How to Make Series Testing Lamp

Series Test Lamp ( Testing Board ) 

How this works is an incandescent light bulb filament has a low resistance when cold, typically under 20 ohms.
 When current begins to flow and filament heats up and resistance rises rapidly to act as a sort of current safety valve.

If the item being tested, in this illustration a transformer, has any shorts or draws excessive current the energy is absorbed by the lamp and doesn't cause further damage to the device or blow expensive fuses.

The transformer could even be connected backwards and would still be protected while the lamp glows nice and bright.

Circuit

How to make a series test lamp
1. Cut the wire into two pieces making each one a meter long.

2. Now you have two pieces of wire. Fix the bulb holder using one end of the two pieces of wires and install a light bulb on to the holder.

3. You have other two ends of the wire free. Fix a two-pin plug on that free pairs of wires. It means you can light up the bulb if you put two-pin plug in a live two-pin socket.

4. Check the continuity of the test lamp by an Avometer; be sure that bulb turns on when the plug is inserted in a live two-pin socket.

5. Now, pull out the plug from the socket.

6. Finally, you need to slice one of the wires in the middle and remove insulation from each of the cut-ends for half an inch so that the bare copper is clearly visible.

7. Your test lamp is ready for experimental test. Always use a cap to cover the bare copper wire to avoid any accidental electrification.