Question

why led bulb is used in electric tester? how it should be connected?

Answer 1

Led bulb is used in electric tester because it glows even if small amount of current passes through it. The shorter lead of the led bulb is connected to the negative terminal of the battery whereas the longer lead of the bulb is connected to positive terminal of the battery.

Answer 2

LED stands for light emitting diode. It works on a completely different principle compared to the neon lamp. The lamps used in the voltage tester have a break down voltage of about 90V. One terminal of the lamp is connected to the tip of the tester through a resistor to decrease the current.
The tip of the tester is touched to the conductor being tested (for instance, it can be used on a wire in a switch, or inserted into a hole of an electric socket).