CH. Ill] MAGIC LANTERN WITH NERNST LIGHT 95 



spending wires must be attached to these lamp binding posts or 

 connections, or the lamp will soon burn out. Unfortunately, one 

 cannot tell by simple observation when the wires are connected 

 properly, as for the arc lamp ; but he must determine the polarity 

 of the wires before connecting them with the lamp (see Ch. XIII, 

 701-703)- 



145. Marking the wires and attachments after determining 

 the polarity with the direct current system. When the polarity of 

 the wires is determined, if one is to use the same place for current 

 repeatedly, it is a good plan to mark the position of the socket and 

 the two parts of the separable plug by a straight line of colored 

 paint when all are in position. Then it will be easy to connect up 

 the parts correctly at any future time. Then if the positive wire 

 has its insulation material colored red, at least at the lamp end, it 

 will enable one to connect up with that particular socket correctly 

 at any future time. It is also a convenience to have polarized 

 separable plugs (fig. 4pA) , then the two parts of the plug cannot be 

 reversed if they should become separated. On the other hand, if 

 the attached part is left in place and the cap or removable part 

 pulled off, one can make the connection correctly at any time with- 

 out trouble, as it cannot be put together wrong. 



Unfortunately, one cannot be sure that a separable plug and the 

 lead wires connected to the lamp properly for one incandescent 

 socket will be so for any other, and one must determine the polarity 

 for each socket. 



An alternating current is more satisfactory for the Nernst lamp 

 than a direct current, for with the alternating current one does not 

 have to trouble about the polarity of the two wires, since both are 

 alike. 



146. Management of an exhibition with the Nernst light. 



This is precisely as for any other magic lantern radiant except that 

 the lamp must be started three to four minutes before it is needed, 

 for it may take that time to get good illumination. Furthermore, 

 it is better to leave the light burning during the entire lecture, so 

 that there will be no delays. The light can be shut off the screen 

 during the intervals with the objective shield (fig. 14). 



