THE RING ARMATURE TELEPHONE RECEIVER 



135 



Magnetic Circuit 



The essentials of the magnetic structure of the ring armature receiver, 

 including an equivalent circuit, are shown in Figs. 18(a) and 18(b). As ex- 

 plained earlier, the magnetic structure includes an L-sectioned ring pole- 

 piece of 45% permalloy having an outwardly extending flange which carries 

 a non-magnetic ring, the latter acting as a support for the permendur dia- 

 phragm. The remalloy magnet, which is also an L-sectioned ring, is assem- 

 bled over the pole-piece assembly so that its inwardly extending flange 

 overhes the diaphragm. This overlying portion of the magnet plays an 

 important part in that it enhances the force factor of the device by securing 

 some of the advantages of a balanced armature type of receiver in a simpler 



•MAGNET 



(, 'DIAPHRAGM 



UPPER 

 AIR GAP ^g2 



LOWER 

 AIR GAP ^g, 



NON-MAGNETIC ^COIL 

 SUPPORT 



(a) (b) 



Fig. 18— Magnetic circuit analogy, (a) Physical arrangement of magnetic structure- 

 (b) Equivalent magnetic circuit, 



type of structure. The auxiliary magnet principle shown here was first used 

 on the simple bipolar receiver,^ and later was applied to the ring armature 

 type structure.^ In both cases, gains in force factor of 3 to 6 dh were realized 

 The equivalent magnetic circuit of Fig. 18(b) shows to a first approxima- 

 tion the relations of the physical elements of Fig. 18(a), neglecting the 

 leakage paths. As shown, the overlying portion of the magnet provides a 

 shunt path so that a part of the d-c. flux flows around the armature and only 

 through its inner marginal portion, while the lower portion of the magnet 

 carries additional flux to the armature and through the main air gap. The 

 magnetic circuit is a type of partially balanced circuit which, if fully bal- 

 anced, will have no d-c. flux flowing through the armature provided the 

 following relations are satisfied: 



5- 



9^2 + «,2 



