334 Prof. S. P. Thompson on an Arc-Lamp 



the internal resistance of the Grove-cells ; and even then the 

 Duboscq lamp does not, for certain reasons, work as satis- 

 factorily as the lamp to be described, and its cost is about 

 three times as great. 



In every arc-lamp for optical purposes there must be 

 mechanism adapted to perform the four following actions: — 



1. To bring the carbons together into initial contact. 



2. To part the carbons suddenly, and with certainty, to a 

 short distance — about 3 millimetres — apart. This action is 

 technically called " striking " the arc. 



3. To supply carbon as fast as it is consumed, by moving 

 one (or both) of the pencils forward into the arc. This 

 action is called " feeding " the arc. 



4. To so move the carbons, or their holders, that the lumi- 

 nous points retain the same position in space at the proper 

 focus of the optical system. This action is called u focusing" 

 the arc. 



It may be remarked, in passing, that the feeding mechanism 

 of many lamps also performs the action, set down as No. 1 of 

 the above, of bringing the carbons into initial contact pre- 

 paratory to striking the arc. 



In many arc-lamps the attempt is made to unite the striking 

 and feeding mechanisms in one ; but in many lamps, and in 

 the one I have to describe, the striking and feeding mechanisms 

 are distinct. The striking mechanism in all the arc-lamps of 

 commerce consists of an electromagnet or solenoid arranged 

 in the main circuit of the lamp, the armature or plunger of 

 the same being mechanically connected with one or both of 

 the carbons, so that when, by the turning on of the current 

 through the touching carbons, there is a great rush of current, 

 the attraction of the electromagnet or solenoid shall instantly 

 part the carbons and strike the arc. In the majority of the 

 commercial arc-lamps it is the upper carbon only that is 

 raised to strike the arc ; in a few other lamps, and in the one 

 I am using, the lower carbon is depressed. In one of the 

 older patterns of the Duboscq lamp the lower carbon was also 

 thus directly acted upon, its holder being attached to the 

 armature of an electromagnet beneath it. The same is true 

 of the Serrin lamp. But in the Duboscq-Foncault lamp the 

 arc is struck in a different way. The two carbon-holders are 

 connected by racks to a clockwork gearing which either 

 parts them or brings them together, the movement being 

 driven by a double train of wheels, either of which can be 

 released in turn. The weight of the upper carbon-holder 

 drives the train that moves the carbons together; a coiled 

 spring drives the train that parts the carbons. Whether 



