The telephone was created through the cross-pollination of two fields of study - electricity and acoustics. Although the invention of the telephone is usually credited to Alexander Graham Bell, many inventors were working on the problem throughout the 1860s and 1870s, most notably Elisha Gray, who filed a patent application for the same device only a few hours after Bell did. Although the basics of electricity were known by the 1830s, nobody suggested transmitting speech electrically until 1854. The telephone was finally invented on March 10, 1876. Into it were spoken the famous words, "Mr. Watson, come here, I want to see you."
The telephone operates on simple principles. A telephone mouthpiece contains a thin metallic coating separated from an electrode by a thin barrier (today we use plastic) which connects to a wire carrying an electric current. When a person speaks into the mouthpiece, the acoustic vibrations from her speech push the metallic coating slightly closer to the electrode, resulting in variations in voltage and therefore a speedy conversion from acoustic to electric energy. The electric pulses are conveyed through a wire to the speaker on the other end, where electric pulses are converted into acoustic energy again.
Converting speech to electrical energy before transmission is far more efficient than conveying speech through a mechanical channel, for example a metal pipe, because the walls of mechanical channels absorb so much of the acoustic energy as it travels. Well-insulated wires, however, are effective at protecting electrical energy from dispersing before reaching its destination. Electrical pulses travel at the speed of light, whereas acoustic pulses are limited by the speed of sound. In his 1627 book New Utopia, Francis Bacon referred to the possibility of a long-distance network of tubes to transmit human speech. Many historians consider this the first reference to something like a telephone. Unfortunately, he lacked the scientific knowledge to understand why this would be impractical.
Three critical developments were necessary precursors to the invention of the telephone. First was the understanding that an electrical current can create a magnetic field, and therefore mechanical or acoustic energy. This is credited to Danish physicist Christian Oersted in 1820, who demonstrated that a compass needle can be manipulated by running an electric wire over it. Second was the understanding that the reverse is possible - electrical induction - that is, generating a current by placing a moving magnet next to a wire. This insight is credited to inventor Michael Faraday in the year of 1821. The last necessary development was that of the battery, a chemical device capable of producing a continuous source of electricity. The first very crude battery was the "Leyden jar", a device for storing static electricity invented by two Dutchmen in 1746. Volta and others developed more sophisticated batteries throughout the 18th and 19th centuries.
Often considered second in impact only to the printing press for revolutionizing human communication, the telephone made it possible to connect people to others without dependency on the postal service. Every domain of human affairs, from business to daily life, was changed radically and permanently. Globally, the telephone has saved humanity trillions of dollars in communications costs. The basic design of the telephone has not changed since its initial invention.