Gravimetry is the measurement of gravitational force, weight, or density. Gravimetry may be used when either the magnitude of gravitational force or the properties of matter are of interest.

Units of measurement

Gravity is usually measured in units of acceleration. In the SI system of units, the standard unit of acceleration is 1 metre per second squared (abbreviated as m/s²). Other units include the Galileo (abbreviated Gal), which equals 1 centimetre per second squared, and the g, equal to 9.806 65 m/s². The value of the g approximately equals the acceleration due to gravity at the Earth's surface (although the actual acceleration varies fractionally from place to place).

How gravity is measured

An instrument used to measure gravity is known as a gravimeter. Since general relativity regards the effects of gravity as indistinguishable from the effects of acceleration, gravimeters may be regarded as special purpose accelerometers. Many weighing scales may be regarded as simple gravimeters. In one common form, a spring is used to counteract the force of gravity pulling on an object. Using Hooke's law, the change in length of the spring may be calibrated to the force required to balance the gravitational pull. The resulting measurement may be made in units of force (such as the Newton), but is more commonly made in units of mass (such as the pound or kilogram) by correcting for the Earth's gravitational acceleration (g).

More sophisticated gravimeters are used when precise measurements are needed. When measuring the Earth's gravitational field, measurements are made to the precision of microGals to find density variations in the rocks making up the Earth. Several types of gravimeters exist for making these measurements, including some that are essentially refined versions of the spring scale described above. These measurements are used to define gravity anomalies.

Besides precision, also stability is an important property of a gravimeter, as it allows the monitoring of gravity changes. These changes can be the result of mass displacements inside the Earth, or of vertical movements of the Earth's crust on which measurements are being made: remember that gravity decreases 0.3 mGal for every metre of height. The study of gravity changes belongs to geodynamics .

See also

• Physical geodesy