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Alexander Graham Bell, Telephone Inventor
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Bell’s variable resistance liquid telephone

Bell’s experimental liquid (variable resistance) telephone transmitter

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Bell’s reed telephone receiver
Bell’s reed telephone receiver

Bell’s diagram telephone
Bell’s diaphragm telephone

The Invention of the Telephone

Bell’s first speech transmission of March 10, 1876, was made with a variable-resistance transmitter similar to this one. This is a reproduction of the actual transmitter Bell displayed at the World’s Fair in Philadelphia two weeks later.

The tin mouthpiece focuses the sound waves of the speaker’s voice. These sound waves cause a membrane stretched across the narrow end of the mouthpiece to vibrate. A needle attached to the center of that membrane has electric current running through it. As the membrane vibrates, the needle moves up and down in a cup of slightly acidic water, and the electrical resistance between the needle’s current and the water varies. The varying vibrations travel through a wire to a receiver, which translates them back into sound.

The reed telephone receiver shown is a reproduction of the telephone receiver used on March 10, 1876. Originally part of a harmonic telegraph, Bell and Watson used it for their historic telephone experiment. Pressing the receiver against his ear, Watson heard Bell’s first successfully transmitted message--“Mr. Watson, come here, I want to see you.”

Bell’s major public demonstrations in 1876 were made primarily with an electromagnetic transmitter like the diaphram telephone.

Here, the vibrations of the speaker’s voice make the membrane move a piece of steel-the armature-near the pole of an electromagnet. The vibrations of the armature cause a corresponding fluctuating current in the coil of the magnet. A distant receiver detects the current and changes it back into sound.

Although the variable-resistance type of transmitter was ultimately more successful, Bell used the electromagnetic transmitter because it worked better and more consistently at the time.

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