When two conductors that are having a large spacing as compared to their diameters are subjected to an alternating potential difference, the atmospheric air surrounding the wires remains unchanged till a certain specific value of voltage.
However, if the voltage surpasses this specific value a faint violet glow called corona appears around the conductors. This specific voltage is termed as critical disruptive voltage,
The phenomenon of the corona is usually accompanied by a hissing sound, the production of ozone, power loss, and radio interference.
As the voltage is increased, the luminous envelope surrounding the conductors becomes larger and higher, and the sound, power loss, and radio noise increase accordingly. If the applied voltage is raised to the breakdown value, the air insulation breaks down, and a flash-over occurs between the conductors.
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