Perhaps one of the first sensors used by man was a compass, which is essentially a magnetic sensor. The compass needle, which is steel, aligns itself with the earth's magnetic field lines and points toward the north. Magnetic sensors use both nonmagnetic and magnetic materials. The magnetic materials used most commonly are iron based and are commonly called ferromagnetic materials. The ferromagnetic materials respond most sensitively to a magnetic field, and the individual dipoles tend to align themselves with the applied magnetic field. This does not happen instantaneously. In particular, sections of the material called domains line up and finally the entire material is aligned. Several magnetic sensors can be realized as a result of the galvanomagnetic effect, which relates to interrelationships that occur between flowing electric current subjected to a magnetic field. Due to the interrelationships between magnetic fields, electric fields (voltage), and current in semiconducting materials, a number of so-called solid-state magnetic sensors can be realized.