270 ILLINOIS STATE ACADEMY OF SCIENCE 



efficient — though in this case we are grateful for the heat 

 also. Chart II shows the efficiency of various light 

 sources calculated on the basis of power consumption for 

 light produced. A candle power of brightness should 

 give a flux of 4rt or 12.56 lumens but in practice a lower 

 value is usually obtained. 



CHART II 



Tungsten at its melting point, 3670°K, would emit 57 

 lumens per watt (an efficiency of 9.24 per cent) but its 

 rate of evaporation is considerable even below this tem- 

 perature. Progress with incandescent filaments depends 

 upon finding a material with a lower rate of evaporation 

 and a higher melting point. Thus carbon, which has been 

 displaced by tungsten as a filament material, may come 

 back into use, since it has a much higher melting point, if 

 certain of its disadvantages can be overcome. 



Another possibility is to find a suitable gaseous envel- 

 ope which will reduce the loss of material due to evapo- 

 ration. Or it may be possible to control the emissive 

 properties of matter to procure proper selective radia- 

 tion. It has been pointed out that the maximum possible 

 efficiency of selectively radiating rare-earth oxides in an 

 ordinary Bunsen flame is thirteen times that of the in- 

 candescent mantle. It is known that most of the white 

 refractory oxides, such as lime and magnesia, as well as 

 the oxides of the rare earths are thermo-luminescent. 

 That is, when heated they emit a greater amount of vis- 

 ible light than can be accounted for by their temperature 



The luminosity of electrical discharge in gases must 

 be considered. Incandescent solids emit continuous spec- 

 tra while incandescent gases give selective emission. If 

 this radiation happens to be in a part of the visible spec- 

 trum that is desirable as illumination, the luminous effi- 

 ciency may be very high. Lamps are being operated on 

 this princple in England at a consumption of 0.5 watt 

 per candle power. Their development has just begun, 

 and since a very high intensity has not been attained 

 they are being used mainly as glow lamps and for adver- 

 tising display. The Germans claim to have developed 

 this method of light production to a still higher efficiency. 



