Prototype telephone constructed by Alexander Graham Bell in 1875. The device consists of a coil of wire, a magnetic arm and a taut membrane. Any sound causes the membrane, and hence the magnetic arm, to vibrate. The movement of the magnet induces a fluctuating electric current in the coil, & this electrical signal can be reconverted into sound by an identical apparatus at the other end of the circuit. Bell filed a patent on the telephone on 14th February, 1876, but his priority as absolute inventor was questioned by Elisha Gray of Western Electric who similarly filed a patent on the same day. After long litigation, Bell won "the most valuable patent ever issued in the US".
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