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

Question: How did that work?
Self-contained rewrite: How did the harmonic telegraph work?