Abstract
A Spring Drive is a highly accurate mainspring driven watch that uses the mainspring as a power source and an electronic regulator to control hand speed. The output torque of the mainspring is highest when the mainspring is fully wound and gradually decreases as the mainspring unwinds. A torque-return system that extends the power reserve by winding the mainspring using energy spent on an electromagnetic brake when torque is high has been developed and provided on the Cal. 7R08. This system extends the power reserve by seven hours, without changing the mainspring.
