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This is a conductivity appartus that indicates electricity is flowing through the material containing the electrodes if the bulb lights up when the switch is closed.
The bulb won't glow when the electrodes are emebedded in solid lead bromide. Lead bromide is an ionc substance, and in the solid form the attraction of positive and negative ions keeps the ions in fixed positions in a crystal lattice. The electrical charge will be unable to move from one electrode to the other so the circuit will be broken.
The bulb will glow when the material surrounding the electrodes is molten lead bromide. When an ionic compound is in the molten (liquid) form the positive and negative ions are free to move around. The ions will carry charge between the two electrodes and complete the circuit.
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