Question

Why we gently tap the cardboard when iron sprinkle over cardboard ?
This question from magnetic effect of electric current.

Answer 1

we tap to cardboard beacause when we tap on cardboard the iron filling will arrenger in form of magnetic field

Answer 2

we gently tap the cardboard so that the iron which has accumulated at one place only can slowly spread over the cardboard and this will ultimately help the iron fillings to arrange in it's magnetic field.