REFERENCE SYSTEM FOR TELEPHONE TRANSMISSION 547 



of 22 db in steps of 0.1 db. As in the case of the reference transmitter, 

 provision has been made for the introduction of distortion networks 

 to simulate the distortion of any receiving system. In the moving 

 coil receiver^ shown in Fig. 4 and in cross-section in Fig. 7, a coil of 

 aluminum ribbon, by vibrating relatively to a fixed permanent magnet, 

 actuates a clamped, unstretched, thin duralumin diaphragm to which 



PERMANENT MAGNET 



MOVING COIL 



CAPILLARY TUBE 

 FOR HYDROGEN 



CONTACT SURFAC ES 



Fig. 7 — Schematic of moving coil receiver, coupler and condenser transmitter. 



it is attached. Air damping in this receiver is secured by an arrange- 

 ment somewhat similar to that employed in the condenser trans- 

 mitter. The structure is simple, rugged, and, from the standpoint of 

 freedom from distortion, its performance is comparable to that of the 

 condenser transmitter. The central surface of the diaphragm is 

 protected by a meshed wire screen instead of by the more usual type 

 of receiver cap. This construction avoids resonance effects which 



' This instrument, which is similar in many respects to one described by E. C. 

 Wente and A. L. Thuras in the Bell System Technical Journal, January, 1928, will 

 be discussed in a future paper by the same authors. 

 36 



