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Updated: May 7, 2025
Interest in the duplex as a field of invention dwindled, however, as the quadruplex loomed up, for while the one doubled the capacity of a circuit, the latter created three "phantom wires," and thus quadruplexed the working capacity of any line to which it was applied.
If this were the only thing that he had ever accomplished, it would entitle him to consideration as an inventor of note. The quadruplex, however, has other material advantages, but how far they and the natural growth of the business have contributed to the investment and earnings of the telegraph companies, is beyond practicable computation.
Edison had never received a check before and he had to be told how to cash it. Edison immediately set up a shop in Newark and threw himself into many and various activities. He remade the prevailing system of automatic telegraphy and introduced it into England. He experimented with submarine cables and worked out a system of quadruplex telegraphy by which one wire was made to do the work of four.
Duplex telegraphy, or the sending of two separate messages in opposite directions at the same time over one line was known and practiced previous to this time, but quadruplex telegraphy, or the simultaneous sending of four separate messages, two in each direction, over a single line had not been successfully accomplished, although it had been the subject of many an inventor's dream and the object of anxious efforts for many long years.
We must return to the days of 1878, when Edison, with at least three first-class inventions to his credit the quadruplex, the carbon telephone, and the phonograph had become a man of mark and a "world character."
As will have been gathered from the above, the principle embodied in the quadruplex is that of working over the line with two currents from each end that differ from each other in strength or nature, so that they will affect only instruments adapted to respond to just such currents and no others; and by so arranging the receiving apparatus as not to be affected by the currents transmitted from its own end of the line.
Then there are the stock printer, the automatic repeating telegraph, quadruplex telegraphy, the phono-plex, the ore-milling process, the railway telegraph, the electric engine, the phonograph. Some of these inventions seem, in the glow of his incandescent light, or with one's ear to the tube of the telephone he improved in its most essential part, to be too small for Edison.
He spent his leisure in the Mechanics' Library, studying works on electricity and general science. He also developed his ideas on the duplex system; and if they were not carried out, they at least directed him to the quadruplex system with which his name was afterwards associated.
The award to Edison was not only for his inventions in duplex and quadruplex telegraphy, and for the phonograph, but for the development of a commercially practical incandescent lamp, and the development of a complete system of electric lighting, including dynamos, regulating devices, underground system, protective devices, and meters.
Thus by combining instruments that respond only to variation in the strength of current from the distant station, with instruments that respond only to the change in the direction of current from the distant station, and by grouping a pair of these at each end of the line, the quadruplex is the result. Four sending and four receiving operators are kept busy at each end, or eight in all.
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