THE RING ARMATURE TELEPHONE RECEIVER 



111 



Early Steps in the Development 



It has long been evident that the effective mass of the magnetic disc type 

 diaphragm used in the bipolar receiver is high, and is therefore a serious 

 limitation to obtaining an extended frequency range without sacrificing 

 efficiency. Attempts were made to reduce this mass by using lightweight cone 

 type radiators driven by relatively small magnetic discs at the center as in 

 Fig. 2(a). The difficulties, however, of controlling the vibrational stabihty 



MEMBRANE 



FERRULE -GRID 



APHRAGM 



MAGNET 



DIAPHRAGM SEAT 



COIL 



POLE PIECE 



TERMINAL 

 PLATE 



^- ACOUSTiC 

 RESISTANCE 



Fig. 1 — Sectional view of the ring armature receiver in a handset. 



of such structures made them impractical. Moreover, the mass of the arma- 

 ture was 100% additive to the moving system. 



By reversing the positions of the armature and the light-weight portion 

 of the diaphragm, putting the latter in the middle and using a ring of mag- 

 netic material around the outside edge as shown in Fig. 2(b), much better 

 results were obtained. ^-^ The armature, having one edge resting on a seating 

 surface, added only 30% of its mass to the moving system. Also, since the 

 large central portion of the diaphragm carries no flux, it could now be re- 

 placed by a lightweight non-magnetic material instead of the relatively 



2U. S. Patent No. 2,170,571, E. E. Mott, Filed August 12, 1936. 

 3U. S. Patent No. 2,171,733, A. L. Thuras, Filed October 6, 1937. 



