CHAP. VIIL] THE ELECTRIC LIGHT. 679 



of the wheels. The two carbons then separate, and an instantaneous 

 formation of the voltaic arc takes place, and the lamp now begins 

 to work. 



But by degrees the carbons consume, and their distance increases, 

 the voltaic arc grows larger, and the intensity of the current diminishes 

 by reason of the increase of resistance ; the energy of the soft iron 

 of the electro-magnet thereby grows less, and the attraction is dimi- 

 nished upon the armature A, which then yields to the action of the 

 antagonistic springs R. The oscillating system then rises, draws up 

 the catch d so that the catch- wheel is disengaged, and the wheels 

 work again. Thus the carbons approach each other once more, 

 increasing the intensity of the current and therefore the attraction of 

 the armature, and so on indefinitely until the carbons are too much 

 worn away and have to be renewed. The working of the lamp and 

 the duration of the light produced are thus insured continuously, and 

 depend only on the carbons being selected of the proper length, con- 

 sidering the time for which the illumination is to continue. 



The current arriving by the connection to the tube A B, passes from 

 the upper to the lower carbon, follows the tube K K, and by an undu- 

 lating band e enters the bobbin of the electro-magnet ; whence it goes 

 to the binding- screw n, which in turn communicates with the negative 

 pole of the battery or of the magneto-electric machine. 



We should add that the diameters of the wheels F and the pulley G 

 are calculated to have the same ratio as the distances passed over 

 by the carbons, which will be unequal if their wearing away is so, 

 so that the luminous point may always be maintained at a constant 

 height. 



II. ELECTRIC LIGHTHOUSES VARIOUS APPLICATIONS OF THE 

 ELECTRIC LIGHT. 



One of the most important applications of the electric light is 

 certainly that of the illumination of lighthouses. First-class .oil 

 lamps, thanks to the admirable lens arrangements of Fresnel, have, in 

 ordinary times, a range quite sufficient for the service of the coasts, 

 but not so on nights when the air is foggy, and which are precisely 



