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CH. II] MAGIC LANTERN WITH ALTERNATING CURRENT 73
should be opposite the craters of the electrodes, so that the position of the carbons can be readily seen (fig. 133, 145).
§ 111. Ammeter for alternating current. — The ammeter serves the same purpose for the alternating as for the direct current; that is, it indicates the amount of current (§7). The construction for the alternating current is somewhat different, so that the one for direct current cannot be used for alternating. On the other hand excellent ammeters are now constructed which can be used for both alternating and direct currents (§ 664, 702a).
§ 112. Mechanical centering in a horizontal axis. — This is done precisely as for the direct current lantern (§51, fig. i , 2 and 40) .
§ 113. Amount of current necessary. — In genera] it requires from two and one-half to three times as many amperes of alternating current to get the same brilliancy of image as of the direct current (see § 755-768). Then for a screen distance of 10 meters (30 feet) one should have a current of about 25-30 amperes; and for a distance of 15 to 25 meters (50-75 ft.) one should use from 30 to 45 amperes. If one can be satisfied with less brilliant screen images, of course the amount of current may be somewhat less.
For a further discussion of the comparative merits of direct and alternating currents, and means of changing alternating to direct current see Ch. XIII, § 755-756, 682-683.
USE OF THE MAGIC LANTERN WITH ALTERNATING CURRENT FOR EXHIBITIONS AND LECTURE DEMONSTRATIONS
§ 114. The suggestions for the lecturer are as in Chapter I (§21-40).
§ 115. Suggestions for the operator. — These are the same as when using the direct current arc lamp (§ 26-42), except that in using the alternating current arc lamp more care is required to get good results.
(i) The carbons must be properly proportioned to each other. If they are of the same composition they should be of the same size. If one is solid and the other cored, the solid one is smaller (§ 7 53 a) .