342 BELL SYSTEM TECHNICAL JOURNAL 



prominent to encourage the user to talk directly into it and in this 

 way reduce the losses which often result when flush type faceplates are 

 employed with desk stand and wall set instruments. A phenol plastic 

 part, equipped with contact springs, holds the unit tightly in the face- 

 plate and provides electrical connections. 



As in the handset the unit of the receiver is held in place by the cap. 

 Springs are provided in the shell for bringing out the electrical con- 

 nections. A metal insert adds sufficient weight to meet the switch- 

 hook requirements of the existing sets. The phenol plastic parts of 

 both the receiver and transmitter are so designed as to insulate 

 thoroughly the units and minimize breakage. 



Transmitter Unit 



The new transmitter unit is of the "direct action" type, that is, one 

 in which the movable electrode serves the dual purpose of contact and 

 pressure surface. As is shown by Fig. 2, this electrode is mounted at 

 the center of a diaphragm of thin aluminum alloy formed into a shallow 

 cone and ribbed to add rigidity. "Books" of thin impregnated paper 

 mounted in a recess in a die-cast frame provide a resilient support for 

 the edge of the diaphragm. The fixed electrode is held in place in 

 the frame by a threaded ring and is insulated from the frame by a 

 phenol fibre washer and a ceramic insulator which also forms one of 

 the surfaces of the carbon chamber. The active surfaces of both 

 electrodes are gold plated. A silk annulus clamped at its outer edge 

 between the ceramic insulator and the frame and its inner edge between 

 the movable electrode and the diaphragm forms a resilient closure for 

 the carbon chamber. Electrical connection between the movable 

 electrode and the frame is provided by means of metal strips of low 

 stiffness. Provision is made for machine filling the carbon chamber 

 through a hole in the fixed electrode and closing this hole by means of 

 a cap which crimps over a projecting shoulder. The exposed surfaces 

 of the cap and the threaded ring are silver plated and form the contact 

 surfaces for the electrical connections. A moisture-resistant mem- 

 brane protects the internal parts of the unit from the effects of con- 

 densed moisture from the breath. This membrane is clamped at its 

 outer edge between a protective grid of perforated metal and the frame. 

 A thin metal ferrule fastens the grid to the frame. The exposed parts 

 of the unit are anodically finished to resist corrosion. 



In addition to being simpler than the earlier transmitter and hence 

 less difficult to produce, the new transmitter unit has characteristics 

 such that: 



