The present invention provides a system for providing an improved compact power transfer unit for use in motor vehicles having a front wheel drive transverse based engine and transaxle, and, more particularly, to a power transfer unit for use with a transaxle arrangement capable of providing all-wheel drive or four-wheel drive. Each power supply circuit includes a direct current power source and a DC to alternating current converter. To read magnetization information from the hard disk, a change of an optically induced tunnel current is used. The volume of the second measuring chamber can vary according to the displacement of a piston.