EMF / K2 Meter
The EMF meter- This is a tool for measuring electo-magnetic fields. Certainly any metal object that has electricity running through it would create an EMF, also Earth itself has a very weak one and this is how compasses work.
There is an argument that spirits, when present, give off a weak magnetic field that can be picked up by these devices.
Certainly, if you’re investigating an area with no electricity and you’re getting EMF spikes, this could be good collaborative evidence.