Bauhaus-Universität Weimar

Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine
Gage, Henry Simon and Henry Phelps Gage
[Ch. 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 burn¬ 
ing, otherwise the correct relative position of the carbons would 
not be maintained (fig. 24-25). On a no volt, direct current cir¬ 
cuit, the lamp will bum 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 bum 
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 bums 
more quietly and will bum longer without being fed. If a step- 
down transformer is used, then the right-angled lamp will not bum 
so long—only one to two minutes—while a lamp with inclined 
carbons will bum 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 are not in contact. Now turn the switch for the room 
lights and the snap switch in the socket where the separable attach¬ 
ment 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 are 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.


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