Conversation so far:
How did Alexander Graham Bell help the deaf?
His research on hearing and speech further led him to experiment with hearing devices which eventually culminated in Bell being awarded the first U.S. patent for the telephone in 1876.
How did Alexander Graham Bell help his father?
They made plans to establish a teaching practice and in 1871, he accompanied his father to Montreal, where Melville was offered a position to teach his System of Visible Speech.
Did Alexander Graham Bell make any inventions for the deaf?
The harmonic telegraph
How did the harmonic telegraph work?
Messages could be sent through a single wire if each message was transmitted at a different pitch, but work on both the transmitter and receiver was needed.
Did Alexander Graham Bell do more work for the deaf after experimenting with the harmonic telegraph?
Unsure of his future, he first contemplated returning to London to complete his studies, but decided to return to Boston as a teacher.

Question: How long did he stay a teacher of the deaf?
Self-contained rewrite: How long did Alexander Graham Bell stay a teacher of the deaf?