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Edison, on dais, makes telephone call in front of a theater filled with people.

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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The Challenge of Success
Before Forty
Changes At Forty
Home Life
The New Technical World
Fame And Its Distractions
Edison In His Eighties