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      Redox Reactions and Electrochemical Potential

The porous barrier between the two solutions can be eliminated in the particularly simple version of the Zn-Cu cell, the Daniell gravity cell.

In this cell the lighter zinc sulfate solution is carefully layered on top of a denser copper sulfate solution, and the electrodes are gently lowered into place, with the copper electrode insulated where it passes through the zinc sulfate layer.

The Daniell cell delivers a dependable 1.10 volts, and at one time was used widely as a stationary power source for telegraph lines and doorbells.

It obviously would be useless in a moving vehicle, for agitation would mix the two solutions. Metallic zinc then could transfer its electrons directly to copper ions in solution, and the electrons would not have to pass through the external wire.

The cell would be ruined by an internal short-circuit, and the free energy released would be wasted as heat. As was pointed out previously, the purpose of the salt bridge, or porous barrier, is to permit the migration of counter ions to maintain charge neutrality, while avoiding mixing the solutions and thereby short-circuiting the cell.

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