BOX 2.1 How do geophysicists measure gravity?


Geophysicists measure gravity using a gravity meter (or gravimeter). There are two different kinds of gravity meters used today. One kind of instrument measures relative changes in g, rather than the actual value. These instruments often use a mass on the end of a spring to measure changes in g (see below). The stronger g is, the longer the spring becomes. These instruments are kept at a constant temperature so that the properties of the spring do not change. This kind of instrument weighs about 10 kg and requires about 3-5 minutes to measure g with a precision of 0.01 mGal.

The other kind of gravity meter (not shown) measures the actual value of g and is called an absolute gravimeter. It measures very accurately how fast a small mass falls by bouncing a laser beam off the falling object. To reduce friction on the mass, the mass is dropped within a chamber which also falls. This procedure is repeated about 1000 times to achieve a precise measurement of g. This instrument, in contrast to the other kind of gravimeter, weighs about 250 kg and takes about 12 hours to obtain a measurement of g to a precision of 0.01 to 0.001 mGal.

Questions

1. What percentage of the Earth's gravitational field is a gravity meter capable of measuring?.
2. What happens to a spring as it is heated? Is it easier to elongate the spring when it is cold or warm?
3. What are the advantages of using an absolute gravimeter? What are the disadvantages?

Answers

Gravometric measurments


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