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Edison making a call from San Francisco at the inauguration of the
transcontinental line, 1915
A telephone call of more than a few hundred miles needs to be amplified. The best amplifiers (or relays), which were used on the first trans-continental line when it opened to the public in 1915, used vacuum tubes--direct descendants of the "Edison effect" discovered by Edison in 1881.
SI negative #87-1697
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