124 THE ELECTRIC ARC 



through B and the inductance F, as indicated by the arrows 

 with circles. 



The efficiency of this rectifier is good even on a hundred- 

 volt circuit, and becomes higher as the voltage is raised. 

 Norden 1 states that the efficiency of such rectifiers giving 

 100 volts is 80 per cent for currents varying from 10 to 

 30 amperes, while the efficiency of the rotary converter 

 gi ving the same voltage is only 54 per cent for 10 amperes 

 and 62 per cent for 30 amperes. In general, he says, the 

 static rectifier is more efficient than the rotary converter 

 when less than 5 kilowatts are being used. 



The "return " current, that is, the current passing through 

 the tube in a direction opposite to the main current, is 

 ordinarily but a few millionths of an ampere. 2 It increases 

 slightly as the voltage is increased and very greatly when 

 the current is increased, so that the tube becomes hot. 



As has been stated it is not possible to pass an alternat- 

 ing current through a vacuum tube having mercury for 

 one of the electrodes. This is true when there are only 

 two electrodes or when there is no inductance in the circuit, 

 but evidently there is no reason why the mercury arc recti- 

 fier, as described above, should not be used to give light, and 

 in fact such a lamp is now being sold. When used for this 

 purpose it is made longer than when used as a rectifier and 

 the voltages are so arranged that the greater part of the 

 energy is used in the tube instead of in the outside circuit. 

 Such an arrangement is a mercury vapor lamp for alter- 

 nating circuits. 



It has been found by Bedell 3 that partial rectification 



1 Allgem. Elek. Ges. ZS., Feb., 1910. 



2 Schultze, Elektrot. ZS., 31, 28; 1910. 



3 Elec. World, 40, 408; 1902. 



