CH. I] MAGIC LANTERN WITH DIRECT CURRENT 1 1 



4. Standard source of light. With all forms of present day 

 projection the direct current arc light is taken as the standard 

 because, next to the sun, it is the most perfect light source available. 

 In many places it is to be had during the entire twenty-four hours, 

 and is the safest and most easily managed light capable of furnish- 

 ing sufficient illumination for use with all kinds of apparatus, from 

 the simplest magic lantern to the moving picture machine and the 

 compound microscope. 



MAGIC LANTERN WITH DIRECT CURRENT ARC LIGHT 



Except the projection table, the room and screen, (for which see 

 424 and Ch. XII,) the essential elements of a magic lantern and 

 their arrangement are shown in fig. i, 2, 3. They are as follows: 



5. Wires for the electric current. There must be two wires 

 for carrying the current extending from the main line to the electric 

 lamp. One wire, the positive ( + ), conveys the current to the 

 upper carbon of the lamp, and the other, the negative ( ), conveys 

 the current from the lower carbon back to the main line (fig. 1,2) 

 (see also Ch. XIII). 



6. Rheostat. This device must be placed in the circuit along 

 either the positive or the negative wire, it makes no difference 

 which. In figures i and 2 it is placed in the negative wire. 

 It serves as a balance, and [limits the amount of current pas- 

 sing through the lamp ( 744-748). 



7. Ammeter. This indicates the amount of current flowing. 

 It is not necessary, like the rheostat, but is very desirable, for with 

 the information it gives, the operator can determine whether any 

 defects in the brightness of the screen image are due to the lack of 

 current, or whether something else is at fault (see Troubles. 

 61-95.) 



The ammeter is placed along one wire the same as the rheostat 

 (fig. i, 2). 



In case no ammeter is used the rheostat can be calibrated and 

 marked when the apparatus is installed (see 729). 



