The Measurement of the Newtonian Constant of Gravitation 


                                               James E. Faller
                                                  JILA-NIST
                                            Boulder, Colorado

The Newtonian Constant of Gravitation, G, remains the poorest known and perhaps
hardest to measure of the fundamental constants.  Newton surmised its value
based on his knowledge of the earth’s surface density and his assumption that
the core was mostly iron.  Since that time researchers have measured G in a
variety of ways-some of which will be described briefly.  The body of this talk
will focus on two JILA determinations of this constant.  The first, which was
completed several years ago, determined G by measuring the effect a nearby
large mass had on the acceleration of a free-falling body. The second-and
presently ongoing experiment-- measures the relative deflections of two freely
hanging masses when 500kg of mass is moved nearby.  These deflections are
measured by a laser locked to a Fabry-Perot cavity created by two mirrors that
are positioned within these hanging masses.  Frequency changes of this laser
precisely measure the relative separation of these two masses.  These length
changes together with the experiment's geometry can then be used to determine G.