When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low. However, when the applied voltage exceeds a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called corona. EasyEDA: Ideas for Circuit Design, Innovation for Electronics Access
The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss
and radio interference. The higher the voltage is raised, the larger and higher the luminous envelope becomes, and greater are the sound, the power loss and the radio noise. If the applied voltage is increased to breakdown value, a flash-over will occur between the conductors due to the breakdown of air insulation.
The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona.
If the conductors are polished and smooth, the corona glow will be uniform throughout the
length of the conductors, otherwise the rough points will appear brighter. With d.c. voltage, there is
difference in the appearance of the two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow.
Reference and bibliography: Power System Analysis, By VK Mehata, AK Mehta.
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