The first types of diodes were called Fleming valves. They were vacuum tubes. They were inside a glass tube (much like a lightbulb). Inside the glass bulb there was a small metal wire and a large metal plate. The small metal wire would heat up and emit electricity, which was captured by the plate. The large metal plate was not heated, so electricity could go in one direction through the tube but not in the other direction. Fleming valves are not used much anymore because they have been replaced by semiconductor diodes, which are smaller than Fleming valves. Thomas Edison also discovered this property when working on his light bulbs.