A piston includes a piston body which is surrounded by a media side, by an oppositely disposed drive side and, at the peripheral side, by a piston jacket, wherein the piston jacket forms a connection between the media side and the drive side. This disc can have a strap which connects to a user’s hand to allow the user to rotate andor hold the computer at a desired position with one hand, thereby allowing the other hand to do other things, such as type on the keypad. The exchange bias field is between 30 to 3600 Oe and the anisotropy direction of the ferromagnetic elements is between 15 to 75\xb0 to an anisotropy direction of the antiferromagnetic elements. The amplifier is configured to compare output voltages of the filter for generating an enable signal, and the control circuit is configured to generate a selection signal according to the enable signal.