126 DELL SYSTEM TECHNICAL JOURNAL 



phone receivers in use in the Bell system if these were directly con- 

 nected to the amplifier. 



In describing the various pieces of equipment which together 

 make up the system, we will follow the order in which the power is 

 carried through the system from the transmitter to the receiver- 

 projector units where the amplified sound waves are propagated. 



Transmitters 



In the early work on the public address system, an air-damped, 

 stretched diaphragm condenser transmitter was employed, having a 

 thin steel diaphragm about 2 inches in diameter, constituting one 

 plate of the condenser. The other plate was a rigid disk, the dielec- 

 tric being an air film l/lOOO of an inch in thickness. Due to the 

 stretching of the diaphragm and the stiffness of the air film, the 

 diaphragm of this transmitter had a natural period of approximately 

 8000 cycles per second which is well above the important frequencies 

 in the voice range. This high natural period, in conjunction with the 

 damping due to the thin film of air, resulted in a transmitter of very 

 high quality of reproduction. However, its extremely small capacity 

 (in the order of 400 micro-microfarads) made it necessary to use leads 

 of very low capacity between the transmitter and the first amplifier, 

 and due to the high impedance of the transmitter and its associated 

 input circuit to voice frequency currents, these leads were very sus- 

 ceptible to electrostatic and electromagnetic induction. It was 

 necessary to limit them to a length of 25 feet, and to provide complete 

 shielding. Moreover the output of this transmitter was less than one 

 five-thousandth of that of the transmitter now used, and for the early 

 installations of the system, it was necessary to provide a preliminary 

 amplifier beneath or to one side of the speaker's stand in order to keep 

 the transmitter leads short and to provide sufficient power to properly 

 operate the main amplifiers. Work was therefore undertaken to 

 provide a transmitter having quality practically as good as the con- 

 denser transmitter, volume output sufficient to operate the main am- 

 plifiers, and not requiring the elaborate precautions as to shielding 

 the leads. 



The high quality transmitter which was the result of this develop- 

 ment work is of the granular carbon type with two variable resistance 

 elements, one on each side of the diaphragm and is commonly known as 

 a push-pull transmitter. It has nearly the same high quality re- 

 production characteristics as the condenser transmitter, due to the 

 use of the same stretched diaphragm and air damping structure. It 



