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When did Bell begin Alexander Graham Bell's early work on the telephone?
By 1874, Bell's initial work on the harmonic telegraph had entered a formative stage.
what led to his discovery of the telephone?
Bell experimented with a "phonautograph", a pen-like machine that could draw shapes of sound waves on smoked glass by tracing their vibrations.

Question: What was happening at the time of his discovery?
Self-contained rewrite: What was happening at the time of Alexander Graham Bell's telephone discovery?