Sub-millimeter tests of the gravitational inverse-square law: A search   for "large" extra dimensions
  Motivated by higher-dimensional theories that predict new effects, we tested the gravitational 1/r^2 law at separations ranging down to 218 micrometers using a 10-fold symmetric torsion pendulum and a rotating 10-fold symmetric attractor. We improved previous short-range constraints by up to a factor of 1000 and find no deviations from Newtonian physics. 
C. D. Hoyle, U. Schmidt, B. R. Heckel, E. G. Adelberger, J. H.   Gundlach, D. J. Kapner, and H. E. Swanson