When you cast
electromagnetic pulse, you send a powerful burst of energy that ruins electronic circuitry within the spell's
area. Any computer, telephone, television, or other device with a computer processor inside ceases to function immediately, and the contents of hard drives and other storage media are scrambled.
The
electromagnetic pulse affects only devices with extensive circuitry, not everything that uses
electricity. Ordinarily the lights stay on, although their electronic switches might not function. Automobiles continue to
run, although their engines will
run more roughly without electronic guidance. Most cars manufactured after the 1980s won't start after an
electromagnetic pulse because they have electronic ignition systems.
Some
military gear is immune to energy effects such as
electromagnetic pulse thanks to a process called tempest hardening. Tempest hardening adds +1 to the purchase DC of an item and makes it Restricted (+2).