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CH. I] TROUBLES WITH THE MAGIC LANTERN 51
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 burn away more rapidly than the upper carbon the upper one would soon be in the position shown in fig. 246. 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 burn 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 experiment 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