John Denker is correct that there is a device called a gravimeter that
uses a spring scale and is very sensitive. Probably the most widely
used is the Worden Gravity Meter. The United States Geological Survey
has made extensive use of this meter. Even in little Bluffton, Ohio
(population 3000) our gravity has been measured on two different
occasions by the USGS using a Worden Gravity Meter.
This meter can detect *changes* in g of about 1 part in 100 million. It
cannot determine g to this accuracy except in comparison to some other
accepted place.
There is a rather old publication from USGS that describes how they
conduct their surveys and how the Worden meter works. I do not have the
reference at hand because I am home right now.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Chair of Sciences
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu