sensitive device for measuring variations in the Earth's gravitational field, useful in prospecting for oil and minerals. In one form, it consists of a weight suspended from a spring; variations in gravity cause variations in the extension of the spring. A number of different mechanical and optical schemes have been developed to measure this deflection, which in general is very small. Gravimeters have been developed that can detect variations in the Earth's gravitational field as small as one part in 10,000,000.