Graham Bell's invention of the telephone was a step-by-step process, sometimes with despair that the invention could succeed. During a period of recuperation at Tutelo Heights from his strenuous workload of teaching and inventing, he contemplated the phonautagraph, a sound writer (remember Professor Higgin's contraption from Pigmalion?) which could receive sound waves on a membrane stretched over a hollow cylinder, and then move a stylus in sympathy with the vibrations. Bell's sudden insight was that a receiver modeled closely to the human ear might produce more accurate tracings of speech vibrations. He also experimented with the harmonic multiple telegraph in which a number of telegraph signals could be sent simultaneously over the same circuitry in either or both directions. For this invention he first used a tuning fork (later substituting a tuned steel reed) with its prong placed between the poles of an electromagnet, producing a dc electric current at each vibration. By operation of a telegraph key in the circuit of each fork, the intermittent current, controlled by the vibration of that particular fork, could be sent over a telegraph line wire to a receiving end where a similar fork, tuned to the same pitch, would be actuated by an electromagnet. Only the fork tuned to that pitch would "receive" the signal. A similar system was used for remote control of substation equipment as "supervisory control" in recent years.

Graham Bell was materially aided by Thomas A. Watson who was adept at building electrical apparatus. On June 2, 1875, Bell, assisted by Watson, was experimenting with the multiple telegraph - Watson sending, Bell receiving. One of the tuned-reed transmitters, which was in close proximity to the pole of an electromagnet, fused its contacts. Bell heard the faint echo of the vibrating fused reed, and then realized the idea of an undulating current that could transmit sound by electrical wire!

The first electric speaking telephone (June 3, 1875) was to utilize a diaphragm of tightly stretched parchment which would vibrate a steel armature of an electromagnet to replicate the sound vibration into an undulating current.

How could the signal be strengthened to produce a practical telephone? Again, the multiple telegraph was a factor. Graham Bell was working on a spark arrester for the multiple telegraph, and he regulated the resistance between the two wires by dipping their ends into a vessel of water, varying the resistance by adjusting the depths to which the wires were submerged. From this came the idea of producing undulating current stronger than those obtained from his magneto-electric instrument. As the diaphragm vibrated, the wire rose and fell subject to its control, and by varying the resistance, varied the magnitude of the current.

With this apparatus, a test was made on March 10, 1876, Bell spoke and Watson heard the first sentence transmitted by the telephone, "Mr. Watson come here, I want you."