Bell's extensive knowledge of the nature of sound and his understanding of music enabled him to conjecture the possibility of transmitting multiple messages over the same wire at the same time. Although the idea of a multiple telegraph had been in existence for some time, Bell offered his own musical or harmonic approach as a possible practical solution. His "harmonic telegraph" was based on the principle that several notes could be sent simultaneously along the same wire if the notes or signals differed in pitch. Почему enabled, had been, offered, was based будут употребляться в таких формах?

Enabled - Past Indefinite Active Voice. In the first sentence we can observe a well-known fact happened in a certain moment of time in the past.

had been - Past Perfect Active Voice. In the dependent clause of the complex sentence action has been completed before the action in the independent clause.

offered - Past Indefinite Active Voice. Action in the main clause of the complex sentence represents ordinary well-known fact in the past.

was based - Past Indefinite Passive Voice. Past Indefinite is used to show simple action happened in the past. Passive Voice is used because the subject "harmonic telegraph" receives the action of the verb. We can understand that Bell based his telegraph on the certain principle.