The batteries

Example: The copper/zinc Daniell cell

A permanent electrolytic bridge (which could be replaced by a semi-permeable membrane) filled with a solution of sodium sulfate prevents the direct contact of $Cu^{2+}$ and $Zn$ If $Cu^{2+}$ and $Zn$ were in contact, they would react normally: $Cu^{2+}$$ +Zn \longrightarrow$ $Cu$$+Zn^{2+}$ The electrical conductor allows the indirect electron exchange

$Zn$ $-$ $2e^-$( pass through the conductor )$\longrightarrow$ $Zn^{2+}$ $Zn^{2+}$ ions pass into the solution, therefore this compartiment takes a positive charge. $Cu^{2+}$$+$ $2e^-$( come from the conductor )$\longrightarrow$$ Cu$ $Cu$ is deposited on the electrode, $Cu^{2+}$ ions disappear, therefore this compartiment takes a negative charge due to the excess of sulfate ions In the electrolytic bridge (or semi-permeable membrane) charges are compensated: - $Zn^{2+}$ ions migrate to the right - sulfate ions migrate to the left See the video: →   Cu/Zn battery

Generalisation

Oxydant 1 is strong enough to take electrons from reductant 2. But they cannot react directly because of the semi-permeable membrane or the electrolytic bridge separating them. So..

In compartment 2, the reductant 2 looses its electrons to the external circuit: It is oxidised. In compartment 1, the oxidant 1 takes electrons from the external circuit: It is reduced.

In this way an electrical current is produced with intensity I whose direction ( conventional direction of physics ) is the opposite of the direction taken by the electrons . In physics, I has always the direction from the positive pole of an external circuit to the negative pole.