A surface of constant energy in the reciprocal lattice (see reciprocal space) of a crystal, with this energy being the energy of the Fermi level. In the case of free electrons, the Fermi surface is the surface of a sphere. In general, however, it is much more complicated and depends on the crystal lattice. The Fermi surface dictates the electrical, thermal, and magnetic properties of metals. There are several ways to investigate Fermi surfaces experimentally.