T54 ELECTRIC LIGHTING. 



Serrin, Gaiffe, Siemens, Carre, Lontin, RapiefT, Brush, and 

 Biirgin are the only ones which are practically used, and 

 therefore we shall describe the details of these only. 



Fo lira ii It and Duboscq's Lamps. Foucault was, as 

 we have seen, one of the first to invent the regulator with a 

 fixed luminous point working by successive electro-magnetic 

 movements. He thus describes his apparatus to the Academic 

 des Sciences : 



"The two carbon holders tend to move towards each 

 other by springs, but they can only move by setting in motion 

 a train of wheels, the last of which is controlled by a catch. 

 It is here that electro-magnetism comes into play; the 

 current that lights the carbons passes through the spires of an 

 electro-magnet with an energy varying with the intensity of 

 the currents. This electro-magnet acts on a piece of soft 

 iron drawn away on the other hand by an antagonist spring. 

 On this piece of soft iron is mounted the detent which en- 

 gages the wheel and allows it to pass when necessary, and 

 the direction of the movement is such that it presses on the 

 wheel when the current becomes stronger, and liberates it 

 when the current becomes weaker. Now, as the current 

 becomes stronger or weaker precisely when the interpolar 

 distance decreases or increases, it will be understood how the 

 carbons have the power of approaching at the instant the 

 distance between has just been increased, and that this 

 approach cannot proceed to actual contact because the in- 

 creasing magnetization resulting from it opposes an insur- 

 mountable obstacle, which rises of itself as soon as the inter- 

 polar distance is again lessened. 



" The approach of the carbons is therefore intermittent; 

 but when the apparatus is well adjusted, the periods of rest 

 and of advance succeed each other so rapidly that they are 

 equivalent to a continuous progressive movement." 



Foucault does not explain how he adjusted the greater or 

 less approach of the carbons ; it was probably by giving the 



