Why use a salt bridge at all in an electrochemical cell?
The use of a salt bridge is to maintain the electrical neutrality throughout the cell.

A salt bridge will allow the movement of ions.
Electrons flow from anode to cathode.Due to this a negative charge will be developed in the cathode and a positive charge in the anode.This charge developed will oppose the electron flow (current from anode to cathode) and at one level the electron flow will be ceased.So the cell's action will be stopped.
So in order to prevent this, salt bridges are used.

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Why is it important to use a salt bridge in a voltaic cell? Can a wire be used?