4i8 



MODELS FROM SERIAL SECTIONS 



[Ch. XI 



multiplied by the amperage used, if the voltage is lowered the amperage 

 is raised proportionally; hence the need of the large wire on the side 

 toward the lamp beyond the transformer where the amperage is 

 increased. 



§ 676. Lamp for 6-volt current. — There are in common use two 

 lamps, one of 72 watts and one of 108 watts. Now as the wattage 

 is the voltage times the amperage, for the 72-watt lamp the amperage 



with a 6-volt current must 

 be 72 divided by 6 or 12 

 amperes. For the 108- 

 watt lamp in hke manner 

 the amperage is the watt- 

 age divided by the volt- 

 age, — 108 divided by 6 = 

 18 amperes. This shows 

 at once why the large 

 wires must be used be- 

 tween the lamp and the 

 transformer. If the usual 

 small wires are used the 

 resistance is too great 

 and part of the energy is 

 used up in heating the 

 wires instead of in heating the filament to supply the hght. 



It is also a good plan to have the wires between the transformer and 

 the lamp as short as possible and not be inconvenient. 



§ 677. Arrangement of the lamp for the large projection outfit. — 

 If the lamp is to be used in the lamp-house instead of an arc lamp 

 for the large projection outfit, it must be centered carefully and put 

 the right distance from the large condenser. The filament takes the 

 place of the crater of the arc lamp and hence should be in the focus 

 of the first element of the condenser, so that the beam between the first 

 and second elements of the condenser will be approximately parallel. 

 If a two-lens condenser is used the lamp filament is slightly within 

 the focus, making the hght slightly diverging between the two lenses 

 of the condenser. 



Fig. 246. Drawing and Projection Outfit 

 WITH Large Mirror on Separate Drawing 

 Table. 



For fuU explanation see Fig. 112. Instead of 

 the arc lamp here shown the 6-voIt incandescent 

 lamp can be used for most purposes (§676). 



