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



outset, the very conditions of modern electric supply are that 

 either e or i is maintained constant, the usual arrangement in 

 commercial lighting being i constant for arc-lamps in series, 

 and e constant for glow-lamps in parallel. One of the two 

 factors being a constant by the conditions of the supply, the 

 other factor must be kept constant by the feeding mechanism. 

 Or, in other words, the variations of the other factor should 

 be made to control the action of the feeding mechanism. The 

 mechanical part of the feed may consist of a train of wheels 

 driven by the weight of the carbon-holder or by a spring, or 

 it may consist of a friction-clutch holding the carbon from 

 sliding forward, or of a worm-gearing or any other ; but it 

 must be controlled by an electric mechanism of one of the two 

 following kinds. For keeping i constant, the feeding mecha- 

 nism must be controlled by an electromagnet (or solenoid) 

 placed in the main circuit, working against an opposing spring 

 or weight. For keeping e constant, the feeding mechanism 

 must be controlled by an electromagnet (or solenoid) placed 

 as a shunt to the arc, and working against an opposing spring 

 or weight. In the latter case, if for any reason the arc grows 

 too long, the potential at the terminals will rise, more current 

 will flow around the shunt, which will then overcome its op- 

 posing spring (or weight), and will release the feeding 

 machinery until balance is restored. The use of the shunt, 

 introduced first by Lontin, enables arc-lamps to be connected 

 two or more in series in one circuit. A less perfect solution 

 is the differential principle introduced by Von Hefner Alteneck, 

 where the difference between the attractions of a series and a 

 shunt-solenoid maintains constant, not the product ei, but the 

 difference e—i. 



The only perfect solution of the problem is a feeding 

 mechanism which, by combining in itself a shunt-coil and a 

 series-coil, shall keep the product ei a constant, however either 

 factor may vary. All the commercial arc-lamps for lighting 

 in series have shunt-circuits to control the feeding mechanism; 

 though often the arrangement takes the form of a shunt-coil 

 wound (differentially) outside the series-coil of the striking 

 mechanism ; so that feeding is accomplished by the shunt-coil 

 demagnetizing the striking electromagnet and momentarily 

 un-striking the arc. 



Returning to the Duboscq lamp, it may be observed that, as 

 it possesses no shunt- coil, it can only feed by a weakening of 

 the current in the main circuit. Hence it is obvious that a 

 Duboscq lamp cannot possibly work in a constant-current 

 circuit. Also two Duboscq lamps will not work in series with 

 one another, as their individual feeding is not independent of 



