The devlopment of so called mining compasses in the 17th century greatly helped the search for the useful ores needed to keep the industrial revolution alive.
Surprisingly the technology of the ore finding magnetometer hadn’t moved on a great deal by the time this New Scientist article from 1960 reveals.
Magnetic ore prospecting
Magnetic ore discovery, or prospecting, works because all ores and minerals are, to some extent at least, magnetic. By knowing and sensing the different magnetic susceptiblities of different materials we can determine where in the rock around us the useful ore can be found.
For example haematite is weekly magnetic, but is often found mixed in with the more strongly magnetic magnetite.
In addition to differences in susceptibility there is also further magnetisation induced by the earth’s magnetic field. When the rock cooled from the earth’s mantle, it was effectively field cooled through its Curie temperature in the earth’s magnetic field, giving the rock a permanent magnetization. This is known as paleomagnetism.
The permanent magnetisation can also be changed by weathering or changes in chemical composition of the rock. One spectacular source of changes is lighning strikes: the large, rapid electric current induces a magnetic field which can in turn cause local variation in magnetic field.
The first dedicated ore prospecting magnetometer or ‘mining compass’ was developed in 17th century Sweden, and was really just a compass needle which was also free to move in the vertical direction. You could walk over your magnetic ore and see deflection of your compass, indicating that there was some magnetically active material below you. This device was further developed in the 1800s with the Thalen-Tiberg magnetometer (also Swedish), which also sensed the magnitude as well as direction of the local magnetic field.
Despite the vintage of this technology it could still be found in use in the 1950s and 1960s, although in general it had been surpassed by equipment such as the Schmidt variometer.
The Schmidt variometer is simply a magnetic balanced on a knife-edge. Weights could be added to the magnet to compensate for local field variations and balance the magnet. In the field measurements could be taken of the deflection of the magnet, which would thus show any changes in field relative to the reference value.
The state of the art in the 1950s was the nuclear precession magnetometer which is based on the precession of protons about the local magnetic field.
Modern Magnetic Prospecting
Magnetic surverying still remains an important tool for both geological archaeological applications (from land, sea and air). The techniques used are essentially similar to those applied in 1950s and 60s, but tend to use more modern equipement.