A dc discharge can behave as an ac current generator; under certain conditions, stable oscillations appear with the same period in the current and in the light emitted by the positive column even if it seems steady to the eye.The current oscillations have been analyzed for their harmonic content. The various wave forms of the light oscillations observed along the positive column can be attributed to the fact that the component oscillations of different frequencies travel with different speeds.
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A. M. Pilon (1957) studied this question.
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