9 o 



THE ELECTRIC ARC 



mercury in the lower bulb flows along and makes contact 

 with that in the upper one. The tube is then tipped back 

 into the position shown in the figure, which breaks the 

 contact and starts the arc. 



FIG. 26. 



The mercury lamp made by the General Electric Com- 

 pany is shown in Fig. 27. Here the anode is a cylinder of 

 carbon and the arc is started by a fine carbon filament 

 running through the lamp and dipping into an iron cup 

 which floats on the mercury at the bottom of the tube. As 

 soon as the current is started this cup is drawn down by 

 a solenoid about the outside of the tube, through which 

 the current flows. As it is drawn down, an arc is started 



