But this spring of 1875 was the dark hour just before the dawn. . . . The date when the conception of the undulatory or speech-transmitting current took its perfect form in Bell’s mind [was] the greatest day in the history of the telephone, but certainly June 2, 1875, must always rank next; for on that day the mocking fiend inhabiting that demonic telegraph apparatus . . . opened the curtain that hides from man great Nature’s secrets and gave us a glimpse into that treasury of things not yet discovered. . . . [Watson’s literary style is hardly what one would expect from a man who left school at the age of thirteen. However, when he was forty, he entered M.I.T. and took courses in literature, geology, and biology—subjects which dominated his interest in later years. He left the American Bell Telephone Company in 1881 and spent a year in Europe. On his return he went into shipbuilding and founded the Fore River Ship and Engine Company in East Braintree, Massachusetts, which had a large share in building the U.S. fleet that fought the Spanish-American War. In 1904, aged fifty, he retired from business and spent the remaining years of his life in travel. He died in 1934.]

In the course of their experiments on the harmonic telegraph, Bell had found the source of their difficulties. The trouble lay in their inability to tune transmitters and receivers into precise and perfect congruence. Since Bell had a musical ear (and Watson did not), it was he who undertook the finicky and seemingly endless job of adjusting the tuning screws. His method was to hold the vibrating spring, or reed, of a receiver close to his ear while the corresponding transmitter in the other room was sending its intermittent current through the electromagnet. He would then manipulate the tuning screw until the vibratory whine emitted by the spring of the receiver appeared to coincide with the whine coming—through the air—from the transmitter.

On the afternoon of June 2, 1875 [Watson continued], we were hard at work on the same old job, testing some modification of the instruments. Things were badly out of tune that afternoon in that hot garret, not only the instruments, but, I fancy, my enthusiasm and my temper, though Bell was as energetic as ever. I had charge of the transmitters as usual, setting them squealing one after the other, while Bell was retuning the receiver springs one by one, pressing them against his ear as I have described.

One of the transmitter springs I was attending to stopped vibrating and I plucked it to start it again. It didn’t start and I kept on plucking it, when suddenly I heard a shout from Bell in the next room, and then out he came with a rush, demanding, “What did you do then? Don’t change anything. Let me see!”

Bell, at the other end of the line, had heard in his receiver a startling, unbelievable sound, a sound quite different from the familiar whine of the vibrating transmitter. Instead he had heard the distinctive metallic twang-g! of a plucked spring, a sound with tones and overtones, a sound that made his heart stand still.

Watson showed him what had happened. The contact screw had been set down so far that it had made permanent contact with the spring. Hence when Watson plucked the spring the circuit remained unbroken. And instead of producing an intermittent current, the spring had acted as a diaphragm and sent an induced, undulating current over the line. In Watson’s words,

That strip of magnetized steel by its vibration over the pole of its magnet was generating that marvellous conception of Bell’s—a current of electricity that varied in intensity precisely as the air was varying in density within hearing distance of that spring. That undulatory current had passed through the connecting wire to the distant receiver which, fortunately, was a mechanism that could transform that current back into an extremely faint echo of the sound of the vibrating spring that had generated it.

What was still more fortunate, the right man had that mechanism at his ear during that fleeting moment, and instantly recognized the transcendent importance of that faint sound thus electrically transmitted. The shout I heard and his excited rush into my room were the result of that recognition.

The speaking telephone was born at that moment. Bell knew perfectly well that the mechanism that could transmit all the complex vibrations of one sound could do the same for any sound, even that of speech. … All the experimenting that followed that discovery, up to the time the telephone was put into practical use, was largely a matter of working out the details.