336 ELEMENTS OF ELECTRICAL ENGINEERING. 



current arc in open air is about 0.85 when the frequency is 60 

 cycles per second. 



Differences between direct-current and alternating-current arc 

 lamps. The differences which depend upon the peculiarities 

 of the direct-current arc and the alternating-current arc are 

 described above. In the alternating-current lamp the electro- 

 magnet that operates the lamp mechanism has a laminated core, 

 and an inductance or reactance coil is used instead of resistance 

 as a ballast when the lamp is connected to constant-voltage mains. 



The alternating-current lamp is inherently noisy. The noise 

 of the arc is, however, quite effectually smothered by the bulb of 

 the enclosed arc lamp. The rattling noises due to the vibration 

 of the cores of the electromagnet and of the reactance coil may 

 be greatly reduced by clamping the laminations firmly together, 

 and by suspending the electromagnet on supporting springs to 

 prevent the transmission of its vibrations to the metal case. 



The slow consumption of the lower, or negative, carbon in the 

 direct-current enclosed-arc lamp causes only a slight downward 

 displacement of the arc as the lamp continues to burn, the lower 

 carbon being fixed in a rigid support. The approximately equal 

 rates of consumption of the two carbons in an alternating-current 

 enclosed-arc lamp, on the other hand, causes a very considerable 

 downward displacement of the arc as the lamp continues to burn 

 if the lower carbon is held in a rigid support. Therefore the 

 alternating-current arc lamp must have either a mechanism which 

 feeds both carbons, or a long enclosing bulb so that after the 

 lamp is freshly trimmed a considerable downward displacement 

 of the arc is allowable. 



138. Enclosed arc lamps versus open arc lamps. A large item 

 of expense in the maintenance of old style arc lamps, in which the 

 arc burns in the open air, is the cost of the frequent trimming 

 (after about eight hours burning) necessitated by the rapid wast- 

 ing away of the carbons. This item of expense is greatly reduced 

 by the use of enclosed arc lamps which require trimming after 

 about 150 hours burning. 



