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
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.