414 



ALTERNATING CURRENTS 



constant for a considerable time if used only occasionally. 

 By means of these lamps, secondary incandescent lamp standards 

 may be calibrated and used. 



180. Unit Solid Angle or Steradian. In order to understand 

 the fundamentals of light emission and distribution, it is neces- 

 sary to know what is meant by solid angle. A unit solid angle is 

 the angle at the center subtended by a unit area on the surface of a 

 sphere which has a unit radius. 



Figure 376 shows a sphere whose radius 

 is 1 ft. An area of 1 sq. ft. on its surface 

 subtends a conical solid angle at the 

 center. This angle is a unit solid angle, 

 L I sq.ft. sometimes called the steradian. 



As the area of the surface of a sphere is 

 equal to 4?rr 2 , there must be 4ir units of 

 376. Unit solid solid angle about the center of a sphere. 



This may be seen by letting r = 1. 



If any area on the surface of a sphere be divided by the 

 square of the radius, the result is the solid angle that this area 

 subtends at the center. 



Example. A certain sphere is 3 ft. in diameter. How many unit solid 

 angles does an area of 3 sq. ft. on its surface subtend at its center. 

 The number of unit solid angles 

 3 



FIG. 



(1.5)' 



= 1.33 steradians. Ans. 



181. Luminous Flux: 

 Lumen. Light may be 

 considered as a flux which 

 emanates from a luminous 

 source in the same way 

 that magnetic flux ema- 

 nates from a magnetic pole. 

 The amount of illumina- 



1 Lumcr. 



Fio. 377. One lumen, the unit of light flux. 



tion emitted by a luminous 



source may be considered as being the total light flux emanating 



from that source. 



Figure 377 shows a candle placed at the center of a sphere 

 whose radius is 1 ft. Assume that this candle emits light uni- 

 formly in all directions, the intensity being equal to that in 



