230 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



course be given to the likelihood of their achievement. In the era when ex- 

 periment was the dominant factor in development, advancement in per- 

 formance was largely expressed in terms of the modification which was 

 tested. 



The results of this era of measurement began to have their effect on the 

 design of commercial telephone instruments about twenty-five years ago. 



Though the idea of a handset goes back to the early days of telephony^ 

 it was not found out until the middle 1920's how to get, in the instruments 

 of a handset, service performance comparable to that afforded by the then 

 available instruments when supported and separated as they were in the 

 wall set and deskstand set. There were two important limitations to achieving 

 this result — one, the so-called "howling" resulting from the coupling be- 

 tween the diaphragm of the transmitter and the diaphragm of the receiver; 

 and the other, the degradation of the performance of the granular carbon 

 transmitter with position. The importance of the amplification provided by 

 this granular carbon on the design of the telephone plant has been indicated 

 and it was the magnitude of this amplification and the resonances in the 

 instruments that caused the howling difficulty in the handset. With such 

 instruments directly coupled mechanically, the howling problem was diffi- 

 cult to solve .^^ 



In the early twenties, in the development of the handset which was made 

 available in 1927, the coupling factor between the diaphragms of the two 

 instruments was measured for a variety of proposed designs of handle; and 

 the development and selection of the design was dn the basis of a handle 

 having resonance out of the range of the instruments used and of such ma- 

 terial as to provide dissipation of energy in this mechanical transmission 

 path.49 



The other factor that made possible the solution of this problem was the 

 development of a transmitter in which the vibratory system was essentially 

 free from resonance and the positional effect of the carbon chamber was 

 materially reduced. This transmitter — the first non-resonant transmitter^^ 

 in commercial telephony — gave a decrease in the magnitude of the electrical 

 output. It was demonstrated, however, by articulation and repetition-rate 

 tests, that the reduction in loudness output was compensated for by the 

 lower distortion of the reproduced sounds; and hence the combination of 

 higher quality with decreased loudness gave a resultant intelligibility in 

 service comparable to that obtained with the then available deskstand trans- 

 mitter. The change in transmitter response is shown by a comparison of 

 curves A and B of Fig. 4(a). The elimination of sharp resonances gives an 

 additional improvement in transient response. 



A comparison of curves A and B of Fig. 4(b) shows that the small receiver 

 of the 1927 handset was made to give the same performance as the preceding 



