PHOTOMETRY OF THE ELECTRIC ARC 133 



source of light is the long luminous vapor. Because of the 

 high conductivity of this vapor, it is possible to have very 

 long arcs and the candle power is ap- 

 proximately proportional to the length of 

 these arcs. 



The distribution of light is much more 

 symmetrical than with the more common 

 forms. In some cases this is a disadvan- 

 tage, as when the light is needed beneath 

 the lamp. On this account the carbons 

 are sometimes placed nearly parallel and 

 above the arc as shown in Fig. 42, the 

 arc being kept below by means of a magnet, 

 same plan as that of the "Carbone" lamp. 1 



The amount of light depends on the amount of substance 

 added to the carbon. For example, the candle power with 

 different percentages of calcium fluoride has been studied 

 by Wedding 2 and his results are shown in Fig. 43. The 

 arc with which he worked was one between converging 

 carbons with a reflector above. In each case there was a 

 current of 9 amperes and a potential difference of 45 volts. 

 The amount of added mineral is given with each curve. 



The efficiency of the flaming arc varies, of course, with 

 different makes of carbons and with different kinds of 

 lamps. Sharp 3 states that Blondel's flaming arc requires 

 between 0.16 and 0.18 watt per mean lower hemispherical 

 candle power, which is less than one-fourth of that of 

 the open carbon arc. Marchant 4 states that the yellow 



1 Lond. Elec., 57, 51, and 129; 1906. 



2 Elektrot. ZS., 23, 702; 1902. 



3 Trans. Intern. Elec. Cong., St. Louis, 2, 765; 1904. 



4 Elec. World, 56, 216; 1910. 



