THE LUMINOUS EFFICIENCY OF ILLUMINANTS 537 



with the object of ascertaining what percentage of the energy 

 given to modern illuminants reappears as useful visible light. 

 These researches have involved the exploration of the spectrum 

 of illuminants with a bolometer or thermopile, the percentage 

 of energy occurring in the luminous form being subsequently 

 deduced graphically from the energy curve thus obtained. 

 In some cases special absorbing solutions intended to separate 

 the non-visible from the luminous energy have also been used. 

 The results are not entirely harmonious, being affected by 

 certain experimental difficulties and by the definition adopted 

 for the "visible region" of the spectrum. A distinction (originally 

 not sufficiently appreciated) has now been drawn between the 

 percentage of radiated energy appearing in a visible form or 

 "radiant efficiency" and "luminous efficiency," by which latter 

 is meant the corresponding percentage of the total energy 

 given to the lamp, including conduction and convection losses. 



The one broad fact which stands out from the researches of 

 different investigators is that the efficiency even of the most 

 modern illuminants is remarkably low. We might perhaps 

 select as typical the following series of figures derived from 

 a table published by Dr. H. Lux, representing the luminous 

 efficiency of some common artificial illuminants : 



Source. Luminous Efficiency. 



(Percentage of total energy 

 radiated as visible light.) 



Petroleum lamp 0*25 per cent. 



Incandescent gas : vertical 0*46 „ 



inverted 0-51 „ 



Electric incandescent lamp : carbon filament . . 2*07 „ 



„ „ „ tantalum „ . . 4-87 „ 



„ „ „ tungsten „ . . 5-36 „ „ 



Arc lamp, D.C. : enclosed i"i6 „ „ 



m 11 >, open 5*6 „ „ 



„ „ yellow flame 13*2 „ „ 



It will be noticed that in the case of the great majority of 

 artificial illuminants all but about 5 per cent, or less of the energy 

 supplied is wasted. When to this prodigious waste is added 

 the considerable loss in the process of conversion of coal into 

 coal-gas or into electrical energy, it will be seen how small 

 a fraction of the original energy is ultimately applied to its 

 intended purpose. 



Yet a glance at the table shows that appreciable progress 

 has been made. Until recently such progress has been in a 



