The fluxgate compass

For many years years the basic instrument for measuring magnetic fields resembled Coulomb's--a magnetic needle suspended at its middle from a fine fiber, or balanced on a needle-point. It was a delicate instrument, of limited accuracy, not suitable for rough handling. Around the time of World War II electronic instruments came into use. One type, now very widely used, is the fluxgate magnetometer, based on the saturation of magnetic materials. A typical electromagnet, such as is used in a relay or machinery, has an iron core around which the current-carrying coil is wound. The coil's magnetic field is greatly strengthened by the iron, because the iron atoms are magnetic.
In ordinary iron, the magnetic axes of its atoms point in random directions, and the sum of their magnetic fields is close to zero. When current flows in the coil, however, its magnetic field lines up the magnetic axes of atoms in the core, and they add their magnetism to the one created by the electric current alone, making it much stronger. But there exists an obvious limit to the process: when all atoms are lined up, a condition known as the saturation magnetization of the iron, the iron core can provide no further help. If one further increases the current in the coil, the magnetic field only increases by the amount due to the electric current itself, with no contribution from the core.
Materials exist - certain ferrites - where saturation occurs abruptly and completely, at a stably defined level. Ferrites are composed of finely divided magnetisable particles suspended in an insulating material, so that no electrical eddy-currents can flow within them. If a large enough alternating current is driven through a coil wrapped around a core of such material, the core's magnetic polarity flip-flops back and forth, and saturation occurs in each half of the cycle, in symmetric fashion. If however such an electromagnet is located in an existing magnetic field, directed (entirely or in part) along the axis of the ferrite core, that symmetry is upset. In the half of the cycle in which the field of the coil is added to the existing magnetization, saturation arrives a bit earlier, because it depends on the total magnetic intensity, external plus that of the coil. In the other half of the cycle, where the magnetization due to the coil opposes that of the existing field, it happens a bit later, because the sum of the two is somewhat weaker than the field of the coil alone. That asymmetry can be sensed electronically, and this is the basis of the operation of the fluxgate magnetometer. Hand-held electronic compasses generally have two sensing coils at right-angles to each other. They are very useful in that they will memorise several bearings, thus avoiding having to try to write them down in the dark or remember them on the way down to the chart-table.