  We report the first isoelectronic differential force measurements between a
Au-coated probe and two Au-coated films, made out of Au and Ge. These
measurements, performed at submicron separations using soft
microelectromechanical torsional oscillators, eliminate the need for a detailed
understanding of the probe-film Casimir interaction. The observed differential
signal is directly converted into limits on the parameters $\alpha$ and
$\lambda$ which characterize Yukawa-like deviations from Newtonian gravity. We
find $\alpha \lsim 10^{12}$ for $\lambda \sim 200$ nm, an improvement of $\sim$
10 over previous limits.
