298 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



higher the temperature the whiter is the color of the light, since the 

 short yiolet rays are added to the long waves only at higher tempera- 

 tures. On the scale of whiteness, the arc light come first; it is similar 

 in its composition and its appearance to sunlight. After this comes 

 the acetylene light, with its dazzling whiteness. Then comes the red- 

 dish-colored lights, such as the gas flame, the incandescent electric light, 

 the candle, and the petroleum lamp, and finally come the red lights, 

 such as the hearth fire. The absolute temperature of the arc, as stated 

 above, is about 3,600° 0. That of the petroleum lamp is certainly below 

 2,000° and the melting point of platinum is, let us say, 1,800°- Then 

 the absolute temperatures of the two are in the ratio of 3,600 to 1,800, 

 or as 2 is to 1. For the same area the carbon of the arc light would 

 emit about 16 times as much light as the petroleum lamp. And yet the 

 price per candle power is almost as great for the arc light as for the 

 petroleum lamp, as shown above. This indicates how large a propor- 

 tion of the energy must be lost in the transformation of the heat 

 equivalent of the fuel into energy of motion of the dynamo and the 

 transformation of this energy into electrical energy and finally into 

 light. It is therefore easily understood why the incandescent electric 

 light occupies almost the last place in the table. In addition to the 

 energy which is lost in the production of the electric current we must 

 consider that the temperature of the filament of an incandescent lamp 

 is much lower than that of the carbon in the arc. In the arc lamp the 

 maximum temperature is practically determined by the volitilization 

 temperature of the carbon. In the incandescent lamp the filament 

 must be kept intact. This can only be done if it is heated to moderate 

 whiteness in the absence of air — i. e., of oxygen. If a long life of the 

 incandescent lamp is not to be considered, by increasing the tempera- 

 ture of the filament to intense whiteness the glow lamp emerges from 

 its darkness and outshines all luminous flames. I now gradually 

 increase the electric current passed through this incandescent lamp. 

 [Exijeriment.] The filament becomes brighter and brighter, and you 

 see what a flood of light is emitted by this single lamp. It, however, 

 lasts only a short while, for the filament disintegrates. While the 

 increase in temperature has quadrupled its brightness, the current 

 passed through it only had to be doubled. I can not make it any 

 plainer to you than by this experiment that an improvement in the 

 incandescent light can only be attained if we succeed in increasing 

 the temperature of the filament without at the same time disintegrat- 

 ing it or volitilizing it. But even now, with the low market price of 

 incandescent lamps, less stress is to be laid on their life than on their 

 luminosity, and it is more economical to use three lamps, each capable 

 of burning three hundred hours and with very bright filaments, than to 

 use one lamp outlasting all the others but with a reddish filament. 

 The radiation of light is proportional to the fifth power of the absolute 

 temperature, while the electric energy which is supplied to the lamp is 

 proportional to the second power of the temperature. 



