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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



The superimposed direct-current characteristics are an indication of the 

 stabihty of the receivers. SHght changes in the air-gap of a receiver may 

 occur during its Hfe due to external mechanical stresses, extreme tempera- 

 ture variations, or magnetic influences. The application of direct current to 

 the receiver winding produces such changes in the air-gap artificially in a 

 controllable and reproducible manner, and shows that within a certain range 

 there is no serious effect on the response level to be expected from slight 

 alterations in the air-gap. The curve also shows that there is an optimum 

 magnetization for the instrument at which the response is a maximum, and 

 the sharpness or bluntness of the response peak is a measure of the stability. 



500 

 450 

 400 



350 

 300 

 250 

 200 

 150 

 100 

 50 



500 1000 1500 2000 2500 3000 3500 4000 4500 5000 



FREQUENCY IN CYCLES PER SECOND 

 Fig. 10 — Impedance of ring armature receiver, measured on a closed chamber of 6 cc 

 volume. 



The comparison between the two receivers shows that the Ul receiver is less 

 sensitive to changes in superimposed direct current than the HAl receiver. 

 The impedance-frequency characteristic of a typical Ul ring armature 

 receiver, measured on a closed chamber of 6 cc. volume, is shown in Fig. 10. 

 The magnitude of the impedance rises with frequency because of the in- 

 ductance of the instrument. The departures of the impedance curve from a 

 smooth upward sweep with frequency represent the contributions of the 

 mechanical and acoustical elements of the receiver to the electrical im- 

 pedance, that is, the motional impedance. The phase angle of the ring 

 armature receiver averages 40°, which is somewhat lower than that of bi- 

 polar receivers such as the HAl because of the larger part played by eddy 

 currents in the impedance of the former. 



