To make a glass tube bridge, a glass tube is heated and bent on a burner to shape like U. Agar and bridge material such as sodium sulphate or potassium chloride is filled into it. The length of the glass tube bridge is adjustable according to the apparatus but its width is generally 0.86mm.
The purpose of using salt bridges in electrochemical cells is to introduce spectator ions into the solutions. These spectator ions preserve the neutral nature of solutions enabling the electrochemical cell to keep running.
Maintenance of electrical neutrality within a cell is the main significance of a salt bridge.
Salt bridges are required in a galvanic cell to keep the electrode potential established and to keep the electrical conductance, plating, etc going on. If there is no salt bridge, the cell will halt and the electrical process will cease gradually.