The THERMATRON is essentially a voltage controlled oscillator and wave shaper controlled by the action of a flame. This is possible because electricity can be conducted through a flame. This is not a new discovery, in fact the electrical properties of flame have been known for hundreds of years and well studied. For example, many hot water heaters have a sensor that detects flame by sensing the current inserted through the gas flame (if the gas flame goes out, the current does not reach the sensor and the heater knows that the flame is out).
Check out the project’s post for a much deeper explanation of the Thermatron’s funciotnality and flame conduction in general.