  We study the ac electric potential induced by the electron sound wave (a
perturbation of the electron distribution function propagating with the Fermi
velocity) in single crystals of high purity gallium. The potential and the
elastic components of the electron sound demonstrate qualitatively different
dependencies on the electron relaxation rate: while the phase of the potential
increases with temperature, the phase of elastic displacement decreases. This
effect is explained within the multiband model, in which the potential is
attributed to the ballistic quasiwave, while the elastic component is
associated with the zero-sound wave. We observed a mysterious property of the
superconducting state: all manifestations of the potential accompanying the
lattice deformations, including usual sound wave, disappear below T_c in almost
jumplike manner.
