356 BELL SYSTEM TECHNICAL JOURNAL 



which are not sufficiently overpolarized before demagnetization to 

 resist further demagnetization under service conditions are rejected. 



Temperature Effects. — The diaphragm of the new receiver is held in 

 place by the force developed by the polarizing flux and hence it is free 

 to expand and contract independently of its seating surface. This 

 feature renders the performance of the receiver independent of the 

 changes in temperature to which it has been subjected. The force due 

 to the polarizing flux is sufficiently high to prevent rattling at input 

 intensities many times those of loud speech. 



Coupling 



Although station circuits can be designed which under ideal con- 

 ditions result in no sidetone, this objective is never fully realized under 

 actual plant conditions, with the result that a part of the electrical 

 output from the transmitter always reaches the local receiver. 

 Whether the electrical coupling between the transmitter and receiver 

 as evidenced by the residual sidetone is of importance from the stand- 

 point of sustained oscillation or "howling," depends upon the degree 

 of mechanical and acoustical coupling between the instruments. 

 Handset and instrument design has advanced to a stage where mechan- 

 ical coupling need no longer be a problem. On the other hand, as 

 the response of the instruments is improved, the acoustical coupling 

 may become an important item in determining the howling margin. 

 This margin is so large under the conditions where the new handset is 

 being used for transmission purposes that there is no tendency for 

 oscillation or distortion to occur. However, if the handset is placed 

 face downward on a desk or table, an air column is created which 

 resonates in the region of 2500 cycles per second. Inasmuch as this 

 is the region where a substantial improvement in the response of the 

 receiver has been effected, a marked reduction in howling margin 

 results. While there is still sufficient margin to meet all of the require- 

 ments of field use, this situation serves to emphasize the fact that such 

 factors as acoustic coupling may limit the transmission improvements 

 which can be effected under a given set of operating conditions. 



Effective Transmission 

 The extent to which the better performance of the new instruments 

 is effective in improving the grade of transmission afforded the tele- 

 phone user is a complex matter and one which is influenced by such 

 factors as the characteristics of the circuits with which the instruments 

 are associated at a given time, the amount of noise present at the 

 transmitting and receiving stations, the reaction of sidetone on the 



