Make four electromagnets with 20, 40, 60 and 80 turns. Connect them one by one to a battery of 2 cells. Bring the electromagnet near a box of pins. Count the number of pins attracted by it. Compare the strengths of the electromagnets.
On increasing the number of turns in the coil, the strength of electromagnets will be increased. The electromagnet having maximum turns i.e. 80 will have highest strength and electromagnet with least no. of turns will have least strength.