The Mark I Magnaprobe's is a simple analog detector of weak magnetic fields. It responds to fields as weak as 0.3 gauss which can help confirm the integrity of magnetically shielded rooms used in Magnetoencephalography.
I have bought several of these over the years, originally from the British company that makes them, and more recently from this company. I have trouble keeping them because everyone seems to want one. They are indispensable for showing the angle of the magnetic field in three dimensions. It is like a 3 dimensional compass, and very sensitive too. It will even show the vertical angle of the Earth's magnetic field! If your business is magnets, you need one of these.
Thanks for the feedback. This version, the Mark I is incredibly sensitive as you noted. We had them tested once & the will respond to a field as weak as 0.3 gauss. Besides declination they are great for detecting magnetic leakage in NMR/MRI labs. Overkill for Q/A though, use the Mark II instead.