128 



THE ELECTRIC ARC 



light was measured. Because of this irregular distribution 

 the average intensity over a part or the whole of the region 

 illuminated is often given. If the average intensity in all 

 directions is given, it is called the mean spherical candle 

 power (abbreviated to m.s.c.p.). Since the part of the light 

 which is thrown below the horizontal plane passing through 



o a 



1500 



60 



FIG. 39. 



the light is all that is useful in the majority of cases, meas- 

 urements are often made of this part alone and the aver- 

 age of this is called the mean hemispherical candle power 

 (m.h.s.c.p.). 



Some of the measurements given in the following pages are 

 taken from German sources and are given in hefner units. 

 One hefner unit is equivalent to 0.88 English candle. 



Distribution of Light. With the common form of the 

 carbon arc the greater part of the light comes from the 



