<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (May 2016) (Learn how and when to remove this template message) </Td> </Tr> <P> Balisor is a system of illuminated beacons for high voltage power lines using a cold - cathode low - pressure neon lamp, used as an aircraft warning light . </P> <P> High voltage power cables, particularly those close to airports, need to be visible day and night . During the day, brightly coloured balls positioned along the length of the cables are sufficient, but during the night, lighting is necessary . These beacons provide this lighting by glowing red, the standard colour used in aviation for warning beacons . </P> <P> The system is extremely simple, efficient and reliable and is considered the most economically viable solution . Its working principle is attractive because it uses a phenomenon usually considered as a weakness . However a large disadvantage is that it does not work when the line is switched off . It cannot also be used on HVDC powerlines, but similar devices may be also used on mast radiators . </P>

What are the balls in power lines for
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