A novel vertical seismometer is pictured below, in which the zero-length spring is itself oriented vertically. The greatly-reduced hinge reaction forces, as compared to the conventional LaCoste-style instrument- should allow significantly improved performance, due to the smaller internal friction of the hinges.

A picture of the prototype instrument that was built and tested is shown in the figure below. The instrument can be adjusted to have a natural period of at least 20 s-the prerequisite for high teleseismic sensitivity. To operate at that period for variable temperature/pressure requires force-feedback, components of which are not pictured. The feedback is not one of `hard' force-balance, but rather `soft' type, based on a long time constant response to mean position changes.