 In the first tests, Woodward simply used a capacitor between two stacks of PZT disks.The capacitor, while being electrically charged to change its internal energy density, is shuttled back and forth between the PZT actuators.Piezoelectric materials can also generate a measurable voltage potential across their two faces when pressed, so Woodward first used some small portions of PZT material as little accelerometers put on the surface of the stack, to precisely tune the device with the power supply.