Power Systems
Corona is accompanied by

 violet visible light
 power loss and interference
All of these
 hissing sound

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Power Systems
Gross head of an hydroelectric power station is

 the height of water level in the river where the tail race is provided.
  the difference of water level between the level in storage and tail race.
 all of above.
 the height of water level in the river where the storage is provided.

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