Apparatus for measuring the magnitude of a magnetic field

摘要

An apparatus for measuring the magnitude of a magnetic field in one embodiment includes a 3He cell having an oscillator connected thereto for producing an electrical discharge for metastable 3He atoms, a 4He lamp having an oscillator connected thereto for producing 4He radiation and an optical path to the 3He cell including a lens for collimating the 4He radiation and a circular polarizer. The imaged 3He atoms and 4He radiation interact to magnetize a portion of the 3He atoms in a direction parallel to the direction of the ambient magnetic field. After the cell magnetization, an oscillatory magnetic field normal to the ambient field is established by coils connected to a frequency synthesizer which is scanned over a frequency interval including the free precession (Larmor) frequency. This action rotates the direction of magnetization of the 3He cell atoms from parallel to the ambient magnetic field to approximately perpendicular to the ambient magnetic field. The cell then is allowed to precess freely and an a.c. is induced in a coil during precession. The a.c. is connected to a frequency discriminator which may be a digital type to digitize the voltage representative of the frequency of the a.c. A very narrow band digital filter filters the digitized output of the discriminator and a digital to analog converter produces a voltage from the digital filter output for a charge recorder for charting.