PHOTOMETRY OF THE ELECTRIC ARC 127 



A second difficulty is due to the continuous fluctuation 

 of the arc. An arc between the better kinds of carbons 

 can be kept fairly steady by constant regulation, although 

 even then sudden changes are apt to occur. But even if 

 satisfactory measurements are made, one has no very 

 definite knowledge of the candle power of the lamp when 

 run under commercial conditions where the regulation of 

 the lamp is not perfect. Fortunately, where cored carbons 

 are used for the anode, this difficulty is not as great as it 

 is with solid carbons. 



A third difficulty is due to the irregular distribution of 

 the light. There is no form of artificial light which is 

 equally luminous in all directions, but the common form 

 of the arc has the disadvantage that there are two direc- 

 tions in which the mechanism of the lamp obstructs the 

 light. Moreover the distribution of light from a direct- 

 current arc is not symmetrical about these two ends. This 

 can well be seen by referring to Fig. 39 taken from the data 

 of Fleming and Petavel. 1 is the arc. The intensity of 

 light in any direction is indicated by the curved line and 

 is proportional to the distance of this line from the arc O. 



Again the center of the arc is usually at one side of the 

 center of the carbons and wanders from one place to an- 

 other. As a result the distribution of the light about a 

 vertical axis is unsymmetrical and the intensity in any 

 direction will vary from time to time. 



Because of the irregular distribution of light from the 

 arc, it means nothing to say that an arc has a certain 

 candle power, unless some definite direction is also given. 

 An arc light of 1000 candle power might be a very good 

 light or a poor one, depending on the direction in which the 

 1 Phil. Mag., (5), 41, 355J 1896. 



