In electricity line i found balls fixed after certain distance. what is it for?

Answer 1

They are fluorescent balls to indicate the position of lines at night, so that aircraft will not fly into them.

Answer 2

Usually to make the lines easier to see from aircraft, so that no one flys into them. Power lines are hard to see from a fast moving aircraft since they are thin, and gray (I.E.: blends into the background color too easily).

Answer 3

They apply weight to keep the lines from vibrating and moving around during high winds...