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CH. I] MAGIC LANTERN WITH DIRECT CURRENT n
§ 4. Standard source of light. — With all forms of present day projection the direct current arc light is taken as the standard because, next to the sun, it is the most perfect light source available. In many places it is to be had during the entire twenty-four hours, and is the safest and most easily managed light capable of furnishing sufficient illumination for use with all kinds of apparatus, from the simplest magic lantern to the moving picture machine and the compound microscope.
MAGIC LANTERN WITH DIRECT CURRENT ARC LIGHT
Except the projection table, the room and screen, (for which see § 424 and Ch. XII,) the essential elements of a magic lantern and their arrangement are shown in fig. i, 2, 3. They are as follows:
§ 5. Wires for the electric current. — There must be two wires for carrying the current extending from the main line to the electric lamp. One wire, the positive (+), conveys the current to the upper carbon of the lamp, and the other, the negative ( — ), conveys the current from the lower carbon back to the main line (fig. 1,2) (see also Ch. XIII).
§ 6. Rheostat. — This device must be placed in the circuit along either the positive or the negative wire, it makes no difference which. In figures i and 2 it is placed in the negative wire. It serves as a balance, and limits the amount of current passing through the lamp (§ 744-748).
§ 7. Ammeter. — This indicates the amount of current flowing. It is not necessary, like the rheostat, but is very desirable, for with the information it gives, the operator can determine whether any defects in the brightness of the screen image are due to the lack of current, or whether something else is at fault (see Troubles.
§ 61-95-)
The ammeter is placed along one wire the same as the rheostat (fig. i, 2).
In case no ammeter is used the rheostat can be calibrated and marked when the apparatus is installed (see § 729).