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, 7o2a). 



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 alter- 

 nating 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 arc as in Chapter I 

 ( 21-40). 



115. Suggestions for the operator. These arc 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 arc of the same composition they should be of the same size. 

 If one is solid and the other cored, the solid one is smaller ( 753a). 



