The Calorelectric Effect in Flames

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Abstract
When two electrodes at different temperatures are placed in a reacting flame gas at atmospheric pressure, a potential difference is established between them. Its magnitude increases with rising temperature difference but it is independent of the electrode material. The cold electrode, usually kept at room temperature, always assumes a positive polarity. With a temperature difference of about 500 degC a calorelectric force of several volts is obtained. The following mechanism is proposed electrons, produced by the hot gas, gain kinetic energy from molecules and radicals in electronically and vibrationally excited states, resulting from combustion reactions. The reacting flame gas represents a non-uniform plasma in a steady state whose electrons acquire a temperature which is considerably above that of the hot neutral gas. As positive ions and electrons diffuse to the electrodes, the electrons lose a small fraction of their energy by collisions in the thermal sheaths at the electrodes where the gas temperature is so low that combustion reactions occur at reduced rates. Since the sheath thickness is larger at the cold than at the hot electrode the electrons suffer a larger energy loss at the former. Hence the electron temperature at the boundary between the thermal and the `electric sheaths' of the cold electrode will be lower and the wall potential less negative than those at the hot electrode. The difference of wall potentials constitutes the calorelectric force. This mechanism has been tested by measuring the electron temperatures with the electrodes acting as a double probe, by calculating and measuring the voltage-current relation of the calorelectric generator and by a two-flame experiment. The results are consistent with the proposed ideas.

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