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. I] 
Irregular or Insufficient Light on the Screen 
§ 78. There may be an insufficient current flowing through the 
lamp. Consult the ammeter (§ 7, 75-77). 
§ 79. Improper relative position of the carbons.—Look at them 
occasionally through the window in the lamp house. They should 
be in the relative position shown in fig. 23. If they are in a wrong 
position (fig. 24, 25), one cannot expect to get a good screen light. 
It sometimes happens that one or both of the carbons has no soft 
core, although the hole in the carbon is present. In such a case the 
crater is liable to jump around as with a solid carbon. Easily 
corrected by substituting a properly cored carbon. 
§ 80. Wrong polarity of the supply wires.—As stated above 
(§5) the positive supply wire should be connected with the lamp 
so that the current passes along the upper carbon and from its tip 
over to the lower carbon, whence by means of the negative wire, it 
passes back to the generator or dynamo. In case the wires were 
reversed in position, the lower carbon would be positive and the 
bright crater would be on it. This would give a poor light, for the 
crater would not face the condenser, and as this carbon would bum 
away more rapidly than the upper carbon the upper one would soon 
be in the position shown in fig. 24B. There would then be a 
double reason for a poor screen image, viz., the crater would not 
face the condenser, and the upper carbon would act as a shield to 
cut the light off the condenser. To determine whether the wires 
are connected to the lamp properly, insert carbons, turn on the 
current, and let the lamp bum a minute or two. Then turn off 
the lamp and watch the hot ends of the carbons. The positive one 
will remain red hot longest. (See also Ch. XIII, § 701-703 for 
determining the polarity). In case the lower carbon remains 
glowing longer than the upper, the polarity is wrong (fig. 271). 
Open the switch and remove both wires from their binding posts 
and insert them in the reverse position. Then repeat the experi¬ 
ment and the upper carbon should remain glowing longest. 
After one has had some experience it is easy to tell whether or not 
the wires are properly connected by watching the carbons through


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