3l6 ELEMENTS OF ELECTRICAL ENGINEERING. 



the lamp along the photometer bar. Thus Fig. 177 shows three 

 mirrors, AA, BB and CC, arranged to reflect the light from a 

 fixed lamp, Z, along a photometer bar. The three mirrors are sup- 

 ported in a rigid frame which may be rotated about the line, DE, 

 as an axis. The figure shows the mirrors in the position to reflect 

 the downward beam from the lamp to the photometer screen. 



Photometer 





screen 



Fig. 177. 



The mirrors, AA, BB and CC, Fig. 1 77, must be large enough 

 so that with the eye placed at the photometer screen one can see 

 the entire luminous surface of the lamp including the globe or 

 shade ; and the distance of the lamp from the screen must be 

 taken as the sum of the distances,/, g, h and z, Fig. 177. The 

 mirrors reflect a certain fractional part, only, of the light from the 

 lamp, and, therefore, the photometer reading must be multiplied 

 by a correction factor. This correction factor is found by ob- 

 serving the photometer readings of the horizontal beam from the 

 lamp with and without the mirrors, making due allowance for the 

 effective distance from lamp to screen in each case. 



If it is feasible the lamp should be rotated steadily about the 

 vertical axis, kf, in Fig. 177, while the photometer readings are 

 being taken. 



131. Measurement of total light flux from a lamp. If a lamp 

 were to emit light of the same conical intensity in all directions, 

 then the conical intensity of the light in hefners (or candles) 

 would be numerically equal to the total light flux from the lamp 

 in spherical-hefners (or spherical-candles), and a single measure- 

 ment of such a lamp by means of a Bunsen photometer would 

 give not only the conical intensity of the light in hefners (or 



