An instrument for measuring variations in gravltational attraction; a gravity meter. Most present gravimeters are of the unstable or astatic type. The gravitational force on a mass in the meter is balanced by a spring arrangement, and a third force is provided which acts when the system is not in equilibrium. This third force intensifies the effect of changes of [[gravity]] and increases the sensitivity of the system. In the LaCoste-Romberg gravimeter, the main spring that balances out the gravitational pull on the weight is a "zero-length spring" inclined at an angle. A zero length spring has a stress-strain curve that passes through zero length when projected back to zero strain. (For example, a spring that requires an initial stress before the coils begin to separate.) Zero-length springs have very long periods and high sensitivity.