The problem of sending two signals simultaneously in opposite directions on the same wire had been solved previously by Julius Wilhelm Gintl and improved to commercial viability by J. B. Stearns; Edison added the ability to double the number in each direction.
To send two signals in a single direction at the same time, the quadruplex telegraph used one signal to vary the absolute strength or voltage of the signal (amplitude modulation) and the other signal to vary the phase (polarity) of the line (phase modulation), i.e., the direction of current flow imposed upon the wire.
1874 Electric pen
1876 Founded Menlo Park laboratory
1879 Electric light & Incandescent light bulb
1880 Electric power distribution
1882 First Electric power plant
It was later on in the year in September 1882 that Edison opened the Pearl Street Power Station in New York City and again it was a DC supply. It was for this reason that the generation was close to or on the consumer’s premises as Edison had no means of voltage conversion.
1887 First Electric power plant
1889 Talking doll
In 2015, the Lawrence Berkeley National Laboratory, in collaboration with the Library of Congress, developed a three-dimensional optical scanning system called IRENE-3D, which allowed surviving discs to be scanned and the audio to be reproduced. As of April 2015, eight recordings had been digitized and may be heard at the National Park Service website