76 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



electrode and decreases in brilliancy and volume as it nears the posi- 

 tive (copper) electrode. 



The peculiarities of the arc are such that Halvorson invented an 

 entirely new principle of control. The electrodes are normally apart. 

 In starting, they are drawn together by a starting magnet with suffi- 

 cient force to dislodge the slag which forms on the negative electrode 



'^^i^ii 



Diagram of Series Magnetite Arc Lamp. 



The method of control, entirely different from that of other 

 arc lamps, was invented by Halvorson to meet the peculiarities of 

 this arc. 



and which becomes an insulator when cold. Current then flows 

 through the electrodes and through a series magnet which pulls up a 

 solenoid breaking the circuit through the starting magnet. This allows 

 the lower electrode to fall a fixed distance, about seven-eighths of an 

 inch, drawing the arc, whose voltage is then about 72 volts. As 

 the negative electrode is consumed, the length and voltage of the arc 



