One answer is the simple electrochemical cell. On the left side, a piece of metallic zinc is immersed in zinc sulfate solution, and on the right, copper is immersed in copper sulfate. The two pieces of metal are connected by a wire, and the two solutions are connected by a porous barrier that allows the migration of ions but prevents the bulk mixing of the two solutions.

At the left metal rod, called an electrode, zinc atoms give up electrons and enter the solution as zinc ions. This electrode is slowly eaten away. At the copper electrode on the right, copper ions from solution combine with electrons and plate out on the electrode as metallic copper.

This electrode slowly increases in bulk as the reaction progresses. The electrons needed to reduce the copper ions at the right come from oxidation of zinc atoms at the left, but to do so they must travel through the external wire circuit.

In the two solutions, as zinc ions enter the left compartment and copper ions are removed from the right, negative sulfate ions must migrate slowly through the porous barrier from right to left, and positive zinc ions from left to right, to preserve electrical neutrality in the two solutions.

The two electrodes, connecting wire, and porous barrier form a closed circuit, with negative electrons moving from left to right through the wire, and positive and negative ions moving through the porous barrier.