Magnetic field sensors are used to measure magnetic flux and/or the strength and direction of a magnetic field. They are used mainly in scientific measurement, navigation, and industrial applications. Magnetic field sensors use several types of sensing technologies. Magnetoresistive devices measure electrical resistance as a function of the applied or ambient magnetic field. Flux gate or coil sensors measure differences in the magnetic field at the ends of a vertical rod and then plot this information on a grid. Hall Effect sensors convert the energy stored in a magnetic field to an electrical signal by developing a voltage between the two edges of a current-carrying conductor whose faces are perpendicular to a magnetic field. Magnetoinductive sensors consist of a coil that surrounds a ferromagnetic core whose permeability changes within the earth's magnetic field.  Proton precession devices use liquids such as kerosene and methanol that have high densities of hydrogen atoms. Overhauser or nuclear precession devices combine an electron-rich liquid with hydrogen and subject the mixture to a radio frequency (RF) signal. Optically-pumped sensors polarize a gaseous alkali with a specific wavelength of light. Superconducting quantum interference devices (SQUID) are very sensitive to low magnetic fields.