On the afternoon of June 2, 1875, two young men bent over work benches in the hot and stifling garret of a five-story brick building occupied by the electrical workshops of Charles Williams, at 109 Court Street, Boston. They did not speak to one another, for they were in separate rooms some sixty feet apart, at opposite ends of the floor. Between the rooms ran a length of wire.

The younger of the two men was Thomas A. Watson, twenty-one years old, a native of Salem who had left school at the age of thirteen but had become, during several years of employment at the workshop, an able and imaginative technician. His skill had been tested in the construction of virtually all the devices required by Williams’ clients—call bells, telegraph keys, galvanometers, annunciators, relays, sounders. He had, moreover, read nearly all of the few books on electricity then available, in the morning of the electrical age. His fingers were deft, his intelligence keen.

The man at the other end of the wire was a tall, rather pale, dark-haired, brown-bearded amateur inventor named Alexander Graham Bell. He was twenty-eight years and three months old. Unlike his collaborator, he had no connection with the Williams workshop. He was, in fact, a teacher of the deaf and a specialist in training teachers of the deaf. He held the title of professor of vocal physiology at Boston University. But for more than a year he had been working with Watson on an invention that he called a “harmonic telegraph.” And he had been thinking about it for more than a decade.

The purpose of Bell’s harmonic telegraph was to make possible the transmission of several messages over a single telegraph wire at the same time without interference. Thirty-one years had elapsed since Samuel F. B. Morse sent his famous message, “What hath God wrought,” over the world’s first telegraph circuit, between Washington and Baltimore. During that interval wires had spread like spider webs across the face of the land, and in 1866 the first successful submarine cable spanned the Atlantic Ocean. But as the demand for telegraph service soared, the capacity of each wire remained precisely the same: one message per wire per unit of time. Bell was well aware of the need for multiple telegraphy, and he had formed a notion as to how it could be achieved.

The germ of his idea had first incubated in his mind when, at the age of nineteen, he was teaching elocution and music at Weston House, a boys’ school near his native Edinburgh, Scotland. As the son and grandson of teachers of elocution, Bell had already acquired a great deal of knowledge about acoustics and the anatomy of the human vocal apparatus. One day when he was alone it occurred to him to attempt some informal experiments to determine how vowel sounds are produced. Shaping his mouth and tongue into position to pronounce a given vowel, he tapped his teeth or cheeks with his fingernail or a pencil. His trained ear easily distinguished the varying resonance pitches of his mouth cavities as they changed form in the production of different vowel sounds. He concluded that every vowel sound is the product of resonances from the changing cavities of the mouth.