For several hours after the unforgettable twang, Bell and Watson repeated the experiment, changing places, changing the circuits, testing each pair of transmitters and receivers, and cross-checking each other’s observations. On through the afternoon and into the night, “there was little done but plucking reeds and observing the effect”—this time the words are Bell’s. But faintly as the signals came through, they were there and they were true. And Bell now knew that his invention could be made to work, for his major and most persistent fear had been resolved. As he expressed it some thirty years afterward, “These experiments at once removed the doubt that had been in my mind since the summer of 1874, that magneto-electric currents generated by the vibration of an armature in front of an electromagnet would be too feeble to produce audible effects that could be practically utilized.”

Before they parted company for the night, Bell gave Watson instructions for making the first speaking telephone. The specifications were simply those of the membrane telephone which he had envisaged at his home in Brantford the summer before. Watson promised to have it ready the next day. Bell walked the streets for some time and when he returned to his lodgings found he could not sleep. Though elated, he felt guilty at having invented the telephone when his sponsors expected him to be hard at work on the harmonic telegraph. Before he went to bed he wrote a letter to Hubbard.

“Dear Mr. Hubbard,” he began. “I have accidentally made a discovery of the very greatest importance. . . . ”

On the next day, June 3, 1875, Watson constructed the first Bell telephone. As a mouthpiece, he arranged a small hollow cylinder, closed at one end by a tautly stretched parchment membrane. To the center of the membrane he attached the free end of a transmitter spring. It was a beautifully simple mechanism. When a person spoke into the mouthpiece, sound waves from his voice caused the membrane to vibrate. The membrane then caused the attached transmitter spring to vibrate. And the transmitter spring, vibrating over one pole of its electromagnet, induced an undulatory current that varied in intensity as the air varied in density during the production of vocal sounds.

That evening Bell and Watson met at the shop, after the workmen had gone home, for the initial tests. Surmising that the signal would be faint at best, and that both he and Bell would doubtless be shouting at the top of their lungs, Watson had taken the precaution of running the wire—the world’s first telephone line–from their fifth-floor garret down to the third floor, to lessen the chance of hearing each other directly through the air. On the first test the new telephone was placed on Watson’s workbench, while Bell stationed himself at a receiver in the garret. Watson shouted; but Bell, straining his ears, could hear nothing. They then exchanged places, with Bell at the transmitter below and Watson upstairs. This time the results were more encouraging.

“I could unmistakably hear the tones of his voice,” Watson recalled later, “and almost catch a word now and then. I rushed downstairs and told him what I had heard. … It was enough to show him that he was on the right track, and before he left that night he gave me directions for several improvements in the telephones I was to have ready for the next trial.”

Watson attributed the one-way transmission that night not to any defect in the system but to Bell’s lifelong training in elocution: “The reason why I heard Bell in that first trial of the telephone and he did not hear me, was the vast superiority of his strong vibratory tones over any sound my undeveloped voice was then able to utter.” He then added dryly, “My sense of hearing, however, has always been unusually acute, and that might have helped to determine this result.”