Q Why don't birds get electrocuted when they settle on overhead power lines. Also why do tennis court nets sag in the middle? Is this not breaking the rules? - G Gregg, Ferryhill.

A If a bird lands on the live wire of an electric overhead cable it will be charged to the same voltage as the cable itself. However it requires more than just a charge to electrocute the birds.

An electric current needs to flow through the bird's body and a body is not as good a conductor as metal.

Electrons flow from an area of high voltage to areas of low voltage and since the bird landing on the cable is instantly charged to the same voltage as the cable there is no need for the current to flow through the bird.

However, the ground has an electrical charge of virtually zero voltage. So anything touching the ground that comes into contact with the bird as it sits on the cable will create a flow of electric current through the bird towards the ground, resulting in death.

In the United States, large birds like eagles are often electrocuted on overhead cables. These birds can sometimes touch two cables at once, creating a circuit that kills them as they come into land.

Much research time has been invested into improving the design of the suspended cables to reduce the risks of electrocution to large birds.

In answering your second question we are once again into cables and posts.

The rules of tennis as specified by the International Tennis Federation do not make any reference to 'a sag' but it is clear from the specifications that the centre of the net is lower than at the ends.

The rules state that the net should be suspended from a cord or cable that has a maximum diameter of one third of an inch.

The cables should be attached to or pass over the tops of the two supporting posts no more than six inches square.

The supporting posts are erected three feet outside the court on each side and here the cord or metal cable that supports the net should be 3ft 6ins above the ground.

This means that if the cord or cable passes over the top of the posts then the posts will also be 3ft 6ins high.

However, if the cord or cable is attached to these posts in some other way the rules specify that the posts must be no more than one inch above the top of the net's cord or cable.

The height of the cord/cable at the centre of the net should be three feet - in other words six inches lower than it is at the posts.

The net should be held down taut at the centre by a white strap of no more than two inches in diameter.

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