I've noticed that power-lines (on the electrical grid)
often have several red balls on them.
I would like to find out what function these balls serve.

Some characteristics :
- I've observed the balls only on high-voltage power-lines, 
  not on inner-city lower-voltage lines.

- Seen in several locations throughout the US.
  e.g. power-lines visible from highway
  680 driving from San Jose to Livermore, CA

- Balls are probably about a foot in diameter, red in color

- A power-line (the wire strecthing between two towers)
  will usually thread through two-or-three balls equally spaced.

- In a cluster of wires stretching between two electrical  towers,
  usually only a single wire will have the balls on it, 
  the others wont.


I've considered the following options :
1. Insulator : Why would only one wire in a cluster of wires 
     need insulation  ? Does one wire carry special voltage/current ?
2. Connector connecting one length of wire to another  :
     Why would you need 2-3 connectors on a single strecth between towers ?
3. Warning indicator for low-flying aircraft:
     Maybe.

I am looking for information
- what these balls are called
- why they are there

Hi yesdi,

It is common to see these red marker balls on powerlines that span a
large distance. The purpose of these marker balls is to indicate to
low-flying aircraft where the powerline is so that they do not fly
into it (called a wire strike). The FAA mandates their usage.

While from the ground it is easy to identify where the powerlines are
against the sky as a backdrop, the thin lines are often difficult to
see from the air with a varied terrain backdrop.

According to this article from the firm of Magaņa, Cathcart & McCarthy
,wirestrikes are the number one cause of helicopter accidents.
[ http://www.mcmc-law.com/wirestrikes.html ]

Small airports, such as McCreary County Airport in KY use markers as
visual indicators:
Additional obstruction remarks: RY 04 MARKER BALLS ON    
                                POWER LINE CROSSING RY   
                                APCH.                    
[ http://www.airnav.com/airport/18I ]


Raychem is a manufacturer of marker balls.  Here is a link to their
catalog page [PDF]:
[ http://www.raychem.com/products/dulm_rrg/11-1.pdf ]
[ http://www.raychem.com/products/dulm_rrg/11-3.pdf ]

Tana is also a manufacturer (includes the FAA guidelines):
[ http://www.tanawiremarker.com/faa.htm ]

There is even software to calculate how much a powerline will sag
under its own weight and the weight of the marker balls:
[ http://www.powline.com/products/sagsec.html ]

Google Search:
marker balls power line
[ ://www.google.com/search?num=20&hl=en&safe=off&q=marker+balls+power+line
]

mit

Thanks for the detailed answer.

They have also been found to reduce "bird strikes," or birds
(specifically migrating whooping cranes) being injured or killed by
collisions with power lines.  Quoting from an article by Tom Stehn of
Aransas National Wildlife Refuge found in 'Journey North', a Global
Study of Wildlife Migration:

"The number one problem of migrating cranes is collision with power
lines. Danger can be a huge string of transmission lines high in the
air or simply a single wire running into a farm house or irrigation
system in an isolated area where practically no one lives. The cranes
simply do not see the lines. Why should they even be looking since the
only natural danger they have are attacks from eagles? Transmission
lines are hard to see when you are looking into the sun, late in the
day when the light is dim, or in bad weather including blizzards or
foggy days. When we radioed 6 whooping cranes back in the early 1980's
and tracked them all the way to and from Canada, two of the six died
hitting power lines. To make power lines more visible, red plastic
balls or similar devices are placed on the lines near airports so
pilots can see any lines as they come in to land. When power lines are
built across wetlands, the U S Fish and Wildlife Service asks the
companies to mark the lines. This reduces bird strike mortality by 50
%. "

http://www.learner.org/jnorth/tm/crane/MigrationDangers.html

Google search terms:  red balls power lines
://www.google.com/search?sourceid=navclient&querytime=mGE93B&q=red+balls+power+lines

The UK Air Accidents Investigation Branch reported on a fatal
helicopter crash in 1998 where an air ambulance had collided with
power lines crossing a valley in Kent, England. The report contains
pictures taken by the investigators which show the valley and the
extreme difficulty in seeing power lines from the air.

Marker balls are generally not used in the UK due to environmental
considerations (they're ugly and spoil the landscape).

Report:
[ http://www.aaib.dtlr.gov.uk/bulletin/feb00/gmask.htm ]
Images:
[ http://www.aaib.dtlr.gov.uk/bulletin/feb00/images/gmaskf3.jpg ]

I consider these the Bill Graham memorials, because they really
started to appear on California power lines just after the helicopter
carrying Bill Graham and others crashed into a power tower in very
adverse weather conditions.

The balls are there to alter the amount the lines swing. They are
matched against the harmonic motion of the wind, and prevent excessive
swinging over long spans. That's why you only see the balls when the
lines cross large spans - like over a river or a highway. This is the
primary reason for the balls -- not to prevent whooping cranes from
hitting them.

You will find that these balls only occur over roads, specifically
main routes / national routes. The reason for this is helicopters
actually follow these routes when flying low especially if following
vehicles for whatever reason, and when in urban areas, where the only
visual landmark is these routes. Medivac helicopters also follow main
routes to hospitals they may not be familiar with. The balls offer
better visual prescence than the cables alone and prevent collisions.
Hope this offers some more information to your answer

At one point in time, there were a combination of red, orange, and
white balls on the high-voltage lines spanning the 15 freeway between
Temecula and San Diego, California.  Perhaps the colored pattern is
even easier to spot than a single color of balls.