INSTRUMENTS FOR THE NEW TELEPHONE SETS 347 



to as the signaling resistance. As is shown by the soHd curves the 

 signaling resistance of the earlier type of transmitter after artificial 

 aging by an amount considered to be the equivalent of four years in 

 service falls outside the acceptable area. On the other hand, the re- 

 sistance of the new type when aged and measured under identical 

 conditions falls well within the limiting curve and hence not only 

 requires less frequent replacement but also permits greater freedom in 

 circuit design and plant layout. 



Moisture condensed from the breath is an important factor in deter- 

 mining the life of a transmitter. A protective membrane is provided 

 in the new transmitter unit which not only is highly moisture resistant 

 but also results in no appreciable transmission impairment. The 

 characteristics of the material employed in this membrane are such 

 that it is not affected by the aging conditions encountered in service 

 such, for example, as the alkaline reaction of water after it has been in 

 contact with phenol plastic parts or tobacco ashes. The exposed 

 metal parts are finished to resist the corrosive action of these agents. 



Response. — Reducing the transmitter to an equivalent electrical 

 circuit provides a useful means for analyzing its performance and deter- 

 mining the extent to which the individual parts contribute to its 

 response. Such a circuit for the new unit is shown on Fig. 6. 



While the diaphragm can be represented as a lumped mass for 

 frequencies in the region below 3500 cycles per second, it is necessary 

 to consider it as being composed of three masses coupled by stiffnesses 

 in order to represent adequately its performance at higher frequencies. 

 These masses consist of the central portion m-o, the ribbed intermediate 

 portion m^ and the outer portion W4. The central portion includes the 

 mass of the movable electrode and is coupled to the ribbed portion 

 by the stiffness s^ which in turn is coupled to the outer portion by the 

 stiffness 52. The paper books which support the edge of the diaphragm 

 have a stiffness 54 and a resistance ri. Their mass is included in the 

 mass of the outer portion of the diaphragm W4. The internal re- 

 sistances of the portions which form the coupling stiffnesses 52 and 56 

 are represented by ^2 and n respectively. A hole is provided in the 

 diaphragm to permit rapid equalization of low frequency pressures of 

 high intensity and prevent damage to the diaphragm and other parts. 

 The mass and resistance of this hole, nisrs, are so chosen that their 

 effect on response is confined to frequencies below 300 cycles per second 

 where the station circuit itself is relatively inefficient. The controlling 

 stiffness, S3, is that of the cavity between the diaphragm and the die- 

 cast frame. As is to be expected the impedance of the carbon granules 

 is a function of amplitude and frequency. However, for the purpose 



