NO. 2 



HISTORY OF ELECTRIC LIGHT — SCHROEDER 



23 



nected with a reservoir of mercury. The pull of the series magnet 

 closed the valve fixing the height of the column. The pull of the 

 shunt magnet tended to open the valve, and v^hen it overcame the 

 pull of the series magnet it allowed mercury to flow from the reser- 

 voir, raising the height of the column bringing the carbons nearer 

 together. This reduced the arc voltage and shunt magnet strength 

 until the valve closed again. Thus the carbons were always kept the 

 proper distance apart. In first starting the arc, or if the arc should 



Lacassagne and Thiers' Differentially Controlled 

 Arc Lamp, 1856. 



The lower carbon floated on a column of mercury whose height was 

 " differentially " controlled by series and shunt magnets. 



go out, current would only flow through the shunt magnet, bringing 

 the two carbons together until they touched. Current would then 

 flow through the contact of the two carbons and through the series 

 magnet, shutting the valve. There were no means of pulling the 

 carbons apart to strike the arc. Current flowing through the high 

 resistance of the poor contact of the two carbons, heated their tips 

 to incandescence. The incandescent tips would begin to burn away, 

 thus after a time starting an arc. The arc, however, once started was 

 maintained the proper length. 



