Hi, Lately I've been concerned about the Ohm's law [tex]J=\sigma E[/tex] and the physical interpretation of this law depending on what is considered the cause and what the effect. More concretely, it is quite natural and intuitive for me the interpretation of this law in one direction: a electric field [tex]E[/tex] over a ohmic conductor will cause a current [tex]J[/tex]. No one would have problems working out that electric force acting upon electrons will cause them to move, i.e. setting up a current. We can certainly say that the electrical field is the cause in this case and the current is the effect, the result. However, in the reverse direction, namely that a current [tex]J[/tex] injected somehow in a ohmic conductor will cause a electric field [tex]E[/tex], is by far more counterintuitive. Anyhow the current is generated previously by other means (for instance photoelectric or thermoionic effects) and injected into the conductor an electric field will arise. What is the explanation that acounts for that electric field caused just by movement of electrons in a conductor (which is nothing but a cristaline structure "covered" by a cloud of electrons)? I would gladly read your explanations. Thanks.