On arriving home he resumed arduous work on many of his inventions--chiefly those relating to duplex telegraphy. This subject had interested him at various times for four or five years previously, and he now returned to it with great vigor. Many inventors had been working on multiple transmission, and at this period a system of sending two messages in opposite directions at the same time over one wire had been invented by Joseph Stearns, and had then lately come into use. The subject of multiple transmission gave plenty of play for ingenuity and was one that had great fascination for Edison. He worked out many plans, and in April, 1873, two applications for patents. One of these covered an invention by which not only could two messages be sent in opposite directions over one wire at the same time, but, if desired, two separate messages could be sent simultaneously _in the same direction_ over a single wire. The former method was called the "duplex," and the latter the "diplex." Duplexing was accomplished by varying the _strength_ of the current, and diplexing by _also_ varying the _direction_ of the current. In this invention there was the germ of the quadruplex, and now Edison redoubled his efforts toward completing the latter system, for, while duplexing doubled the capacity of a line, the quadruplex would increase it four times. He was working also on other inventions, but the quadruplex claimed most of his attention. He says: "This problem was of the most difficult and complicated kind, and I bent all my energies toward its solution. It required a peculiar effort of the mind, such as the imagining of eight different things moving simultaneously on a mental plane without anything to demonstrate their efficiency." It is, perhaps, hardly to be wondered at that, when notified he would have to pay twelve and one-half per cent, extra if his taxes in Newark were not at once paid, he actually forgot his own name when asked for it suddenly at the City Hall, and lost his place in the line! He succeeded, however, in inventing a successful quadruplex system by a skilful combination of the duplex and diplex with other ingenious devices. The immense value of this invention may be realized when it is stated that it has been estimated to have saved from fifteen million to twenty million dollars in the cost of line construction in America. But Mr. Edison received only a small amount for it. We will let him tell the story in his own words: