jh wrote in news:no-080071.01572204112005@localhost:

So my question is this: is it possible to build a device, or modify a
microphone, so that it picks up ONLY the electromagnetic interference,
but no actual sound?

if you remove the magnet from the microphone, it will do just that.

or go to radio shack and buy one of those telephone pick ups. you could
probably also use an audio transformer (high impedence side) with the other
side open.

--
stealthaxe