If a cathode is chemically treated with a common household chemical and put into an alcohol flame, electron current can flow to a positive anode that is placed inside of the flame. This creates a diode rectifier.
What is very intriguing though, is that electron current can still flow to the anode even when it appears to be well outside of the flame.
A grid placed within this space between the flame and anode can control the amount of current flow. This creates a triode with gain, that has similarities to a vacuum tube triode.