Extrapolating further, Bell reasoned that if the transmitting fork were also connected to a telegrapher’s key that could open and close the circuit, the fork would then become in effect a telegraphic sender, capable of transmitting a series of Morse code signals–dots and dashes–at its own particular frequency. Now suppose further that instead of just one sending fork, you had perhaps six, each with a different pitch, or frequency, and each one paired with a receiving fork of exactly the same frequency at the other end of the line. Then if all six forks began transmitting Morse signals along the same wire at the same time, a complex electrical current carrying six different frequencies would flow through the wire to the receiving end. There each of the six receiving forks, each with its electomagnet and each tuned exclusively to the pitch of its sending partner, would vibrate in resonance with its partner—and only with its partner. The complex signal would thus be unscrambled and each of the six messages sent simultaneously over the same wire would be clearly received. This train of thought led Bell to his conception of the harmonic telegraph.

It was this conception that Bell carried with him from Brantford to Boston in the spring of 1871 when, fully recovered from his illness, he resumed his career as a teacher. He had been offered a fee of five hundred dollars by the Boston school board for a series of lectures on “Visible Speech”—a code of written symbols indicating the position and action of the vocal organs in the production of various sounds, which had been devised by his father as a valuable aid in teaching the deaf to speak. Apart from the money involved, the prospect appealed to Bell enormously for several reasons. He was anxious to return to active professional life, he enjoyed teaching, and he had always been profoundly interested in the problems of the deaf. His mother was deaf.

During the ensuing months Bell made up his mind to remain permanently in the United States, and in October, 1872, he opened a school of vocal physiology and the mechanics of speech in Boston, where he demonstrated his father’s methods before teachers of the deaf. A year later he received an appointment as professor of vocal physiology at Boston University, and transferred his students there. Through his work with the deaf, Bell met two men who would prove tremendously important to him in the years that lay just ahead. One was Gardiner Greene Hubbard, a Boston lawyer and president of Clarke Institute for Deaf-Mutes in Northampton, Massachusetts; Hubbard’s daughter Mabel had lost her hearing through an attack of scarlet fever when she was barely four years old. The other was Thomas Sanders, a prosperous leather merchant of Salem, whose five-year-old son George, born deaf, became one of Bell’s private students. Grateful to Bell for his interest in their children, both men became his close friends and within a year of their first meeting, upon learning of his electrical experiments, offered to cover his expenses in return for a share in future patent rights.

Despite his crowded daily teaching schedule, Bell continued his experiments, working far into the night in an effort to perfect the harmonic telegraph. Sometime during the winter of 1873-74 he conceived the idea of improving his device by substituting flexible strips of metal—like organ reeds or flattened clock springs—for the tuning forks. As he envisaged the new apparatus, one end of each reed would be clamped firmly to one pole of an electromagnet; the other end, extended horizontally, would be free to vibrate over the other pole. Each reed would be provided with a tuning mechanism.

Lacking the time and mechanical skill to construct the necessary parts, Bell sought help at Williams’ workshop in Court Street. At that time the electrical industry was still in its infancy, and the Williams establishment with its thirty-odd employees ranked as one of the best-equipped shops in the country. Among its employees was Thomas A. Watson, and it was he who came to assist Bell in the modification of his harmonic telegraph. From their initial encounter grew a long and rewarding professional association.

In a memoir written years afterward, Watson recalled: “I made half a dozen pairs of the harmonic instruments for Bell. He was surprised, when he tried them, to find that they didn’t work as well as he expected.” The failures, however, were blessings in disguise. For, as Watson pointed out, “Had his harmonic telegraph been a well-behaved apparatus that always did what its parent wanted it to do, the speaking telephone might never have emerged from a certain marvellous conception that had even then been surging back of Bell’s high forehead for two or three years.”