CH. XIII] ENERGY FOR MOVING PICTURES 571 



form below, together with the corresponding values when alternat- 

 ing current is used. 



780. Radiant energy passing the aperture plate when using 

 right-angle lamp with direct current. Power passing Powei for each 



through aperture square centi- 

 plate meter of fi'.m 



Total radiation of arc, of which 10% is light. 98 watts 23 .4 watts 

 Radiation passing through water-cell, of which 



43% is light 22.8 watts 5.44 watts 



Visible radiation only, 377 lumens per watt . 9.8 watts 2.34 watts 



781. Radiant energy passing aperture plate when using right- 

 angle lamp with alternating current. Power passing Power for each 



through aperture square centi- 

 plate meter of film 



Total radiation of arc, of which 6.4% is light 222 watts 53.0 watts 

 Radiation passing through water-cell, of which 



41% is light 34. 4 watts 8.2 watts 



Visible radiation only, 264 lumens per watt . 1 4 watts 3 . 2 watts 



782. Effect of opacity of the film. When a nearly trans- 

 parent film is used, a large proportion of this radiation passes 

 through, but when a nearly opaque film, such as the title is shown, 

 almost all of this energy is absorbed and converted into heat. 

 From these tables it is not difficult to understand why, if there is 

 no water-cell used, the film is likely to spoil or ignite if it is stopped 

 for a few seconds while the light is falling on it. Take the example 

 of the light furnished by the alternating current arc such as is used 

 in a great many places. Here the film is absorbing energy at the 

 rate of 53 watts per square centimeter, which is faster than the 

 surrounding air can cool it. If now a water-cell is used, the energy 

 rate is reduced to 8.2 watts. Experiment has shown that under 

 these conditions with the water-cell, the heating effect is not great 

 enough to ignite even a black celluloid film if for any reason it 

 should stop moving. But it must be remembered that even if a 

 water-cell is used the film would catch fire if held in an extremely 

 concentrated beam. (For the time of ignition of film see 596) . 



779a. If the new standard size for the opening in the aperture plate 

 ( 5?oa) were used, the figures in the example would be slightly different, but 

 the principle is shown just as well in the statement here given. 



