Bell had been studying new methods to improve the telegraphic transmissions. However, during his spare time it was devoted to develop techniques for teaching deaf people to speak and to read lips.
During his research work he had studied several voice-actuated instruments, like the Koenig manometric capsule as well as the Phonoutograph invented by Martinville.
While conducting these experiments, Bell was struck by the similarity of the Phonautograph to the human ear. In this way he concluded that a similar apparatus modeled after the structure of the human ear probably could produce more acurate register of the speech vibrations.
Since Bell had any knowledge on reproducing the structure of the human ear he called on the acquaitance of Dr. Clarence J. Blake, a famous American otologist, who advised him to start a meticulous study of human ear anatomy from a dead man available from the morgue of Harvard Medical School.
In mid 1874, Bell carried on several experiments in such way when speaking into the anatomical piece coupled to a Phonautograph, it made tracings of the speech vibrations over a piece of smoked glass.
Considering those early fundamental experiments for the future development of the telephone, Bell begun to develop the transmission of the spoken words on wires through the use of the harmonic telegraph.