VOLTAIC ARC LAMPS. 163 



first of which engages a rack, making part of the upper carbon- 

 holder, and has its axle provided with a drum, round which 

 is wound a Vaucanson chain ; this chain, after having passed 

 over a pulley, is attached to a piece connected with the lower 

 carbon-holder. It follows that when the oscillating system, 

 in consequence of the inactivity of the electro-magnet, has 

 liberated the wheel, the upper carbon-holder is free to de- 

 scend, and in descending not only causes all the wheels to 

 turn, but also raises, by means of the Vaucanson chain, the 

 lower carbon-holder. This action goes on until the current, 

 by becoming stronger, produces a more powerful action on 

 the electro-magnet, which thereupon engages, by means of the 

 oscillating system, the fly in the train of wheels. The ap- 

 paratus is then stopped until the energy of the electro-magnet 

 again diminishes. A second opposing spring, acted on by 

 an adjusting screw, allows the sensitiveness of the instrument 

 to be increased or diminished at pleasure. Lastly, a chain 

 hanging down is so arranged as to act as a counterpoise to 

 compensate, in the action of the oscillating system, for the 

 loss of weight occasioned by the consumption of the lower 

 carbon. 



The current is transmitted to the lower carbon through a 

 bent flexible plate which follows the carbon in its move- 

 ments, and it is transmitted to the upper carbon through 

 the mass of the apparatus and the electro-magnet which has 

 the free extremity of its coil connected with an external 

 binding screw. 



There is nothing special about the lower carbon-holder: it 

 is merely a socket provided with a pressure screw for hold- 

 ing the carbon ; but the upper carbon-holder is more com- 

 plicated, in order that it may have two rectangular move- 

 ments so as exactly to fix the two carbons in the desired 

 relative position. The positive carbon is maintained above 

 the negative by means of a tube, supported by two horizontal 

 jointed arms controlled by two screws. One of the screw? 

 allows the carbon-holder to be displaced in one vertical 



II 2 



