Magnetometers
Magnetometers are miniature electro-mechanical devices that measure the magnetic field they are placed in.
Interference
Soft-Iron
Soft-iron materials are ones which are not magnetic themselves, but generate a magnetic field.
This includes:
- Ferrous EMI shielding
 - Screws
 - Battery contacts
 
Hard-Iron
Hard-iron materials are ones which are naturally magnetic.
This includes:
- Speakers
 - Proximity switches
 - Hall-effect sensors
 
Electromagnetic
Magnetometers are not directly affected by electromagnetic waves. However, if these waves pass through conductive substances (including those inside the magnetometer itself), these can induce a current which in turn creates a magnetic field.