page_title: Invention of the telephone
section_title: Alexander Graham Bell
hierarchical_section_title: Invention of the telephone / Electro-magnetic transmitters and receivers / Alexander Graham Bell
caption_attribution_description: English: Pages 40-1 of Alexander Graham Bell's unpublished laboratory notebook (1875-76), describing first successful experiment with the telephone.
context_page_description: The invention of the telephone was the culmination of work done by many individuals, and led to an array of lawsuits relating to the patent claims of several individuals and numerous companies.
context_section_description: Alexander Graham Bell had pioneered a system called visible speech, developed by his father, to teach deaf children. In 1872 Bell founded a school in Boston to train teachers of the deaf. The school subsequently became part of Boston University, where Bell was appointed professor of vocal physiology in 1873.
  As Professor of Vocal Physiology at Boston University, Bell was engaged in training teachers in the art of instructing the deaf how to speak and experimented with the Leon Scott phonautograph in recording the vibrations of speech. This apparatus consists essentially of a thin membrane vibrated by the voice and carrying a light-weight stylus, which traces an undulatory line on a plate of smoked glass. The line is a graphic representation of the vibrations of the membrane and the waves of sound in the air.
  This background prepared Bell for work with spoken sound waves and electricity. He began his experiments in 1873–1874 with a harmonic telegraph, following the examples of Bourseul, Reis, and Gray. Bell's designs employed various on-off-on-off make-break current-interrupters driven by vibrating steel reeds which sent interrupted current to a distant receiver electro-magnet that caused a second steel reed or tuning fork to vibrate.
  During a June 2, 1875, experiment by Bell and his assistant Thomas Watson, a receiver reed failed to respond to the intermittent current supplied by an electric battery. Bell told Watson, who was at the other end of the line, to pluck the reed, thinking it had stuck to the pole of the magnet. Watson complied, and to his astonishment Bell heard a reed at his end of the line vibrate and emit the same timbre of a plucked reed, although there were no interrupted on-off-on-off currents from a transmitter to make it vibrate. A few more experiments soon showed that his receiver reed had been set in vibration by the magneto-electric currents induced in the line by the motion of the distant receiver reed in the neighborhood of its magnet. The battery current was not causing the vibration but was needed only to supply the magnetic field in which the reeds vibrated. Moreover, when Bell heard the rich overtones of the plucked reed, it occurred to him that since the circuit was never broken, all the complex vibrations of speech might be converted into undulating (modulated) currents, which in turn would reproduce the complex timbre, amplitude, and frequencies of speech at a distance.
  After Bell and Watson discovered on June 2, 1875, that movements of the reed alone in a magnetic field could reproduce the frequencies and timbre of spoken sound waves, Bell reasoned by analogy with the mechanical phonautograph that a skin diaphragm would reproduce sounds like the human ear when connected to a steel or iron reed or hinged armature. On July 1, 1875, he instructed Watson to build a receiver consisting of a stretched diaphragm or drum of goldbeater's skin with an armature of magnetized iron attached to its middle, and free to vibrate in front of the pole of an electromagnet in circuit with the line. A second membrane-device was built for use as a transmitter. This was the "gallows" phone. A few days later they were tried together, one at each end of the line, which ran from a room in the inventor's house, located at 5 Exeter Place in Boston, to the cellar underneath. Bell, in the work room, held one instrument in his hands, while Watson in the cellar listened at the other. Bell spoke into his instrument, "Do you understand what I say?" and Watson answered "Yes". However, the voice sounds were not distinct and the armature tended to stick to the electromagnet pole and tear the membrane.
  In a March 10, 1876, test, between two rooms in a single building in Boston showed that the telephone worked, but so far, only at a short range.
  In 1876, Bell became the first to obtain a patent for an "apparatus for transmitting vocal or other sounds telegraphically", after experimenting with many primitive sound transmitters and receivers. Because of illness and other commitments, Bell made little or no telephone improvements o
caption: Bell's March 11 1876, laboratory notebook entry describing his first successful experiment with the telephone
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