Use an Arduino microcontroller to sense invisible electromagnetic fields using wire, a resistor, and an LED.

Aaron ALAI’s EMF detector project is awesomely simple to make and fun to use. An upgraded version can be built using an LED bargraph for more ‘meter-like’ functionality.

original project can be found here –
http://www.aaronalai.com/emf-detector

additional info, and Arduino sketch –
http://blog.makezine.com/archive/2009/05/making_the_arduino_emf_detector.html

audio/video by Collin Cunningham