222 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



principle of receiver operation and the 1950 set incorporated the invention 

 of a radically new receiver structure. 



Experiment 



The solid-back granular-carbon transmitter and the bipolar hand re- 

 ceiver which were introduced in the last decade of the nineteenth century 

 typify in their general structures the telephone instruments of the first 

 quarter of the twentieth, in the Bell System and elsewhere. Throughout 

 most of this period the progress made in telephone instruments may be 

 characterized largely as improvements determined by experiments on modi- 

 fications in details of form. Many important results were obtained in this 

 field by this empirical or "cut and try" method, as was also the case with 

 the other elements of the telephone set and with apparatus in many other 

 fields in this period. That was generally the technology of the time. Progress 

 by "cut and try," however, tends to be cumbersome, slow and unsystematic. 



The vibrating diaphragms of both transmitter and receiver had their 

 primary resonances in the transmitted range of voice frequencies. By Hsten- 

 ing to the speech sounds reproduced by various structures, judgments w^re 

 made as to their relative merits on the sources of loudness, intelligibihty 

 and naturalness. By such quaUtative tests, the primary resonances of these 

 structures were moved to around one thousand cycles as being the most 

 advantageous location in the audible frequency range. 



Since the resonance of the vibratory elements of these instruments con- 

 tributed so much to their overall conversion efficiencies, the changes in the 

 design tended to enhance these resonance effects. Improvements came in 

 efficiency, reUability and form, but the resonance effects remained peaked 

 around one thousand cycles. Loudness of the sounds reproduced at the 

 other end of the circuit was of great importance and it was early realized 

 that the ear and mind of the listener can do an amazing job in associating 

 distorted reproduced sounds with those spoken by the talker. So amplifica- 

 tion by the granular carbon in the transmitter and the fostering of efficiency 

 by resonances in both instruments were features of development in this 

 period and played a large role in keeping down the cost of the circuits re- 

 quired for the expansion of telephony. 



The undesirable effects of resonance were increasingly appreciated 

 throughout this period of experiment, but no practicable means were dis- 

 covered of reducing resonance without sacrificing unduly the loudness of the 

 sounds reproduced in the ear of the listener. Since resonance had to be — 

 and the importance of the loudness was so readily recognized — the efforts 

 were directed to making the most of resonance. 



An outstanding feature of this period in the progress of the telephone was 

 the difficulty of measuring performance. The technical people then working 



