We now know that the filament was emitting electrons, which were attracted to a positively charged foil, but not a negatively charged one. This one-way current was called the "Edison effect" (although the term is occasionally used to refer to thermionic emission itself). He found that the current emitted by the hot filament increased rapidly with increasing voltage, and filed a patent application for a voltage-regulating device using the effect on November 15, 1883 (U.S. patent 307,031, the first US patent for an electronic device). He found that sufficient current would pass through the device to operate a telegraph sounder. This was exhibited at the International Electrical Exposition in Philadelphia in September 1884. William Preece, a British scientist, took back with him several of the Edison effect bulbs. He presented a paper on them in 1885, where he referred to thermionic emission as the "Edison effect." The British physicist John Ambrose Fleming, working for the British "Wireless Telegraphy" Company, discovered that the Edison effect could be used to detect radio waves. Fleming went on to develop the two-element vacuum tube known as the diode, which he patented on November 16, 1904.