114 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



seat has been transferred from the main magnet to a non-magnetic nickel - 

 chromium alloy ring. 



The details of this construction are shown in Figs. 1 and 3. The diaphragm 

 consists of a flat ring-shaped vanadium permendur^ armature, to the inner 

 periphery of which is cemented a lightweight, plastic-impregnated and 

 molded cloth dome. The dome is placed in a concave downward position 

 with respect to the receiver cap, which greatly enhances its strength with 

 respect to suddenly applied air pressures. At the outer periphery, the arma- 

 ture rests on the non-magnetic seat, and is driven at the inner margin by 

 the magnetic field produced at the tip of the ring-shaped pole piece. The 

 45% permalloy^ pole piece, at its end opposite the air-gap, has an integral 

 flange extending outward, to which the armature seat is welded. The magnet 

 also rests on the pole piece flange, outside of the armature seat. This magnet 

 is made of a material similar to remalloy,** but of a modified composition in 

 which the molybdenum content is increased, which results in a higher co- 

 ercive force but lower residual induction. It is hot formed from sheet material 

 into the shape of a cup and then punched to provide an opening in the base 

 of the cup, the diameter of the opening being slightly smaller than the inner 

 diameter of the cylindrical portion of the pole piece. The inwardly extending 

 flange of the magnet forms the auxiliary portion. 



Acoustic controls of the response-frequency characteristic of the receiver 

 are provided in the same manner as in former telephone receivers of the 

 controUed-diaphragm type,^ except that lower values of acoustic impedance 

 are used which are easier to control. CoupHng chambers on each side of the 

 diaphragm connect to constricted passageways having acoustic mass and 

 resistance. The coupling chamber under the diaphragm exhausts into the 

 handset handle cavity through four holes, molded in the phenol plastic 

 terminal plate, which are covered with an acoustic resistance fabric ce- 

 mented to the terminal plate. This acoustic mesh serves to extend the fre- 

 quency range of the receiver because of its negative reactance character- 

 istics, and the acoustic resistance damps out the diaphragm resonance. The 

 coupling chamber above the diaphragm exhausts into the listener's ear cav- 

 ity through the acoustic mass and resistance of the holes in the receiver cap. 

 Proper selection of the acoustic impedances of the elements of this mesh 

 serves still further to extend the frequency range of the receiver. The rela- 

 tionships of all the acoustic and mechanical elements is such as to produce 

 the desired response frequency characteristic (See section entitled "Network 

 Representation"). 



There are several parts of the ring armature telephone receiver whose 



•"Survey of Magnetic Materials and Applications in the Telephone System," V. E. 

 Legg, The Bdl System Technical Journal, Vol. XVIII, July 1939. 



instruments for the New Telephone SeU, W. C. Jones, The Bell System Technical 

 Journal, Vol. XVII, July 1938. 



