In a galvanic cell, a salt bridge is a tube filled with an electrolyte to allow ion flow and maintain neutral charge over the bridge without extensive mixing. In particular, anions flow into the anode compartment, while cations flow into the cathode compartment.
A common type of salt bridge is a U-shaped glass tube connecting both compartments over the top. Other methods may include a filter paper bridge (soaked in an appropriate electrolyte beforehand) or other porous barriers (e.g. porous disk).
With a U-shaped glass tube as the salt bridge:
With a filter paper as a salt bridge:
