  We demonstrate gravitational coupling between two gold spheres of
approximately 1mm radius and 90mg mass. By periodically modulating the source
mass position at a frequency f=12.7mHz we generate a time-dependent
gravitational acceleration at the location of the test mass, which is measured
off resonance in a miniature torsional balance configuration. Over an
integration time of 350 hours the test mass oscillator enables measurements
with a systematic accuracy of 4E-11m/s^2 and a statistical precision of
4E-12m/s^2. This is sufficient to resolve the gravitational signal at a minimal
surface distance of 400mum between the two masses. We observe both linear and
quadratic coupling, consistent in signal strength with a time-varying 1/r
gravitational potential. Contributions of non-gravitational forces could be
kept to less than 10% of the observed signal. We expect further improvements to
enable the isolation of gravity as a coupling force for objects well below the
Planck mass. This opens the way for precision tests of gravity in a new regime
of isolated microscopic source masses.
