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392 CINEMATOGRAPHIC ANNUAL
use of four tubes, two as amplifying tubes and two as rectifying tubes. The two rectifying tubes supply a plate voltage of 400 volts in the case of the 42-A amplifier and 800 volts in the case of the 43-A amplifier. No means are provided for controlling the gain of these amplifiers. These amplifiers are operated by means of a three-position snap switch which controls the A.C. supply. In starting the switch is turned to the first position which lights the filaments only. After they have had time to become heated, the switch is turned to the next position which supplies the plate voltage, making the amplifier ready for operation. This procedure in starting reduces the strain on the vacuum tubes which would occur if a high voltage were applied while the filaments were still cold.
The 42-A amplifier is capable of amplifying the power it receives approximately 325 times, which is equivalent to 25 decibels. The 43-A amplifier has a power amplification of approximately 36 times or 15 decibels. The 41A, 42-A, 43-A combinations of amplifiers are capable of a power amplification of 100,000,000 times or 80 decibels.
For some houses, that is up to about 1200 seats, the 42-A amplifier will deliver sufficient power without distortion to get satisfactory results. In larger houses one or more 43-A amplifiers may be necessary, the number depending upon the size of the house.
LOUD SPEAKERS OR RECEIVERS
After the sound has been taken from the record and transferred into electrical power and amplified, it is then led to the loud speakers which convert it into sound. The types of receivers most used at the present time for talking picture work are the electro-dynamic coil type. There are many other types of receivers, but the above mentioned type has to date best fulfilled the requirements of talking pictures. The operations of these electro-dynamic receivers depend upon the force which exists between a coil of wire carrying a current and a surrounding uniform magnetic field. The magnitude and direction of this force depends upon the magnitude and direction of the current flowing in the coil, and upon the magnitude and direction of the uniform external magnetic field. Hence, as the speech current is applied to this coil it will tend to move in and out in