176 E. Merritt — Light from Incandescent Lamps. 



The curves in fig. 4 show the relation between candle power 

 and light energy at different candle powers. Abscissae are the 

 candle powers at which the lamps are run, and ordinates meas- 

 ure the energy of the light per candle power, or, in other 

 words, the mechanical equivalent of one candle power. The 

 numerical values of the ordinates are given in the columns 



headed en?. °f the tables. It will be seen that the energy per 

 candle power varies from 1-5 Watts at 0*5 C. P. to about 0*3 

 Watts at 16 C. P. In every case the intensity of the light, as 

 measured by its candle power, increases more rapidly than the 

 energy of the light. 



In this connection it is interesting to compare the value of 

 the mechanical equivalent of a candle power of lamp light, 

 found by Dr. J. Thomson some twenty years ago.* His 

 method was similar to that used in these experiments. A cell 

 of distilled water was used instead of an alum solution, and no 

 correction was made for the dark heat passing through. To 

 get absolute measurements of energy he standardized his ther- 

 mopile by means of Leslie tubes. He found the energy of 

 one candle power of light from an oil lamp to be 2*5 Watts, 

 and for a gas flame and a standard candle the value was very 

 nearly the same. This is considerably larger than the greatest 

 value that I found. A part of the difference might be ac- 

 counted for by the correction for dark heat passing through 

 the water, which Dr. Thomson neglected ; and the standards 

 of light used in the two cases may have been different. It is 

 hard to believe that the temperature of the incandescent mat- 

 ter in an oil flame is less than that of a lamp filament at 0*5 

 C. P. Recent spectrophotometry determinations by Dr. E. L. 

 Nichols and Mr. W. S. Franklinf show that the light from a 

 gas flame is of almost exactly the same quality as that from an 

 incandescent lamp at 16 C. P. This would seem to indicate 

 that the incandescent matter in a gas flame is at about the same 

 temperature as the lamp filament. 



* Julius Thomson. "Das mechanische Aequivalent des Lichtes;" Pogg. Ann. 

 exxv, p. 348. 



f Proc. A. A. A. S., Cleveland meeting, 1888. 



