system
Given the following post, respond as if you're commenting on Reddit:
human
What is the biggest EMP we have? And if someone set it off, how big of an area would it affect?
gpt
If we were strictly going for largest EMP, it would be to detonate one of our largest thermonuclear warheads approximately 300 miles above the surface of the earth.  If detonated above southern Canada, for example, the EMP would affect most of the continental united states.

Its a common misconception that a nuclear detonation creates an EMP.   While it does to some small extent, the real EMP is created by the effects of the detonation on the earth's natural magnetic field.  A huge release of energy inside the earths magnetic field creates a brief local distortion in the field itself, effectively reshaping it for a small amount of time.  Its this sudden reshaping of an enormous magnetic field that completely F's up any electronics that aren't shielded.  Moving a magnetic field around a wire forces current to run through it.  Heaving the Earth's magnetic field around willy nilly induces a significant current in any length of metal that isn't shielded appropriately.

By comparison, a powerful nuclear weapon detonating at lower altitude has an EMP that is relatively inconsequential.  Anything close enough to the blast to be effected by EMP has bigger problems.