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. X] 
Sh Shield to stop stray light and to aid in centering. 
C Carbons with alternating current. They are of the same size. 
D Carbons with direct current. The upper one is 8 mm. and the lower one 
6 mm. in diameter. 
E Shield or disc at the end of the condenser tube showing the opening of 
the condenser (C) and the spot of light at the right. 
§ 488. Arc lamp and small carbons.—The form of arc lamp to 
use on the house circuit is not of particular importance. It may 
be very conveniently one of the small lamps shown in fig. 41-44, 
201, 205, or it can be an ordinary arc lamp for greater currents, 
but supplied with long clamping screws, bushings or adapters for 
the small carbons (§ 127). The small lamps are generally of the 
hand-feed type and move the upper and the lower carbons equally. 
§ 489. Size of carbons for direct current.—A.—The carbons 
found useful for direct current are as follows, all being of the soft- 
cored variety: 
(1) Upper or positive carbon 7 mm. in diameter, lower or nega¬ 
tive carbon 5 mm. 
(2) Upper carbon 8 mm., lower 6 mm. 
(3) Upper carbon 11 mm., lower 8 mm. 
B.—The carbons for alternating current with an equal feed for 
the upper and the lower carbon, should be of the same size, and this 
size should not exceed 8 mm. in diameter for 5 to 6 amperes. If 
only three or four amperes are used, then it is better to have carbons 
not greater than 6 mm. in diameter. 
§ 490. Reason for using small carbons.—In order to have the 
light steady and thus have the field continuously bright, the entire 
end of the upper carbon should be white hot. 
If the carbon is so large that the crater covers only a part of the 
tip, the crater will wander about on the end of the carbon. Every 
change in the position of the crater changes the direction of the 
light beam. While the crater is in one position the entire field of a 
high power objective may be brilliantly illuminated; if the crater 
wanders to a new position, the field will be only partly or not at all 
illuminated. In such a case, one must constantly change the posi¬ 
tion of the mirror of the microscope to keep the field bright. If, 
however, the crater is nearly as large as the end of the carbon, it


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