VOLTAIC ARC LAMPS. 185 



able to the Jablochkoff candle, for in this the current in- 

 creases in energy as the candle burns on account of the 

 decrease in the length of carbon the current has to traverse, 

 whilst in the Rapieff lamp that length is always the same. 



In order that the extinction of one lamp may not cause 

 that of other lamps, Rapieff arranges the electro-magnet that 

 separates the carbons in such a manner that it acts as a com- 

 mutator. When the current passes through the electro- 

 magnet, the commutator is not brought into action, and the 

 circuit is completed through the lamp ; but- when this is extin- 

 guished or withdrawn from the circuit, the electro-magnet in 

 question becoming inactive causes the current to pass through 

 a derivation on which a resistance equal to that of the lamp 

 circuit is introduced, and the circuit of the other lamps is not 

 interrupted thereby. This effect is obtained by means of a 

 second armature, which, being attracted when the current 

 passes, acts as a keeper, thus increasing the electro-mag- 

 netic action exercised on the lamp, and sets the commutator 

 in action, when the current no longer passing, the armature 

 yields to the antagonistic action. 



In a new pattern, Rapieff has replaced the upper carbons 

 of the regulator we have described by a large piece of carbon, 

 which, as in Werdermann's lamp, does not burn. This 

 arrangement had, however, been indicated by Rapieff in his 

 patent of 1877, so that he cannot be accused of having 

 imitated Werdermann in his new arrangement. It is this 

 form that is now most employed. 



Baro's Lamp. This apparatus is composed simply of 

 two metallic tubes placed vertically one beside the other, and 

 in these slide freely two sticks of carbon, which rest on a 

 block of magnesia. These tubes are separated by an insu- 

 lating substance, but a screw regulates the distance between 

 the carbons at the end in contact with the magnesia, so that 

 the arc may be produced at this point of contact under the 

 desired conditions, and be kept there in spite of the con- 



