CH. I] TROUBLES WITH THE MAGIC LANTERN 51 



IRREGULAR OR INSUFFICIENT LIGHT ON THE SCREEN 



78. There may be an insufficient current flowing through the 

 lamp. Consult the ammeter ( 7, 75-77). 



79. Improper relative position of the carbons. Look at them 

 occasionally through the window in the lamp house. They should 

 be in the relative position shown in fig. 23. If they are in a wrong 

 position (fig. 24, 25), one cannot expect to get a good screen light. 



It sometimes happens that one or both of the carbons has no soft 

 core, although the hole in the carbon is present. In such a case the 

 crater is liable to jump around as with a solid carbon. Easily 

 corrected by substituting a properly cored carbon. 



80. Wrong polarity of the supply wires. As stated above 

 (5) the positive supply wire should be connected with the lamp 

 so that the current passes along the upper carbon and from its tip 

 over to the lower carbon, whence by means of the negative wire, it 

 passes back to the generator or dynamo. In case the wires were 

 reversed in position, the lower carbon would be positive and the 

 bright crater would be on it. This would give a poor light, for the 

 crater would not face the condenser, and as this carbon would burn 

 away more rapidly than the upper carbon the upper one would soon 

 be in the position shown in fig. 246. There would then be a 

 double reason for a poor screen image, viz., the crater would not 

 face the condenser, and the upper carbon would act as a shield to 

 cut the light off the condenser. To determine whether the wires 

 are connected to the lamp properly, insert carbons, turn on the 

 current, and let the lamp burn a minute or two. Then turn off 

 the lamp and watch the hot ends of the carbons. The positive one 

 will remain red hot longest. (See also Ch. XIII, 701-703 for 

 determining the polarity). In case the lower carbon remains 

 glowing longer than the upper, the polarity is wrong (fig. 271). 



Open the switch and remove both wires from their binding posts 

 and insert them in the reverse position. Then repeat the experi- 

 ment and the upper carbon should remain glowing longest. 



After one has had some experience it is easy to tell whether or not 

 the wires are properly connected by watching the carbons through 



