Electric Tester 

An  electric tester consists of an electric cell, a torch bulb fitted in a holder and some connecting copper wires. The material to be tested is connected to the gap, left between the ends A and B of the connecting wires. If the bulb lights up at once, the given material is a conductor. If the bulb does not light up at all, the given material is an insulator .

Checking An Electric Tester: 
We need to check the electric tester before we conduct any activity or experiment .Join the free ends of the tester together for a moment . This completes  the circuit of the tester and the bulb should glow. However, if the bulb does not glow, it means that the tester is not working the possible reasons are :
    Reason 1 : Connections are loose ,Check that all the connections are tight.
    Reason 2 : The bulb is fused , Replace the bulb with another new bulb.
    Reason 3 : The cell is used up , Replace the cell with a fresh one.

 Modifications In Electric Tester : 
The bulb glows when the electric current passes through an electric tester .When the current passes through the bulb due to the heating effect of current , the filament of the  bulb gets heated to  a high temperature and it starts glowing. However, if the current through a circuit is too weak, the filament does not get heated sufficiently and  it does not get heated sufficiently and it does not glow. So, we may use an LED in place of the electric bulb in the tester.

An LED means a light emitting diode. It is similar to a bulb but it runs on very low voltages. LED glows even when a weak electric current flows through it The coloured lights that you see in retail stores, stages, bridges and even in the keypads of  mobile phones are all LEDs.

There are two wires (called leads) attached to an LED. One lead is slightly longer than the other while connecting to a circuit the longer lead is always connected to the positive terminal of the battery and the shorter lead is connected to the negative terminal of the battery.
When a standard bulb in the electric tester is replaced by LED, it can easily detect even a weak electric current in  a circuit.

How Does an Electric Torch Work ?
An electric torch has one or more dry cells inside, which act as the source .These cells are connected through a switch to a small bulb. When the switch is pushed to the ‘on’ position, the circuit is complete and the bulb glows. When the switch is pushed to the ‘off’ position , the circuit is incomplete (broken). 

 Circuit diagram of torch :
There are two electric cells a switch and a lamp the torch bulb. The lines in the diagram represent the metal conductors which connect the system together.

A circuit is a closed conducting path. in the torch, closing the switch completes the circuit and allows current to flow.

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