Physicist: The weapon itself doesn’t cause the EMP (or not much of one). The pulse is actually generated by the weapon’s effect on the Earth’s ionosphere.
An EMP is just a sudden change in the electric and magnetic fields, which on its own isn’t too bad. It doesn’t hurt people at least. However, changing EM fields induce currents in anything capable of carrying a current. This is especially true of power lines, where the current can really “build up some steam”. The problem with current suddenly showing up where it’s not expected is that it can arc or overload circuits. The kind of components and wiring you find in today’s electronics (and the last 30 some odd years) can be destroyed by the sort of sudden surge you get from regular old static electricity.
Of course because of that, sensitive electronics are generally grounded (with ground lines!) and/or shielded with Faraday cages.
The way you generate an EMP (or any interesting electromagnetic effect) is you get a bunch of charge and suddenly move it. A nuclear weapon on its own doesn’t have a bunch of extra charges to move around, but luckily (unluckily?) the Earth abides! Between about 40 and 300 miles above your head (about 39 and 299 miles for our Denver readers) there’s a layer of charged particles called the ionosphere. It’s created by radiation from space (mostly the Sun) knocking electrons free of their host atoms. A nuke releases enough heat, suddenly enough, that the resulting upward and outward “puff” of air literally moves the ionosphere overhead. That moving charge is what causes the bulk of the EMP. To a lesser extent, a nuclear device also ionizes the surrounding air, and then moves that.
The Soviets and Americans, being good at this sort of thing, have done a number of tests that involved setting off nuclear devices in and just above the ionosphere. The best known are America’s “Starfish Prime” (you’d think the black ops people would hire better namers-of-things) and the USSR’s “test 184″ (classy name).
According to a review for the US Energy Research and Development Administration, EMP damage was recorded almost a thousand miles away:
“Starfish produced the largest fields of the high-altitude detonations; they caused outages of the series-connected street-lighting systems of Oahu (Hawaii), probable failure of a microwave repeating station on Kauai, failure of the input stages of ionospheric sounders and damage to rectifiers in communication receivers. Other than the failure of the microwave link, no problem was noted in the telephone system.”
The Soviet tests sound even more fun, but the relevant details are a little harder to track down:
“…it knocked out a major 1000-kilometer (600-mile) underground power line running from Astana to the city of Almaty. Several fires were reported. In the city of Karagandy, the EMP started a fire in the city’s electrical power plant, which was connected to the long underground power line.” (ref.)
It’s worth mentioning that nuclear bombs aren’t the only thing that cause these sorts of large-scale electronic nastiness. Any bomb big enough will have a similar effect (if there were other bombs big enough). Also, every now and again, the Sun belches out a cloud of ionized gas that pushes the Earth’s magnetic field around. The results are the similar to a nuclear EMP, but global and toned way down. These “geomagnetic storms” mostly just mess with communication channels, which are often already bumping up against their signal-to-noise limit.