Redox reactions eventually reach equilibrium and E goes to zero. We can capture this energy by setting up an electrochemical cell to separate the two half reactions. The Zn electrode appears corroded due to loss of Zn into solution, and the Cu electrode is being plated with Cu from solution. The salt bridge is a tube filled with a saturated KNO 3 solution. It has frits on the ends that prevent mixing of the solutions but allowing ions to pass through.
Ions move through the salt bridge to keep the individual cells electrically neutral.
The driving force is the same for the case of just mixing the reactants. By separating the two half reactions, the electrons must travel through the wire and we can use the electrical energy. In the sketch above the electrical energy powers a light bulb. When we first set up this electrochemical cell and complete the circuit, we could measure a voltage through the wire. As the reaction proceeds, the system approaches equilibrium and the voltage would eventually go to zero.