makes it very unsteady and changes the vapor in 

 the arc to some extent, so that the values given below can 

 not be considered accurate. They are, however, of some 

 value. These observations were made with a constant 

 length and constant current. 



In the first case both electrodes are carbon. In the 

 second the anode is iron and the vapor from this is highly 

 conducting and causes a large decrease in the drop in 

 potential near the anode. The cathode drop is slightly 

 greater than when both electrodes are carbon, the reason, 

 no doubt, being that the iron anode is cooler than the 

 carbon anode, and allows the cathode to cool off more 

 rapidly, requiring a large cathode drop to maintain the 

 temperature of the cathode. 



In the third case there is still enough iron vapor in the 

 arc to cause a small anode drop, but the heat is conducted 

 away from the iron cathode so rapidly that it requires a 

 still higher cathode drop to maintain the temperature of 

 the iron at the required point. 



