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ARC LAMPS WITH SMALL CURRENTS [Cn. Ill
For direct current the two carbons must be of different size if the feeding mechanism of the lamp moves the carbons equally. With an equal feeding mechanism, the upper or positive carbon can be 7 mm., the lower one 5 mm., or the upper 8 mm. and the lower one 6 mm.
One could use carbons of the same diameter for direct current, but it would be necessary to feed the upper or positive one more rapidly than the lower one on account of the unequal rate of burning, otherwise the correct relative position of the carbons would not be maintained (fig. 24-25). On a no volt, direct current circuit, the lamp will burn about six minutes without going out. The carbons should be fed up every three to five minutes.
For alternating current of no volts, the small lamps will burn from eight to ten minutes, sometimes longer. It is well to feed the carbons every five to seven minutes.
In case a choke-coil is used (Ch. XIII, § 736), the lamp burns more quietly and will burn longer without being fed. If a stepdown transformer is used, then the right-angled lamp will not burn so long — only one to two minutes — while a lamp with inclined carbons will burn three minutes, because it takes a higher voltage to maintain the right-angled than the inclined carbon arc (see Ch. XIII, § 753, 768).
TURNING THE ARC LAMP ON AND OFF
§ 132. Lighting the small arc lamp. — For this, make sure that the carbons arc not in contact. Now turn the switch for the room lights and the snap switch in the socket where the separable attachment plug for the lamp wiring is screwed in. Feed the carbons together until they touch. There should be a flash of light. Separate the carbons two or three millimeters as soon as the flash is seen and the arc will be established and the light will be at full brilliance. Sometimes it is necessary to keep the carbons almost in contact for a half minute or so, until the tips arc well heated, before the arc will bum. If on separating the carbons the light goes out, they must be brought together again as at first.