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Ionizing the Air
Voltages of hundreds of thousands of volts can be generated with a demonstration model Van de Graaff generator. This is sufficient to ionize the air, which has primary constituents nitrogen and oxygen. An applied electric field will polarize air molecules, and if sufficient to force electrons off the molecules, those electrons can collide with other molecules and can start a discharge, or electric arc. The voltage necessary to start an arc depends upon the pressure and gap length, and is modeled in Paschen's law(Wikipedia). For long gaps in air at STP, the threshold for arc discharges is often quoted as 3.4 million volts per meter.
When an atom is ionized, it will tend to collect a replacement electron which may cascade down though the available atomic energy levels, emitting light quanta characteristic of those level separations. The characteristic spectra of Nitrogen and Oxygen would be observed in a visible arc in air.
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