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CH. II] ALTERNATING AND DIRECT CURRENT LANTERNS 69
That is, if 10 to 12 amperes of direct current give satisfactory illumination in a given case, it would require from 25 to 30 amperes of alternating current to give the same brilliancy of screen image. Naturally also the heating with the larger alternating current is greater than with the smaller direct current (see also § 768).
§ 103. The difference between direct and alternating current is, in general terms, this: the direct current has a constant polarity and one carbon is always positive; while the alternating current has an alternation of polarity, as the current flows in one direction for an instant and then in the opposite direction. The result is that each carbon is positive half the time and negative half the time, hence both carbons have brilliant craters from which light for the screen image might be obtained. Sometimes an effort is made to utilize the light from both craters by the arrangement of the carbons in the form of a V, the apex of the V pointing toward the condenser (fig. 230).
INSTALLATION OF A MAGIC LANTERN WITH AN ALTERNATING CURRENT ARC LIGHT
§ 104. Wiring from the supply to the lantern. — This is precisely as for the direct current lamp. If the lantern is to be used for experimental purposes it is advantageous to have an incandescent lamp inserted in the circuit as shown in fig. 2.
§ 105. Rheostat or other regulating device. — There must be introduced along one of the supply wires to the lantern some form of balancing device. This may be in the form of a rheostat like that used for the direct current (§ 6) ; an inductor or choke-coil, a transformer, or a mercury arc rectifier may be used. For the special advantages and disadvantages of the different balancing devices (see § 736-738)
§ 106. Wiring the lamp. — For the alternating current it makes no difference which supply wire is connected with the upper carbon, as each carbon has an approximately equally brilliant crater.