The FG5 Measurement System
Schematic Diagram
Principles of Operation
The FG5 operates by using the free-fall method. An object is dropped
inside a vacuum chamber (called the dropping chamber). The descent of
the freely-falling object is monitored very accurately using a laser
interferometer. Click here for some
diagrams of the process.
The free-fall trajectory of the dropped object is
referenced to a very stable active-spring system called a
Superspring. The Superspring provides seismic-isolation for the
reference optic to improve the noise performance of the FG5
The optical fringes generated in the interferometer provide a very
accurate distance measuremen t system that can be traced to absolute
wavelength standards. Very accurate and precise timing of the
occurence of these optical fringes is done using an atomic rubidium
clock that is also referenced to absolute standards.
The fact that the measurement is directly tied to absolute
standards kept at all of the national and international laboratories
around the world is the reason why the FG5 is an ABSOLUTE
GRAVIMETER. Absolute standards of length and time provide the
means to achieve a calibrated gravity value that does not drift over
time.
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