Correct Answer - Option 4 : A bright white glow appears around the conductor
Corona:
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 the corona.
The phenomenon of violet glow, hissing noise, and the production of ozone gas in an overhead transmission line is known as the corona.
Advantages:
(i) Due to corona formation, the air surrounding the conductor becomes conducting and hence the virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors.
(ii) Corona reduces the effects of transients produced by surges.
Disadvantages:
(i) Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
(ii) Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
(iii) The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.
