Dielectric and conduction effects in electrorheological (ER) fluids with ohmic conductivity of the host liquids and with an applied AC electric field are considered. it is found that the conductivity ratio Gamma(sigma), the dielectric constant ratio Gamma(epsilon) and the applied field frequency f are three important parameters which determine the ER effect. If Gamma(sigma) > Gamma(epsilon), a stronger ER response is obtained with a low applied field frequency than with a high frequency. If Gamma(sigma) < Gamma(epsilon), the opposite occurs. Empirical equations are given for the attractive force between the particles in an ER fluid and for the shear yield stress. The calculated current density is independent of the applied field frequency when the frequency is smaller than a critical value, but becomes a linear function of the frequency beyond the critical value. The predicted attractive force of two nearly touching polymer spheres in mineral oil is in good agreement with that measured; also the predicted shear yield stress of some common ER suspensions is in accord with measurements.
