126 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



istic, the effect of proposed design changes may be predicted, and other 

 characteristics determined without building large numbers of physical 

 models. 



(b) Damped Electrical Impedance 



The damped electrical impedance of the receiver, that is, the electrical 

 impedance when the armature is blocked so that it cannot move, is repre- 

 sented by Ze in the block diagram of Fig. 11. The damped impedance of the 

 ring armature receiver plotted against frequency is shown in Fig. 13. The 



240 



2 

 O200 



160 



120 



80 



40=^ 



400 



800 



1200 1600 2000 2400 2800 3200 3600 4000 4400 

 FREQUENCY IN CYCLES PER SECOND 



Fig. 13 — Damped impedance of ring armature receiver without varistor. 



i-^M0^ 



r I 



\I\N — ^KJ^y^' 



R 

 Fig. 14 — Ze. Network representation for the damped impedance. 



rise in resistance and the departure from linearity of the reactance are due 

 to the effects of eddy currents in the metallic parts of the instrument. 



A circuit representation for Ze is shown in Fig. 14. This circuit is derived 

 on the assumption of a single eddy current path coupled to the receiver 

 winding. The electrical resistance and inductance of the winding are repre- 

 sented by r and ^, and the eddy current circuit by R and L. Analysis of this 

 circuit is useful in determining the extent to which eddy currents have a 

 detrimental effect on the efficiency of the .receiver. In general, the effect of 

 eddy currents is greater in the ring armature receiver than in the bipolar 

 types, largely because of the toroidal form of the motor element. Slotting of 

 the ring-shaped parts has been found to be ineffective in reducing the eddy 



