Salt bridge confusion :(

How exactly does a salt bridge close the circuit? I understand its role of neutralizing the solustions but I can't see how it forms a complete circuit. Do charges flow back to the anode through it? But why? Isn't the purpose of a battery to carry charge from high potential (anode) to low potential (cathode)? Isn't the external connecting wire enough? 




It is the same principle as why there is not going any current from a voltage source to a powered device, if there is currupted galvanic connection anywhere. The initial current **very quickly** balances potentials of power outlets with potentials on the wires and no current is flowing after charging the wire parasitic capacitance.


Let consider the classical Daniel cell $\ce{Zn|ZnSO4||CuSO4|Cu}$ with $||$ being a salt bridge ( or a diaphragm permeable for ions, but avoiding mixing of solutions).


$\ce{Zn}$ anode has more negative potential than the $\ce{Cu}$ one and provides excess of electrons by dissolving and hydrating $\ce{Zn^2+}$ ions.


The electrons flow via the wiring to the copper cathode with higher potential, where they reduce $\ce{Cu^2+}$ to the metallic copper.


In case of the absence of the bridge or the diaphragm, the accumulating negative charge at the cathode compartment **very quickly** creates solution potential difference that balances the electrode potential difference and the current vanishes.


In case of the presence of the salt bridge or the diaphragm, the potential difference between the solutions makes ions to migrate to the respective electrodes, according to their charge and direction of the potential gradient. 


This keeps the compartment potential difference lower than electrode potential difference and the current is free to flow via the wire.


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