33 2 ELEMENTS OF ELECTRICAL ENGINEERING. 



The instability of the electric arc when the current is supplied 

 from constant-voltage mains. An arc cannot be maintained be- 

 tween carbon rods connected to constant-voltage mains unless a 

 resistance is connected in series with the arc. This is evident 

 from the volt-ampere characteristic of the arc as follows : If one 

 could imagine an arc burning steadily between constant-voltage 

 mains, then a slight decrease of current would produce a con- 

 traction of the arc and an increased voltage would be required to 

 maintain the arc ; but this increased voltage would not be forth- 

 coming hence the current would continue to decrease and the arc 

 would go out almost instantly. On the other hand, a slight in- 

 crease of current would produce an increase in the sectional area 

 of the arc, and the constant voltage would be more than sufficient 

 to maintain the arc, hence the current would continue to increase, 

 rising almost instantly to an excessively large value. These 

 changes are much too rapid to be controlled by the lamp mechanism 

 which moves the carbon. 



If a resistance is connected in series with the arc, a decrease of 

 current causes a decrease of the voltage-drop, Ri t in the re- 

 sistance, and an increase of current causes an increase of voltage- 

 drop, so that, if the resistance is sufficiently large the change of 

 Ri will overbalance the change of voltage across the arc due to 

 any assumed change of current, and the arc will be stable. A 

 resistance must always be connected in series with an arc lamp 

 which is supplied with current from constant-voltage mains. A 

 resistance used in this way is called a ballast. When an arc lamp 

 is operated from constant-voltage alternating-current mains, an 

 inductance, consisting of a coil of wire wound on a laminated iron 

 core, may be and generally is used as a ballast instead of a 

 resistance. 



When arc lamps are connected in series and supplied with cur- 

 rent from a constant-current generator (or, in the case of alter- 

 nating current, from a constant- current transformer), no ballast 

 need be used. 



The direct-current arc. The foregoing paragraphs apply in a 



