Turn on the power supply to 4 volts.  Outside of the tank, touch the electrodes together to show that the bulb will glow brightly with no resistance in the circuit.

Start with a tank filled with about 12-15 cm of distilled water. Insert the electrodes about 10 cm apart.

Because the distilled water has no free ions, it will not conduct electricity, and the light bulb will not light.

While stirring, slowly add salt -- maybe up to a tablespoon.  The bulb will begin to glow.

Move the electrodes apart and closer together to show that the brightness of the bulb depends on the electrode separation (the resistance in the circuit).