92 MAGIC LANTERN WITH NERNST LIGHT [Cn. Ill 



^139. Management of an exhibition with the Mazda lamp. 



The exhibition should be managed as for the arc light ( 26-41). 



One must remember that with this relatively weak light only a 

 small screen image should be attempted, and that the room must 

 be relatively darker than for the arc light. In brilliancy the screen 

 images will be more like that of the old lanternists with their weak 

 lights. Clear lantern slides are especially desirable. The very 

 opaque lantern slides sometimes met with can only be well shown 

 by a large arc lamp. 



MAGIC LANTERN WITH A NERNST AUTOMATIC LAMP 



140. This is also an excellent lamp to use with a magic lantern 

 in a small room. Some forms are automatic in starting when the 

 current is turned on, and some have to be specially heated. The 

 automatic form is to be preferred, for it is no more trouble to run 

 than an ordinary incandescent lamp. It takes some time, usually 

 one to three minutes, for the glowers to come to full brilliancy after 

 the current is turned on. They are made for the lantern with one, 

 two, three and four filaments or glowers. The single glower 

 approximates most closely to the arc lamp in the smallness of the 

 source of light. Of course, with the multiple glower lamps a greater 

 amount of light is given off, but they make an extended source. 

 Whether the lamp has one or more filaments it can be attached 

 directly to the house lighting system through any incandescent 

 bulb socket as described for the Mazda lamp ( 137). 



141. Rheostat or ballast for the Nernst lamp. This lamp 

 like the arc lamp is always used with a balancing device, but unlike 

 the arc lam]), the ballast is an integral part of the lamp as pur- 

 chased, and not a separate apparatus as with the arc light (fig. 54, 

 55 and i). 



The glowers and the ballast must be adapted to each other 

 and both must be adapted to the line voltage. 



The ballasts, which are enclosed in a vacuum glass, as with an 

 incandescent bulb, sometimes burn out. The filaments will not 

 then glow when the current is turned on. If a ballast burns out it 

 must be replaced by a perfect one. 



