Magnetic field instruments are used to measure the magnetic field or flux around permanent magnets, coils, and electrical devices. They use several types of sensing technologies. Magnetoresistive instruments measure electrical resistance as a function of the applied or ambient magnetic field. Flux gate or coil instruments measure differences in the magnetic field at the ends of a vertical rod and plot this information on a grid. Hall Effect devices 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 devices 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 then subject the mixture to a radio frequency (RF) signal. Optically-pumped instruments polarize a gaseous alkali with a specific wavelength of light. Superconducting quantum interference devices (SQUID) are very sensitive to low magnetic fields.