In previous class you have used a tester as shown is figure given below to test whether electric current passed through a particular material or not? When this tester is used for liquids sometimes even though the liquid allows the electric current to pass and the circuit is complete, bulb does not glow. What can be the reason?

Due to the heating effect of current, the filament of the bulb gets heated to such a high temperature that it starts glowing. However, if the current through a circuit is too small, the filament of the bulb does not get heated to a high temperature and it does not glow. The reason why is the current in the circuit small is that though a material may conduct electricity, it may not conduct it as easily as a metal. As a result, though the circuit of the tester becomes complete and the current flows through it, the current is too small.