RECTIFYING DEVICES 397 



Use of the Mercury Tube for Arc Lamps. Flaming arc 

 lamps, similar to the magnetite lamps, require continuous 

 current for their operation. Series arc lamp systems are 

 most conveniently operated from constant-current trans- 

 formers, which of course, require an alternating-current 

 power supply. The secondary of the transformer, instead 

 of being connected to the lamp circuit directly, is connected 

 through a mercury tube so that the arc lamps operate 

 on a rectified current. The constant-current transformer 

 operates to give practically constant current just as though 

 the mercury rectifier were not in the circuit. 



A view of a mercury tube outfit for charging storage 

 batteries is shown in Fig. 262; this tube is designed to 

 carry 30 amperes and the tube is generally guaranteed 

 to give 400 hours life. As a matter of fact these tubes 

 generally give more than 1000 hours life in actual operation; 

 their failure is due to a change in the vacuum. 



