• A salt bridge is a means of connecting the oxidation and reduction half–cells of a galvanic cell (voltaic cell), which is a type of electrochemical cell. The most common type of salt bridge is a U–shaped glass tube, which is filled with an electrolyte solution. The electrolyte may be agar or gelatine to prevent intermixing of the solutions. Another way to make a salt bridge is to soak a piece of filter paper with an electrolyte and place ends of the filter paper in each side of the half–cell. Other sources of mobile ions work too, such as two fingers of a human hand with one finger in each half–cell solution.
• The salt bridge is filled with a solution of salt, usually potassium nitrate (KNO₃), or other salt bridges are made with filter paper that is saturated with potassium nitrate. The U–tube is plugged on both ends with glass wool or porous plugs, so that the salt solution does not interfere with redox reactions that take place in a cell (Voltaic or Galvanic).
• The salt bridge in general, prevents the cell from short–circuit by acting as a screen. As the current is drawn from the cell, metal from the left hand electrode (anode) loose electrons and go into the solution. The electrons travel through the external wire to right hand electrode (cathode), and get deposited on the metal as metal ions. The salt solution in the salt bridge uses its own anions (NO₃⁻) and its own cations (K⁺) to substitute for the change in charges at anode and cathode.