Electromagnetic pulses explained

The scale of the damage depends on the height of the explosion. A detonation at an altitude of 30 miles is calculated to cause damage for 480 miles around. At an altitude of 300 miles, the damage would radiate to 1470 miles - a field that would cover much of North America or Europe.

The strength of the pulse depends on the distance of the explosion from the earth's equator.

It is also possible to produce electromagnetic pulses without the use of nuclear weapons. Such devices have been fitted into bombs and missiles, but their range is far more limited because the explosion is far less powerful

Ships and military aeroplanes, particularly those carrying VIPs or nuclear weapons, can be hardened against EMP. In Britain equipment is tested on an EMP simulator at Farnborough airbase in Hampshire.

EMP pulses can also be naturally occuring. Solar flares see the release of large bursts of energy, including electrons and atoms, from the sun's corona.