Technical developments underlying the toll services of the bell system

ELEPHONY involves the transmission of speech to a distance by electrical means. Speech itself, physically considered, consists 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 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.