  In mean-field magnetohydrodynamics the mean electromotive force due to
velocity and magnetic field fluctuations plays a crucial role. In general it
consists of two parts, one independent of and another one proportional to the
mean magnetic field. The first part may be nonzero only in the presence of mhd
turbulence, maintained, e.g., by small-scale dynamo action. It corresponds to a
battery, which lets a mean magnetic field grow from zero to a finite value. The
second part, which covers, e.g., the alpha effect, is important for large-scale
dynamos. Only a few examples of the aforementioned first part of mean
electromotive force have been discussed so far. It is shown that a mean
electromotive force proportional to the mean fluid velocity, but independent of
the mean magnetic field, may occur in an originally homogeneous isotropic mhd
turbulence if there are nonzero correlations of velocity and electric current
fluctuations or, what is equivalent, of vorticity and magnetic field
fluctuations. This goes beyond the Yoshizawa effect, which consists in the
occurrence of mean electromotive forces proportional to the mean vorticity or
to the angular velocity defining the Coriolis force in a rotating frame and
depends on the cross-helicity defined by the velocity and magnetic field
fluctuations. Contributions to the mean electromotive force due to
inhomogeneity of the turbulence are also considered. Possible consequences of
the above and related findings for the generation of magnetic fields in cosmic
bodies are discussed.
