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When the external magnetic field formula_3 is increased in an isolated system, the Landau levels expand, and eventually "fall off" the Fermi surface. This leads to oscillations in the observed energy of the highest occupied level, and hence in many physical properties (including Hall conductivity, resistivity, and susceptibility). The periodicity of these oscillations can be measured, and in turn can be used to determine the cross-sectional area of the Fermi surface. If the axis of the magnetic field is varied at constant magnitude, similar oscillations are observed. The oscillations occur whenever the Landau orbits touch the Fermi surface. In this way, the complete geometry of the Fermi sphere can be mapped.