Bell's patent, U.S. Number 174,465, has been called the most valuable ever issued. Probably no means of communication has revolutionized the daily lives of ordinary people more than the telephone. Simply described, it is a system which converts sound, specifically the human voice, to electrical impulses of various frequencies and then back to a tone that sounds like the original voice.
In 1831, Michael Faraday, an English scientist, proved that vibrations of metal could be converted to electrical impulses. This was the technological basis of the telephone, but no one actually used this system to transmit sound until 1861. In that year, Johann Philip Reis, a German inventor, is said to have built a simple apparatus that changed sound to electricity and back again to sound. A crude device, it was incapable of transmitting most frequencies, and it was never fully developed.
A practical telephone was actually invented independently by two inventors working in the United States, Elisha Gray, and Scottish-born Alexander Graham Bell. Incredibly, both men filed for a patent on their designs at the New York patent office on February 14, 1876, with Bell beating Gray by only two hours! Although Gray had built the first steel diaphragm / electromagnet receiver in 1874, he was not able to master the design of a workable transmitter until after Bell had. Bell had worked tirelessly, experimenting with various types of mechanisms, while Gray had become discouraged.
According to the famous story, the first fully intelligible telephone call occurred on March 6, 1876, when Bell, in one room, called to his assistant, Thomas Watson, in another room. "Come here, Watson, I want you." Watson heard the request through a receiver connected to the transmitter that Bell had designed, and what followed after that is a history of the founding of the Bell Telephone Company (later AT&T), which grew to be the largest telephone company in the world.
Source: The Great Idea Finder
More about the history of the invention of telephone:
Antique Telephone History Web Site
Telephone History - Tom Farley
Telephony Museum
Alexander Graham Bell: Patent for the Telephone - about.com
The Telephone - PBS
The History of the Telephone - about.com
The History of the Telephone- Casson Herbert
The Speaking Electric Telegraph - Scientific American Supplement, 1877
Bell Notebooks Project - Charles R. Twardy
Bell's First Phones - Cybersound
Inventing the Telephone - AT&T
Bell theorized in 1876 that if he could somehow vary the resistance in an electrical circuit at the frequency of sound it would cause the current to fluctuate in exact step with the sound. And if he sent that fluctuating current through one of the electromagnetic relays he was using for his telegraph system (the relay functions as a primitive loudspeaker), he should be able to hear the original sound coming from the relay. If he could do that, he believed, he was very close to sending the human voice through that same circuit. He sketched out this idea in his laboratory notebook as shown in Figure 1.
You can easily perform this experiment. Tuning forks can be obtained from music stores for about five dollars. A six-volt lantern battery works well. For the acid water use 5% white vinegar straight from the bottle and pour it into a 1-cup metal measuring cup. The listening device (used in place of Bell’s relay) can be just about anything that can produce sound: an old telephone receiver, a headphone or a loudspeaker from an old radio.
Be sure to give the tuning fork a good rap and then quickly lower it to the vinegar, keeping it as parallel as possible and ensure it just barely makes contact with the surface of acid water in order that the the tip of the vibrating tuning fork is being rapidly immersed and withdrawn from the vinegar. This changes the resistance between the fork and the vinegar, which in turn causes the current through the listening device to fluctuate at the same frequency of the fork. Try the experiment again, but with the fork perpendicular to the vinegar; you may not hear any sound because there is no change in resistance.
More information about this experiment:
Alexander Bell’s Famous Tuning Fork Experiment - Ed Evenson
Bell's Variable Resistance Microphones - Jim & Rhoda Morris
Shortly after his successful tuning fork experiment, Alexander Bell put his plans in motion for the final experiment - transmitting the human voice over a telegraph wire. This is the experiment that gave us the world’s most historic phone call, “Mr. Watson come here. I want to see you.”
This is a simplified diagram of Bell's liquid transmitter. The diaphragm vibrated with sound waves, just like the tuning fork did, causing a conducting rod (takes the place of the tuning fork) to move up and down in a cup of acid water while the rod is adjusted to just barely make contact with the surface of acid water. As the voice-driven membrane causes the rod to advance and retreat into and out from the liquid, ever so slightly, the circuit resistance will alternately increase and decrease in perfect step with those vocal sounds. And the resulting alternating current with the help of a battery (not shown) will reproduce the original sounds in the receiver (not shown). Bell used a telegraph relay for his receiver but you can use a loud speaker or any other hearing device like in the fork tuning experiment.
Although Bell used a water/acid solution as the liquid, vinegar will perform a similar function.
More information about this experiment:
Alexander Bell’s Liquid Transmitter - Ed Evenson
Telephone History 1876 to 1892 - Tom Farley
Bell's Variable Resistance Microphones - Jim & Rhoda Morris
