74 MAGIC LANTERN WITH ALTERNATING CURRENT [Cn. II 



(2) As there are two sources of light it is necessary to take 

 special pains to focus the lantern slide very sharply on the screen, 

 or, when the carbons burn away so that the sources of light are 

 relatively far apart, the image on the screen will appear partly 

 double like print that has slipped on the press, or like color printing 

 when the impressions do not register, thus giving two partly super- 

 imposed images, especially if the carbons are arranged like a V. 



If the image is sharply focused and the carbons kept close 

 together this trouble will be avoided. 



(3) The carbons must not be allowed to burn away too far 

 before they are fed up, or the lantern will become very noisy. The 

 carbons should be kept about three mm. (Y% in.) apart. This will 

 involve feeding them toward each other every five minutes (see 

 also 131, 753a). 



A pair of gloves with asbestos patches (fig. 5) should be at hand 

 when working about the alternating current lamp. 



Practically all of the magic lanterns found in the open market 

 may be used with an alternating lighting system, provided a lamp 

 designed for the alternating current is used ( 102, fig. 3). 



TROUBLES WITH A MAGIC LANTERN WITH ALTERNATING 

 CURRENT ARC LAMP 



116. Noisy arc. There is no way of entirely obviating the 

 noise in an arc lamp with alternating current. It may be kept at a 

 minimum by using carbons of the proper size for the amperage used 

 ( 753a) and by keeping them relatively close together. As the 

 carbons burn away, increasing the length of the arc, the noise 

 increases. If a heavy current (much amperage) is used the noise 

 becomes very loud and disagreeable. 



The noise is also increased if there is any loose part around the 

 rheostat or lamp which can vibrate in unison with the alternations 

 of the current. 



117. Managing the arc lamp. Practically all of the arc 



lamps used for the magic lantern with alternating current are of the 

 hand-feed type, hence besides all the other things the operator 



