In 1873, while doing work on charged objects, Professor Guthrie discovered that a red-hot iron sphere with a negative charge in a vacuum would discharge. He also found that this did not happen if the sphere had a positive charge.
In 1880, while trying to discover the reason for breakage of lamp filaments and uneven blackening (darkest near one terminal of the filament) of the bulbs, Edison built a bulb with the inside surface covered with tinfoil. When he connected the foil to the negative terminal of the lamp filament, inserting a galvanometer between the filament and the foil connection, nothing happened. But, when he connected the foil to the positive terminal of the filament, a small current flow was indicated on the galvanometer.
Edison saw no use for this effect, and although he patented it, did not study it any further.
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.