THE RING ARMATURE TELEPHONE RECEIVER 113 



used to advantage, even though it had a relatively higher reluctance. With 

 this design, magnetic fields were produced in the two air gaps above and 

 below the diaphragm. In addition, the auxiliary ring magnet acted to shunt 

 a portion of the d-c. flux around the armature, which resulted in reduced 

 saturation in the middle portion of the armature, which permitted increased 

 flux density in the air gap below the diaphragm. This partially separated the 

 paths of the a-c. and d-c. flux in the magnetic structure, and adjusted the 

 magnetic forces exerted on the diaphragm, so that lower stiffness armatures 



VARISTOR 



TERMiNAL PLATE 



Fig. 3 — i'hotograph of the ring armature receiver and its parts. 



could be used than in the structure of Fig. 2(b). Application of the auxiliary 

 ring magnet in front of the diaphragm contributed very substantially 

 toward the development of a suitable motor element. The coil and magnet 

 relationship shown in Fig. 2(c) also resulted in a lower axial height and 

 a more compact structure. 



Description of the Production Design 



The main and auxiliary magnets of the early designs were Alnico castings, 

 and were expensive to produce. In the production design, they have been 

 combined into a single L-sectioned remalloy ring magnet, and the diaphragm 



