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.
This is a diagram of an electrochemical cell with a salt bridge made using potassium nitrate in a glass tube.
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The most common type of salt bridge is a U-shaped glass tube, which is filled with an electrolyte solution. The electrolyte may be contained by agar or gelatin 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.