Candle^Power of Incandescent and Arc Lamps. 147 



almost touch the bulb in one place, the factor is only altered 

 by 1 per cent. 



The unsatisfactory nature of the present method of photo- 

 metering- incandescent lamps is seen in the second reduction 

 M.S.C.P. 

 or T\P — 



This shows that the mean intensity of the light emitted is 

 only about 75 per cent, of the nominal or marked intensity, 

 and also that the mean and marked intensities bear no definite 

 relation to one another, even in the case of lamps of the same 

 type of filament. 



A more satisfactory method would be to determine the 

 mean horizontal candle-power of each lamp by using a rotator 

 similar to that described in the paper, and then to obtain the 

 M.S.C.P., employing a factor previously determined for the 

 particular type of filament by means of an integrating 

 photometer as explained. 



The marking on lamps thus photometered would then be 

 proportional to the amount of light emitted, and hence would 

 be a figure definitely determining their utility as a means of 

 illumination. 



In this manner the suggestion made by Dr. Fleming in 

 his paper on the "Photometry of Electric Lamps " * could 

 be practically carried out with but little more trouble than 

 the present unscientific method of marking incandescent 

 lamps. 



Arc Lamps. 



The Arc lamp being symmetrical about the vertical will 

 not require revolution in the integrator, but the readings 

 will give directly the ratio of the mean spherical to the 

 horizontal candle-power. 



Preliminary experiments only have been made with arc 

 lamps, but these have been sufficient to show that arc lamps 

 may be dealt with in the integrator with bat little more 

 trouble than incandescent lamps. 



In conclusion, the author wishes to express his indebtedness 

 to Dr. J. A. Fleming, F.R.8., and to Mr. W. C. Clinton, B.Sc, 

 for their constant advice and assistance during the progress 

 of the work. 



* Jo urn. Inst. Elec. Eng. vol. xxxii. p. 165. 

 L2 



