The Electrostatics of the Scale
To know the reason for tumbling of a plastic scale.
a plastic scale, dry comb
Place a \(12''\) scale at the edge of the table in such a way that the \(6''\) mark is in line with the edge of the table and the scale just balances.
Rub a dry comb with your dry hair. Bring the comb below the end of the scale which is outside the table. You will observe that the comb will attract the scale and it may tumble down.
When you rub your hair, the comb becomes charged. When the charged comb is brought near the scale, the opposite charge is induced in the scale and the scale is attracted by the comb. This attractive force disturbs the equilibrium of the scale and it tumbles down.