If you were to detonate an atomic bomb, firstly you would be killed in the blast, and secondly you would notice a great deal of gamma rays
, which would kill you again. Then the gamma rays would mess with the upper atmosphere and produce really bad electrons
which would react with the Earth's magnetic field, creating masses
of electrical noise
, but on a dangerously grand scale.
An EMP degrades all systems within potential eyesight of the detonation (including those for which a line-of-sight is blocked by, say, mountains). The worst of the pulse lasts for only a second, but any unprotected electrical equipment - and anything connected to electrical cables, which act as giant lightning rods - is blasted by the nastiest case of static any observer will be likely to see in his or her lifetime. Imagine rubbing your feet on a synthetic carpet and then handling DRAM, and then imagine this affecting all electrical and microelectrical equipment. Military gear combats EMP with the use of a Faraday cage, which disperses the charge harmlessly.
Older, valve-based equipment is much less vulnerable to EMP, which is one of the reasons why a lot of Soviet cold war military hardware - such as the radar on the MIG 25, said to be able to kill a man at a range of ten feet - used valves (the Soviet invasion plan of Europe envisaged Russian forces advancing their troops, with the aid of armoured personnel carriers, through tactically-atomised areas).
Although an EMP is usually associated with nuclear armageddon, there has been much hype in recent years about the flux compression generator, a simple device consisting of copper wire, a battery, and conventional explosives which can, supposedly, produce smaller-scale EMP effects.