The laser wavelength is known to ☐.025 ppb and the clock is stable to ☐.03 ppb as well. A test mass is allowed to fall freely inside a vacuum chamber and its position is measured with a laser interferometer and timed with an atomic clock. Absolute gravimeters provide such measurements by determining the gravitational acceleration of a test mass in vacuum. A relative instrument also requires calibration by comparing instrument readings taken at locations with known complete or absolute values of gravity. Spring gravimeters are, in practice, relative instruments which measure the difference in gravity between different locations. The test mass is sealed in an air-tight container so that tiny changes of barometric pressure from blowing wind and other weather do not change the buoyancy of the test mass in air. Quartz and metal springs are chosen for different reasons quartz springs are less affected by magnetic and electric fields while metal springs have a much lower drift (elongation) with time. This detunes the test mass from most local vibration and mechanical noise, increasing the sensitivity and utility of the gravimeter. The special property of these springs is that the natural resonant period of oscillation of the spring–mass system can be made very long – approaching a thousand seconds. Zero-length springs do not follow Hooke's law instead they have a force proportional to their length. The majority of modern gravimeters use specially-designed metal or quartz zero-length springs to support the test mass. The study of gravity changes belongs to geodynamics. 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: near the Earth's surface gravity decreases 0.308 mGal for every metre of height. These measurements are used to define gravity anomalies.īesides precision, stability is also an important property of a gravimeter, as it allows the monitoring of gravity changes. Several types of gravimeters exist for making these measurements, including some that are essentially refined versions of the spring scale described above. 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. Researchers use more sophisticated gravimeters when precise measurements are needed. The resulting measurement may be made in units of force (such as the newton), but is more commonly made in units of gals or cm/s 2. The change in length of the spring may be calibrated to the force required to balance the gravitational pull. In one common form, a spring is used to counteract the force of gravity pulling on an object. Many weighing scales may be regarded as simple gravimeters. Thus, gravimeters can be regarded as special-purpose accelerometers. For a small body, general relativity predicts gravitational effects indistinguishable from the effects of acceleration by the equivalence principle.
Gravimeters Gravimeter with variant of Repsold-Bessel pendulumĪn instrument used to measure gravity is known as a gravimeter. The value of the g n is defined approximately equal to the acceleration due to gravity at the Earth's surface (although the value of g varies by location). Other units include the cgs gal (sometimes known as a galileo, in either case with symbol Gal), which equals 1 centimetre per second squared, and the g ( g n), equal to 9.80665 m/s 2. In the SI system of units, the standard unit of acceleration is 1 metre per second squared (abbreviated as m/s 2). Gravity is usually measured in units of acceleration.
Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest. Gravimetry is the measurement of the strength of a gravitational field. Geoid undulations based on satellite gravimetry. JSTOR ( December 2009) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources.
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