  We formulate the statistical mechanical description of liquid systems for
both polarizable and polar systems in an electric field in the
$\mathbf{E}$-ensemble, which is the pendant to the thermodynamic description in
terms of the free energy at constant potential. The contribution of the
electric field to the configurational integral $\tilde{Q}_{N}(\mathbf{E})$ in
the $\mathbf{E}$-ensemble is given in an exact form as a factor in the
integrand of $\tilde{Q}_{N}(\mathbf{E})$. We calculate the contribution of the
electric field to the Ornstein-Zernike formula for the scattering function in
the $\mathbf{E}$-ensemble. As an application we determine the field induced
shift of the critical temperature for polarizable and polar liquids, and show
that the shift is upward for polarizable liquids and downward for polar
liquids.
