3 io 



ELEMENTS OF ELECTRICAL ENGINEERING. 



lamps usually show differences of color, and these differences of 

 color do not disappear when the attempt is made to adjust a 

 photometer to equality of brightness. 



The results of the spectro-photometric * comparison of gas 

 light, lime light, and day light are shown by the curves in Fig. 

 174-t The curves refer to beams of gas light, lime light and 

 day light all of which have the same intensity at Fraunhofer's D 



line (the middle of the yellow 

 region of the spectrum) ; and 

 the curves show, for ex- 

 ample, that the beam of day 

 light is about six times as 

 bright as the beam of gas 

 light at Fraunhofer's F line 

 (in the blue region of the 

 spectrum), and only about 

 one fourth as bright as the 

 gas light at Fraunhofer's B 

 line (in the red region of 

 the spectrum). 



F b E 

 Fig. 174. 



C B A 



127. Standard lamps. - 



The British standard candle 

 is a sperm candle made ac- 

 cording to exact specifications and burning 1 20 grains of sperm 

 per hour. The Hefner lamp, so called from its inventor, is a 

 lamp which burns pure amyl acetate. The wick and its con- 

 taining tube are of prescribed dimensions and the wick is turned 

 up to give a flame of prescribed height. J 



* A simple form of spectro-photometer is described in Nichols and Franklin's Ele- 

 ments of Physics, Vol. III., p. 130. 



f See Nichols and Franklin. A spectro-photometric comparison of sources of 

 artificial illumination, American Journal of Science, Vol. 38, pp. 100-114, De- 

 cember, 1889. 



JSee Zeitschrift fur Instrumentenkunde, Vol. XVIII., p. 257, July, 1893. A 

 very satisfactory discussion of the Hefner lamp may be found in Photometrical Meas- 

 urements, by Wilbur M. Stine, The Macmillan Co., 1900, pp. 145-160. 



