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$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 connected. 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 illuminated, when, in fact, more light may be falling on it. To be effective the light must reach the screen from the lantern and from no other source.