Electronic Fermi surfaces have been measured through observation of the oscillation of transport properties in magnetic fields H , for example the de Haas–van Alphen effect (dHvA) and the Shubnikov–de Haas effect (SdH). The former is an oscillation in magnetic susceptibility and the latter in resistivity. The oscillations are periodic versus 1 / H and occur because of the quantization of energy levels in the plane perpendicular to a magnetic field, a phenomenon first predicted by Lev Landau. The new states are called Landau levels and are separated by an energy ℏ ℏ ω ω c where ω ω c = e H / m ∗ ∗ c is called the cyclotron frequency, e is the electronic charge, m ∗ ∗ is the electron effective mass and c is the speed of light. In a famous result, Lars Onsager proved that the period of oscillation Δ Δ H is related to the cross-section of the Fermi surface (typically given in Å) perpendicular to the magnetic field direction A ⊥ ⊥ by the equation