208 BELL SYSTEM TECHNICAL JOURNAL 



from the mouth of the horn; that is, from the screen. If the horn 

 is replaced by a loud speaker of otherwise identical characteristics 

 but which radiates its sound over a very wide angle, there is a tendency 

 for the sound to appear to come from a point some distance back of 

 the screen, thus tending to destroy the illusion. 



The power supply equipment has been fairly completely covered in 

 discussing various parts of the system. Under ordinary conditions 

 the requisite power is obtained from the electric mains in the theater 

 except for the 12-volt battery required for some of the vacuum tube 

 filaments and for the electromagnets in the loud speakers and the 

 dry cells used with the photo cell and photo cell amplifier. Where 

 110- volt D.C. only is available there is a projector-driving equipment 

 which operates on this voltage, but a D.C. motor driving a 60-cycle 

 generator is required for supplying the amplifiers. Where 110-volt 

 A.C. is available, it is only necessary to connect the projector motor 

 and the amplifiers to this supply. 



"^ Bell System Technical Journal, January, 1928 — -"A High Efficiency Receiver of 

 Large Power Capacity for Horn-type Loud Speakers," by E. C. Wente and A. L. 

 Thuras. 



