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Firstly, the video of the THERMATRON is at: http://www.youtube.com/watch?v=58lX3Uu9OOs

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).

It’s not as simple as lighting a fire and sticking it into the input of a synthesizer or a speaker, though. There are a number of things going on here and the impedance of the flame (how much electricity it conducts) depends upon air pressure, chemicals (gas and ionic environment), heat, applied voltage, surface area, etc. A propane flame alone is a poor conductor and does not react as well until more ions are introduced (you see that I add a chemical mixture which introduces ions to the flame like salt in water). The added ions allow the current to flow from point A to B much more easily.

Also, the hotter the flame and electrodes get, the better the conductor. This is because there is simply more energy in the system, more ions whizzing around faster, and more plasma. Within the flame there is some plasma (which is highly energized gas which gives off its electrons and conducts quite well), but only a small amount compared to the plasma generated in an argon gas tube, for example. The addition of voltage, or electrical potential through the electrodes also facilitates the creation of a more energetic flame-conductor and donates electrons to the mix.

So, as the flame heats everything up, including the added ions and metal electrodes, the whole space becomes more energetic and conductive. This space, like the space within a vacuum tube, can conduct electricity, so the 200+ volts applied to the top electrode are attracted toward the bottom electrode. Only a small number of electrons actually make it through this flame environment,though, because it has a high electrical impedance. The current which does conduct is on the order of micro-amperes, but the voltage potential is still strong…

Many vacuum tubes behave in a VERY similar way – low current, high impedance, high voltage potentials. Even the concept of “heating” a vacuum tube’s cathode to create free electrons is remarkably similar. Indeed, the flame, in optimal operation is acting like a diode of sorts. This is due to “The Edison Effect” which you’ll have to read about somewhere else.

So the flame is similar to a vacuum tube and shares predilections to both high impedance circuits and high voltage circuits. So, we logically connect the flame to a vacuum tube’s high impedance grid or plate to make it into an interesting control voltage source.

The Electrons from the flame develop as charge on the grid of a tube, changing the path of electrons within it — allowing more or less current to flow. Tubes don’t mind the small currents, high voltages and high temperatures, so we don’t need a lot of power from the flame to affect the tube circuitry.

Interestingly enough, the distance of the electrodes from one another is not as critical as the temperature. When I insert a chemical coated rod into the flame and touch an electrode, the change in pitch / modulation is not due to the electrode moving as much as it is to the deposit of new molten ions. As long as the electrodes are in the path of the ionized particles or plasma, conduction will occur.

Also notice that the flame does flicker to the pattern of the sound. This is due to the voltage differences making it shrink and grow as it feeds the circuit. It is also partially a response to the acoustic energy emitted by the speaker in the room.

Needless to say, this is not the easiest thing to set up with household materials. It is also dangerous. So, don’t play with gas fire combined with 200+ volts. I am experienced with this stuff and absolve myself of any burns or shocks others may suffer. Also, don’t ruin your solid state stuff by applying high voltage and heat.