THE RING ARMATURE TELEPHONE RECEIVER 115 



functions are almost completely mechanical, as compared with the mag- 

 netic, electrical, or acoustical functions of other parts. The armature seat, 

 which already has been mentioned, is one of these. An interesting design 

 feature of this part is the necessity for high electrical resistivity since most 

 of the a-c. flux links the seat and it is therefore subject to eddy current 

 losses. A nickel-chromium alloy has been found suitable for this part. 

 Another part having a purely mechanical purpose is the coil stop. This part 

 consists of a flat strip of metal punched in a curved shape so as to fit on top 

 of the coil, and having three prongs bent at right angles to the strip. The 

 tips of the prongs pass through slits in the pole-piece flange and are bent over 

 in assembly to hold the coil in place. A membrane is mounted between the 

 protective grid and the magnet flange to keep dust and other foreign sub- 

 stances out of the instrument. In this receiver the protective grid and the 

 clamping ferrule, which is crimped over in the final assembly, are combined 

 into one part. 



Low manufacturing costs are realized by the use of multiple-purpose 

 parts. Some examples have been noted already. The single magnet serving 

 as both main magnet and auxiliary magnet is an example. The combined 

 ferrule-grid, which eliminates the fabrication, finishing, and handling of one 

 part as compared with previous designs, is another. The terminal plate also 

 falls into this class of parts. It not only serves as an electrical termination 

 for the receiver, but also is molded in such a way as to provide the correct 

 coupling air volume in back of the diaphragm; it contains the acoustic 

 passageways leading out of the back of the instrument and provides a 

 mounting surface for the acoustic resistance fabric cemented over these 

 passageways; it mounts and protects a click-reduction varistor which is 

 made a part of the receiver; it is molded with projections which prevent the 

 spade tip terminals of the handset cord from shorting against the varistor 

 case or turning in such a manner as to cause the cord conductors to be 

 pinched between the receiver and its handset seating surface; it has other 

 projections which key into the coil lead holes of the pole-piece and provide 

 insulation between the wires and the pole-piece and at the same time orient 

 and prevent rotation of the terminal plate with respect to the pole-piece; 

 and it is provided with two slots into which the crimped edge of the ferrule 

 is staked to prevent rotation of the ferrule. 



Perhaps the most interesting example of a multiple-purpose part is the 

 diaphragm dome. Its primary purpose is, of course, to radiate sound energy 

 in its capacity as a lightweight, rigid closure for the central opening of the 

 armature. In addition, it has six small projections molded to its top surface 

 just outside of the dome portion, which will touch the inner edge of the 

 magnet flange if the diaphragm is lifted upward off of its seat by mechanical 

 shock. Thus the projections prevent the armature from coming close enough 



