  We compare the long-term fractional frequency variation of four hydrogen
masers that are part of an ensemble of clocks comprising the National Institute
of Standards and Technology,(NIST), Boulder, timescale with the fractional
frequencies of primary frequency standards operated by leading metrology
laboratories in the United States, France, Germany, Italy and the United
Kingdom for a period extending more than 14 years. The measure of the assumed
variation of non-gravitational interaction,(LPI parameter, $\beta$)---within
the atoms of H and Cs---over time as the earth orbits the sun, has been
constrained to $\beta=(2.2 \pm 2.5)\times 10^{-7}$, a factor of two improvement
over previous estimates. Using our results together with the previous best
estimates of $\beta$ based on Rb vs. Cs, and Rb vs. H comparisons, we impose
the most stringent limits to date on the dimensionless coupling constants that
relate the variation of fundamental constants such as the fine-structure
constant and the scaled quark mass with strong(QCD) interaction to the
variation in the local gravitational potential. For any metric theory of
gravity $\beta=0$.
