1887.] 



On Photometry of the Glow Lamp. 



255 



The following is an example of measuring by nsing known currents 

 and cutting off more or less of the comparison light by the sectors. 

 The observations have been given in full to show the deviation of 

 individual observations from the mean : — 



Table IV. 



c. 



P. 



Watts x 



Aperture of disks 



Mean 

 observed 

 aperture. 



Calculated 



constant. 



to balance light. 



aperture. 



7-9 



28 -1 



222 



90* 



90 



91 -2 



7-75 



27 -4 



212 



81, 81, 80 



80-7 



80-6 



7-6 



26-8 



204 



72-5, 72-5, 72-5 



72-5 



72 -5 



7 25 



25-5 



185 



56 -5, 56, 56 -0,55, 55 '5 



55-8 



55-6 



7*00 



24-8 



174 



46 -5, 47, 46 "5, 45 -5 



46-4 



46-6 



6-7 



23-8 



159 



37, 35, 36, 36- 



36-0 



36-0 



6-25 



22-2 



139 



23, 24, 23-5, 24 



23-8 



23-6 



5-85 



20-6 



120-5 



14-15, 15, 14-5, 14 



14-5 



14-6 



5-7 



20-1 



114 



11-75, 11-75, 11-75 

 8, 8, 7-5, 8, 7-5 

 6 -5, 6-5, 6-5, 7 



11-75 



11-75 



5*4 



19-1 



102 



7-6 



7-7 



5-3 



18-7 



99 



6-6 



6-8 



m = 53. n = 17*7. 



The foregoing examples will give an idea of the accuracy with 

 which measurements may be made by either method, and of the 

 exactness with which the parabolic curve is followed. It seems that 

 the photometry of incandescence lamps may be well carried out by 

 measuring the watts. It may be objected that each observation 

 requires readings of the galvanometers, but this is avoided by the use 

 of the formula given in the beginning of this paper. Two observations 

 of current and potential enable the constants to be calculated, and after 

 that one galvanometer alone need be used ; by preference that one 

 giving comparative volts. The current is calculated from such a 

 reading and subsequently the watts. 



Mr. W. H. Preece, in his paperf already alluded to, came to 

 the conclusion that the intensity of the light emitted from a glow 

 lamp varied as the sixth power of the current. This formula is fairly 

 exact within limits, but it is obviously empyric, since where the 

 current is small enough only to cause dark radiation it must fail. 

 The example that he gives would require some slight rectification 

 before it can be used as in the method given above ; since the small 

 distances at which the candle he employed was placed from the 

 screen make it necessary to apply corrections for the thickness and 

 length of flame. 



* The light was fixed so as to balance as nearly as possible when the sectors were 

 at their full aperture. 



t ' Roy. Soc. Proc.,' vol. 36, p. 270. 



