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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 superimposed 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. (3^ 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