Parasitic electromagnetic fields can cause troubles when working with sensitive measuring equipment. One of these are ECG – an electrocardiograph. It deals with very low voltage bio-signals, so any small interference leads to output fluctuations. Modern electrocardiographs have quite effective filters and methods that can deal with most common noises, but it is better to avoid than post process.
This Arduino Mega based EMF tester simply tries to capture 60Hz EMF noise by using simple antenna attached to analog input. When antenna captures some significant 60Hz electromagnetic noise LEDs indicate its intensity. So it may be handy tool to hunt for noisiest equipment around. As it is easy to build it can be interesting to play around. Probably it could be used to find hidden wires in the wall that are less than 1inch deep.