VOLTAIC ARC LAMPS. 165 



electro-magnet by a substance which cannot be changed by 

 the most elevated temperature, Besides, the large section 

 of the spire,> thus formed makes the heating more difficult 

 than with ordinary arrangements, and that is not dne of the 

 least advantages of this kind of electro-magnet 



I ought, in justice, to state that previously to Serrin an 

 electro-magnet of this kind had been invented by Duboscq 

 for his regulator, but he had riot taken care to enamel the 

 parts in contact with the helices, considering that precaution 

 as useless, on account of the great section of the spires of the 

 helix, which prevented them from being raised to redness. 

 Nor did he form his spires in the same way: they were 

 simply a strip of copper hammered into a spiral. 



Siemens' Lamp. The last of Siemens' lamps, which is 

 much used in England and in Germany, is represented in 

 Fig. 41. Like that of Serrin, it can be lighted automatically, 

 and the two opposite actions required for the separation and 

 approach of the carbons are determined by the weight of the 

 upper carbon-holder and by the electro-magnetic vibration 

 of a rocking lever which acts on a clockwork mechanism 

 driven in the opposite direction by the weight of the car- 

 bon-holder. This mechanism, composed of four wheels, is 

 arranged nearly as in the regulators we have just described, 

 and it is on the last wheel i, furnished with a ratchet and a 

 fly with wings, that the vibrating electro-magnetic acts. This 

 last is formed of a bent lever L, jointed at Y, and carrying at 

 M the armature of the electro-magnet E. This is the principal 

 organ of the apparatus, for on one side it carries a contact 

 piece which forms with the stud x the vibrating circuit- 

 breaker, and in the second place the antagonistic spring of 

 the system, the tension of which is regulated by means of the 

 screw R, and finally the driving and stopping catch Q, which 

 acts on the clockwork mechanism by means of the ratchet 

 wheel i. A fixed piece s supports the end of this catch, in 

 order to liberate the wheel i, at a suitable inclination of the 



