The influence of an electric field on the theological properties of su
spensions of small particles in an insulating liquid is first recalled
. Then general arguments are given which show that, under DC fields, t
he particle/particle interaction is controlled by the conduction prope
rties of both constituents. The electrorheological effect occurs only
for solid that present a higher conductivity than the liquid (in the r
everse case, a ''negative'' effect is obtained). Then a model is propo
sed which gives estimates of the attraction force between particles of
an ER fluid. This model is qualitatively confirmed by results on a la
rge scale experiment (spheres of radius R = 0.7 cm) as well as on a re
al fluid. The case of AC fields is also examined.