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CORONA

Corona:

When an Alternating Potential Difference is applied across two conductors,whose spacing is large in comparison with the diameter,then the atmospheric air surrounding the conductors is subjected to electrostatic stresses.At low voltage there is no change in the condition of atmospheric air around the conductor.However,when the potential difference is gradually increased,a stage arrives when a faint luminous glow is violet colour appears together with a hissing noise.This phenomenon is called VISUAL CORONA and is accompanied by the production of OZONE which is readily detected because of its characteristic odour.The glow is due to the fact that the atmospheric air around the conductor becomes conducting due to electrostatic stresses.
DISCHARGE OF CORONA
                     If the potential difference is raised still further,the glow and the noise will increase in intensity until eventually a spark over will take place.The whole phenomenon,the hissing noise,the Violet glow and production of ozone gas is known as CORONA.
                          If the conductor is uniform and smooth,the same conditions will hold throughout the length of the conductor,otherwise the rough point will appear brighter.If the spacing between the conductors is not very large as compared with their diameters,the spark-over may take place before any luminous glow is observed.It happens due to the fact that the distance between the conductors being smaller,there is no time for the glow to occur.In case of DC system the positive conductor will have uniform glow and will appear brighter than the negative one.In case of AC system the current due to corona is non-sinusoidal.Corona is accompanied by a loss of energy,which is dissipated in the form of light,heat,sound and chemical action.

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