CHAPTER VII. 

 PHOTOMETRY OF THE ELECTRIC ARC. 



THE chief use of the arc is to give light and its commercial 

 value depends on the amount and color of its light. These 

 subjects will be considered briefly in the following chapter, 

 but they belong more properly to books on photometry and 

 spectroscopy and the reader is referred to such books for 

 their more adequate treatment. 



There are three difficulties met with when making 

 measurements of the candle power of electric lamps. The 

 first and most radical one is due to the difference between 

 the color of the arc and that of the light standards in 

 common use. Nearly all other forms of illumination are 

 similar in color to the candle and can be compared with it, 

 but this can not be said of the arc. The open arc is dis- 

 tinctly bluer. The enclosed is somewhat less blue due 

 to the absorption by the glass, but it is still bluer than the 

 candle. The flaming arcs vary greatly, but none of them 

 have the same color as the candle, and the mercury arc is 

 less like the candle than any of the others. 



Not only is it difficult for an observer to compare two 

 unlike colors in a photometer, but different observers arrive 

 at results differing often by many per cent. The ideal 

 method would be one giving relative values for practical 

 purposes, such as reading a paper or seeing an obstacle in 

 the road, but unfortunately we have no way of making 

 definite comparisons of this sort. 



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