Electroscope
How It Works
When a plastic comb is rubbed against a woolen material, static electricity is produced. When this charged material is kept near the aluminum ball, the positively charged comb attracts the negative charge of the ball. Hence only positive charge remains in the ball and it is passed down the wire and attracts the negative charge of the aluminum strip. As both the flaps of the strip are positively charged, they repel each other and the strip gradually becomes flat.