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n8 SOUND MOTION PICTURES
feet sound illusion. The 13-A horns are intended to give the necessary depth and fullness to the reproduced sound with the lower frequency. In theatres having more than one thousand seats, two 12-A and two 13-A horns are used. Theatres having fewer than one thousand seats are provided with two 12-A and one 13-A horns. The present 15-A horns replace the two 12-A horns and the one 13-A horn, and are installed alongside each other, instead of on top of each other, as in the case of the 12-A and 13-A horns. The 12-A, 13-A, and 15-A horns have a single throat. The 15-B type horns have a double throat and use four units. Two 15-B type horns are used for theatres of more than one thousand seats. They are also placed side by side and installed on the screen three quarters of the distance up. In exceptionally wide theatres three 15-B type horns are used. These are installed all together, alongside each other, to give even distribution throughout the auditorium.
The units for these horns are termed 555-W Western Electric Receivers. The sound current enters the receiver through the two terminals at the centre marked L-l and L-2, and the magnetizing current enters through the two terminals at the side marked 7-V-plus and 7-V-minus. The magnetizing current required for each horn is iT^ amperes. The circuits for the horns, including the speech circuits and the magnetizing circuits, pass through a metal cutout box located back stage and called the B-box.
VI. Amplifier and Rectifier Equipment
Before operating the equipment, it is necessary that the amplifier panel controls, which control the filament current, be turned on until the tubes warm up. This usually takes at least five minutes. The filament current for these tubes is therefore first switched on and regulated