Logging your search for magnetic fields that might reveal hidden artifacts

Magnetometers are instruments for measuring the direction and/or intensity of magnetic fields. Such fields are created by electrical current flow and also exist naturally in ferromagnetic substances, such as iron and nickel. It is the latter fields that this magnetometer has been designed to detect, particularly those associated with man’s activities, principally in relation to iron-based artifacts, although not solely so.

This is a great improvement over the usual situation where a MIDI keyboard is used to trigger sounds – normally it is not possible with a keyboard to mimic the quick “rolls” that drummers play. The Bongo Box makes this technique possible, without having to go to the expense of buying an electronic drum kit!

Anthony Clark in his book "Seeing Beneath the Soil" says that, “Iron constitutes about six per cent of the Earth’s crust, but little of it is readily apparent. Most of it is dispersed through the soils, clays and rocks as chemical compounds which are very weakly magnetic. “Man’s activities in the past have redistributed some of these compounds and changed others into more magnetic forms, creating tell-tale patterns of anomalies in the Earth’s field, invisible to a compass but detectable with sensitive magnetometers.”

Several sophisticated techniques exist for sensing magnetic fields. Perhaps the most well-known implementation, and probably the most sensitive, is known as the proton magnetometer. Hall Field Effect devices can also be used, although they are less sensitive and are prone to temperature drift. Fluxgate sensors are in widespread use, too, but they are notoriously difficult for the hobbyist to construct from scratch.

However, Speake & Co manufacture a range of fluxgate devices, the FGM-X series. Speake describe them as “very high sensitivity magnetic field sensors operating in the +/-50 microtesla range (+/-0•5 oersted).” This range covers the Earth’s magnetic field (they can also be used in electronic compasses).

Browsing the web, it is apparent that one of the series, the FGM-3, is the device “of choice” in many magnetometer designs, and two FGM-3 sensors are used in this project.