130 THE ELECTRIC ARC 



and because the direction is nearly horizontal. For ex- 

 ample, if a lamp giving the distribution shown in Fig. 26 

 were hung at a height of 15 ft., the light striking the ground 

 at a distance of 150 ft. would make an angle with the 

 horizontal of less than 6 degrees, and the candle power in 

 this direction is not more than a fourth of what it is in a 

 direction making an angle of 50 degrees with the horizontal. 



Effect on Candle Power Produced by Length of Arc, 

 Current, Etc. From the statement given above concern- 

 ing the distribution of light it follows that any cause affect- 

 ing the amount of the crater that is visible from any given 

 direction affects the intensity of light in that direction. 

 Thus the shorter the arc the more the light from the crater 

 is cut off by the cathode. One would expect from this that 

 the longer the arc the greater would be the candle power. 

 There is the more reason to expect this, since there is more 

 luminous vapor with the longer arcs. It has been found, 

 however, that the candle power does not increase indefi- 

 nitely as the arc is lengthened, but reaches a maximum and 

 then decreases with greater length. Thus Ayrton found the 

 maximum candle power to be with lengths of about 4 mm. 

 Mrs. Ayrton plotted curves taken from the data of Blondel. 1 

 These are reproduced in Fig. 40. The numbers near the 

 curves indicate the diameters of the electrodes. Those 

 above the dash indicate the diameter of the anode in milli- 

 meters; those below, the diameter of the cathode. 



From this it would appear that there is a maximum 

 candle power at about 6 mm. of length. Mrs. Ayrton 

 believed the decrease with greater lengths to be due to 

 the absorbing power of the vapor between the terminals. 



1 Mrs. Ayrton's "Electric Arc," p. 33.3. Eel. Elec., 10, 289, 496, and 

 S39J 1897. 



