CHAPTER II. 



THE MAGIC LANTERN WITH AN ALTERNATING 

 CURRENT ARC LAMP AND ITS USE 



100. Apparatus and Material for Chapter II: 



Suitable room with screen (Ch. XII) ; Magic lantern with lan- 

 tern table ( 102) ; Arc lamp for alternating current with suitable 

 carbons ( 1 08); Alternating current supply ; Rheostat, choke-coil 

 or other balancing device ( 105-106); Ammeter for alternating 

 current ( in); Incandescent lamp, flash-light, gloves with asbes- 

 tos patches, testing lamp, fuses, extra condenser lenses, screw 

 driver, pliers, opera-glasses, lantern slides as in Ch. I ( i). 



101. For the historical development of the alternating cur- 

 rent arc lamp see the Appendix ; and for the character and advan- 

 tages and disadvantages of alternating current see 652-653, and 

 modern works on the subject. 



The same books of reference given in 2, Ch. I, are available for 

 this chapter. 



COMPARISON OF ALTERNATING AND DIRECT ELECTRIC 

 CURRENTS AND LANTERNS 



102. A magic lantern for alternating current may be pre- 

 cisely like one for direct current, the only essential difference being 

 that the arc lamp must be of the hand-feed type and the mechanism 

 for feeding the carbons gives equal movement to the upper and to 

 the lower one, both carbons being of the same size. 



One would never use an alternating current with the magic 

 lantern if direct current were available. It frequently happens, 

 however, that the lighting system of a place is of the alternating 

 current type, and no direct current is available. In such a case 

 one must make the best of it, or use a motor-generator set or a 

 rectifier (see 682-683). 



The objections to an alternating current for the arc lamp in 

 projection are: (i) The lamp is noisy; (2) It requires about two 

 and one-half times as much current for the same effective light. 



68 



