Fleming, based upon observations of Edison that when a metal plate with an attached wire was placed inside a light bulb would produce a positive current flow revived the idea and discovered that the same effect would occur with alternating current, but only produce a positive flow. This was in effect a diode tube. When connected to a Marconi detector, the alternating signal could be turned into a perceptible signal. Unfortunately, Fleming lost interest in the valve and decided to try to develop a galena detector system instead. See: http://www.igs.net/~chrisharr/jaf/vacuum.html