CH. XIII] THE ELECTRIC ARC 539 



arc which consists of a copper positive electrode and a negative 

 electrode made of a mixture of iron and titanium oxides, the lowest 

 arc voltage is about 30 volts. The lowest arc potential between 

 electrodes of other substances than carbon are, magnetite 30; 

 platinum 27; iron 26; nickel 26; copper 23; silver 15; zinc 16; 

 cadmium 16; mercury 13. 



The potential differences in the arc lamp are practically constant 

 no matter what current is flowing, but there is a small change with 

 change in current. This is generally such that the greater the 

 current the less the potential difference, and may be explained as 

 follows : 



Suppose a current of 10 amperes to be flowing between the two 

 electrodes of an arc lamp. This will be carried by a small cone 

 shaped mass of conducting gas (fig. 293 A). If the current is 

 increased to 20 amperes the extra heat developed is sufficient to 

 bring more air to a high enough temperature to conduct current, 

 and the cone of conducting gas increases in diameter (fig. 293 B). 



A large cone of conducting gas will be losing heat at a relatively 

 less rate than will a small cone, hence its temperature will be higher 

 and its resistance will be less. As a result of the increased con- 

 ductivity of the hot gases of the arc stream, the greater the current 

 the lower will be the potential difference between the electrodes. 

 There is also a slight lowering of the contact potential difference 

 between the electrodes and the arc stream as well as a lessening of 

 the potential drop in the arc stream. 



THE USE OF BALLAST 



744. The need of a ballast in series with the arc to control the 

 current. On account of the peculiar electrical behavior of the arc 

 lamp it is necessary to use a ballast such as rheostat, or an inductor 

 in series with the arc, or else to use an especially designed generator. 



743a. While the two electrodes of an arc lamp may be of any conducting 

 material, with projection arc lamps the electrodes are always made of carbon 

 and are generally referred to simply as carbons. 



743b. These figures are approximations and vary slightly with arc 

 length and current but are general averages for the usual arc lengths employed : 

 3 to 10 mm. 



See Mrs. Ayrton, The Electric Arc. 



