542 



USE OF BALLAST WITH ARC LAMPS 



[CH. XIII 



ing out of the fuses before any serious damage could result. Even 

 after the arc is burning, if one were to remove the resistance by 

 short-circuiting it, the current would increase to an enormous 

 value. 



745. Example with 110 volt supply, using a rheostat. If we 

 assume that the arc is of such a length that the potential difference 

 between the electrodes is 10 volts, and that this potential difference 



FIG. 296. LATERAL AND FACE VIEWS OF THE RIGHT-ANGLE CARBON ARC 

 WITH 25 AMPERES OF ALTERNATING CURRENT. 



By comparing this picture with fig. 295 it will be seen that in this both 

 craters are of the same size; and that, although 25 amperes of current are 

 flowing, the crater on the upper carbon from which the light is derived is much 

 smaller than with the direct current. The sizes of the upper crater give a good 

 idea of the amount of illumination furnished in the two cases. 



An additional exposure was made to bring out the carbons as in fig. 292-293. 



remains practically the same if the current is diminished or in- 

 creased, and if the supply is no volts, and that this voltage is 

 practically independent of the current used, it is evident that 

 between one of the electrodes and one of the supply wires there must 

 be a potential drop of 60 volts. By using a rheostat at this point 

 the current is controlled. Thus suppose that the rheostat has a 

 resistance of 6 ohms, then according to Ohm's law ( 724a), as the 

 potential difference across its terminals is 60 volts, the current will 



