A seismometer, in which the moving parts are easily visible, is a more appropriate teaching tool. A vertical component seismometer is difficult for an amateur to build because the long-period response requires a zero length spring, which is difficult to find. To a first order approximation, the dual-spring seismometer has a restoring force that can be set to zero at its equilibrium position, theoretically giving it an infinite period. The torque of the first spring counters the torque applied by the mass. The second spring is positioned to exert no torque in the equilibrium position, but negative torque out of equilibrium.
Tension is adjusted on the second spring to adjust the period of system.
We easily achieve stable free periods of 3 to 6 seconds for small motions. The seismometer can be constructed entirely from materials found in a hardware store. Also, the design is appropriate for high-school teachers and students to assemble, and to directly observe its operation. Electronic feedback and digitizing can be added for long-term recording. Electronic feedback can then be use to generate a response that is flat to velocity for periods of 20 to 0.1 second.