(gr∂ vim’ i t∂r) An instrument for measuring variations in gravitational attraction; a gravity meter. Most gravimeters are of the unstable or astatic type. The gravitational force on a mass in the meter is balanced by a spring arrangement. A third force is provided that acts when the system is not in equilibrium; it intensifies the effect of changes of gravity and increases the sensitivity of the system. Usually a zero-length spring is used; it has a stress-strain curve that passes through zero length when projected back to zero strain. See Figure G-6.