Horizontal Candle-Power of Incandescent Lamps. 213 



where (/> is half the angle subtended by the whole filament at 

 the centre of! the photometer-disk. 



If the filament has a double or multiple loop, then the 

 precise correcting factor for angular magnitude of the whole 

 loop is not quite so sharply defined. It will, however, 

 always operate to increase the fraction or percentage which 

 represents the ratio of mean spherical to mean horizontal 

 candle-power. It will always be a small correction if the 

 whole filament subtends an angle, say, of not more than 

 10° at the centre of the photometer-disk, as in the ordinary 

 photometry of an incandescent electric lamp. 



These considerations show that incandescent lamp manu- 

 facturers can without much difficulty furnish the figures 

 required to give the true photometric efficiency of any type 

 of lamp. Instead of stating, as at present, the value of the 

 maximum candle-power taken in one direction, viz., the 

 direction perpendicular to the axis of the lamp in which the 

 whole of the filament is seen exposed, they should furnish the 

 mean horizontal candle-power taken in the same direction 

 when rotating the lamp around its vertical axis, using for 

 this purpose well-known mechanical devices. 



If this reading is taken at such a distance from the lamp 

 that the whole filament does not subtend a greater angle than 

 5° to 10° at the photometer-disk, then the mean spherical 

 candle-power of the incandescent lamp is for practical pur- 

 poses very nearly obtained by multiplying this mean horizontal 

 candle-power by the factor 7r/4. From the measurement of 

 the electric power in watts supplied to the lamp, we have at 

 once the means of calculating the total flux of light per watt 

 which fixes scientifically the efficiency of the lamp as an 

 energv- transforming device. 



Appendix. 



To enable a iudgment to be made as to the validity of the 

 . . . 



assumption that the intensity of the light proceeding from a 



filament in any direction varies as the cosine of the angle 

 between the ray and the normal to the element, the following- 

 experiments were tried by Mr. Gr. B. Dyke at the suggestion 

 of the writer. 



A tubular incandescent carbon-filament lamp was prepared 

 with a straight filament about 10 inches in length. This was 

 placed on a photometric gallery at a distance of about 40 inches 

 from a Lummer-Brodhun photometer, and two baffle-screens 

 interposed having circular holes in them about 1 inch in 

 diameter. These screens were placed so as to divide the 



