What is a salt bridge and how does it work? Are there other devices used in galvanic cells instead of a salt bridge?

A salt bridge is an inverted U-tube that contains an electrolyte and connects the two half-cells in a galvanic cell.

The salt bridge maintains electrical neutrality in the internal circuit.

See

Consider the galvanic cell shown below.

If there were no salt bridge, an excess of positive charge (##"Zn"^(2+)## ions) would build up in the left hand cell. This would oppose more ##"Zn"^(2+)## ions going into solution.

The ##"Cu"^(2+)## ions plating out on the copper would lead to an excess of negative charge (##"SO"_4^(2-)## ions) in solution. This would oppose the loss of more ##"Cu"^(2+)## ions.

The flow of electrons through the wire would soon come to a halt

.

The salt bridge keeps the contents of each half-cell separate and allows the flow of ions to maintain a balance of charge between them.

As positive charge builds up in the left hand cell, ##"SO"_4^(2-)## ions can flow in from the bridge and ##"Zn"^(2+)## ions can flow into the bridge.

As negative charge builds up in the right hand cell, ##"K"^+## ions can flow in from the bridge and ##"SO"_4^(2-)## ions can flow into the bridge.

Cations are flowing in one direction, and anions are flowing in the opposite direction. This countercurrent flow of ions is the internal circuit.

The process continues until the system reaches equilibrium.

Instead of a salt bridge, you can use a porous barrier of clay or fritted glass.

Some labs use beakers joined by a fritted glass disk.