$2 TROUBLEvS WITH THE MAGIC LANTERN [Cn. I 



the lamp-house window when the lamp is burning. The upper 

 carbon should always be considerably brighter than the lower one. 

 When one has found the correct polarity it is wise to mark the 

 positive wire red and the negative wire black. It is also a good 

 plan to mark the positive switch connections plus with red and the 

 negative connections minus with black. But one must not forget 

 that the polarity is liable to be changed by the changing of the 

 wires in the main line when repairs are made, so one must be on the 

 alert to detect polarity change. 



81. Non-registering of the direct current ammeter. In first 

 installing an ammeter if the hand does not register on the dial when 

 the current is turned on and the arc lamp started, either the 

 instrument is out of order, or more likely the wires are wrongly con- 

 nected. Remember that the ammeter must be inserted in one 

 wire, then if it does not register when the lamp is burning the wires 

 were inserted wrong. Turn off the current and reverse the wires 

 in the binding posts of the ammeter. If now the wires are properly 

 connected both to the ammeter and the arc lamp, the polarity in 

 both will be changed by a change in polarity in the main line, and 

 the wires must be changed around in the binding posts in the 

 ammeter and in the arc lamp to get the polarity correct in both. 

 As the lamp and the ammeter are wholly independent instruments, 

 the polarity may be correct in both or wrong in both, or correct in 

 one and wrong in the other. (See also Ch. XIII, ;o2a for ammeter 

 which can be used with both alternating and direct current) . 



DEFECTIVE OPTICAL RESULTS 



82. There may be direct light falling on the screen from some 

 window or some lighted lamp in the room . This will make the disc 

 of light, or the lantern picture on that part of the screen receiving 

 the adventitious light, look faded or gray instead of brilliant. It 

 will look as if that part of the screen were not so brilliantly illumin- 

 ated, when, in fact, more light may be falling on it. To be effec- 

 tive the light must reach the screen from the lantern and from no 

 other source. 



