Technical Developments Underlying the Toll Services of 



the Bell System* 



Early Developments 



General — Telephone Instruments 



'T^ELEPHONY involves the transmission of speech to a distance 

 •^ by electrical means. Speech itself, physically considered, con- 

 sists of rapid longitudinal variations in air pressure, or acoustic waves 

 as they are called, traveling out from the mouth of the speaker or to 

 the ear of the listener. Each sound has its characteristic wave form 

 or group of wave forms and as a result these acoustic waves are of 

 complicated and rapidly changing wave form as is illustrated by the 

 oscillograms on Fig. 1 showing the structure of the electrical current 



• ^^■^'-7^^l^''/■JV/^/^^•^^.//^sy^/\y^/^V^,/l^/^-V>|^ 



\-'^^jK^''^^\J 



S-O.OI SECOND ■ 



Fig. 1- 



-Oscillograms showing electrical current in a telephone circuit resulting from 

 spoken word "Harvard" and vowel in "Har." 



in a telephone circuit resulting from the spoken word "Harvard," and, 

 in more detail, the vowel in "Har." The telephonic transmission of 

 speech requires, therefore, three fundamental elements: 



(a) An instrument which, when acted upon by the acoustic waves 



of the speaker's voice, produces in an electrical circuit 



oscillations or waves suitable to represent the voice of the 



speaker. This is the telephone transmitter. 



* At the request of the Federal Communications Commission, this pamphlet was 

 recently prepared to give the Commission a brief account of some of the principal 

 technical developments which have led to telephone toll service as given by the Bell 

 System. As it brings together in concise form a summary of a large amount of infor- 

 mation of interest to communications people generally, it is being issued with a few 

 minor editorial revisions as a supplement to the Technical Journal. 



H. S. Osborne 



