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The base unit is powered by a transformer connection to a standard electric outlet.
When the telephone is “on hook,” contact with the local loop is broken.
Such a system could not generate enough voltage to produce audible speech in distant receivers, so every transmitter since Bell’s patented design has operated on a direct current supplied by an independent power source.
The first sources were batteries located in the telephone instruments themselves, but since the 1890s current has been generated at the local switching office.
Within 20 years of the 1876 Bell patent, the telephone instrument, as modified by Thomas Watson, Emil Berliner, Thomas Edison, and others, acquired a functional design that has not changed fundamentally in more than a century.
Since the invention of the transistor in 1947, metal wiring and other heavy hardware have been replaced by lightweight and compact microcircuitry.