Read this: Returning home to Brantford after six months abroad, Bell continued his experiments with his "harmonic telegraph".[N 12] The basic concept behind his device was that 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.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): For  his system to work, Bell had to change what about each code?
pitch