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BELL SYSTEM TECHNICAL JOURNAL 



horn is used, of rectangular cross section, lOj/2 feet long, the walls 

 being stiffened to prevent lateral vibration. For most installations 

 these large horns are not required, and two types of fibre horns are 

 used. One of these is straight in the body, with a flaring open end, 

 while the other used in the control room, is bent. 



The grouping of projector units on the volume control switches 

 differs with the type of installation. In outdoor performances, the 

 necessity of correcting the volume in certain directions due to varying 

 winds makes it advisable to group adjacent projectors on a single 

 switch. This is not the case with indoor performances, as no wind 

 effects are possible. Instead, symmetrically placed projectors which 

 will always require equal volume are grouped on a single switch. 



Power Supply Equipment 



In order to convert the commercial electric power supply to forms 

 suitable for supplying the filament and plate current for the vacuum 



Fig. 14a. 



tubes in the amplifiers, two types of power equipment have been made 

 available. When the installation is of a size requiring the high-power 

 amplifier, a vacuum tube rectifier taking its supply at 110 or 220 



