22 ELECTRIC LIGHTING. 



houses it is better to use these last, for the angle within 

 which the electric light has to be projected seldom exceeds 

 1 80, and if the carbons are placed as stated above, the 

 light is increased in a proportion the mean of which is i'66 

 to i. It is, of course, necessary that the positive carbon 

 should be above, and the negative carbon below." 



In a report made by an American Commission on the light 

 produced by different induction machines, the following 

 values are given for the luminous intensities yielded in dif- 

 ferent directions by this arrangement of the carbons : 



In front 2,218 candles, or 231*0 Carcel lamps. 



On one side ... 578 6o'2 



On the other side 578 6o'2 



At the back ... in 11-5 



This gives a mean of 87 r candles. Now the light produced 

 by this same machine under the same conditions, but with 

 the carbons placed in a line with each other, yielded an in- 

 tensity of only 525 candles. "This would seem to show," 

 says the report in question, " that an increase of nearly 66 per 

 cent, of light is due to the arrangement of the carbons; but 

 a close examination has shown that the fact is not so, and 

 that upon the whole there is no advantage in using this 

 arrangement except when the light is to be transmitted in 

 one direction only." * 



The resistance of the voltaic arc is rather variable, beinr; 

 dependent upon the state of ignition of the carbons, and 

 therefore upon their size and the distance between them ; 



* In Douglass's report these differences of illuminating power were re- 

 presented by the following figures (the illuminating power of the light pro- 

 duced by the carbons placed in a line being 100) : 



In front 287, or 2-87 times stronger. 



On one side : ... 116 ,, i'i6 



On the other side 116 ,, i'i6 ,, 



Behind 38 ,, 0-38 ,, 



The mean would therefore be in favour of the light emitted under the con- 

 ditions we are speaking of, a smaller luminous power than that obtained by 

 the American Commission. 



