New tests for variations of the fine structure constant

We describe a new test for possible variations of the fine structure constant, /spl alpha/, by comparisons of rates between clocks based on hyperfine transitions in alkali atoms with different atomic number Z. H-maser, Cs and Hg/sup +/ clocks have a different dependence on /spl alpha/ via relativistic contributions of order (Z/spl alpha/)/sup 2/. Recent H-maser vs. Hg/sup +/ clock comparison data improves laboratory limits on a time variation by 100-fold to give /spl alpha//spl dot///spl alpha//spl les/3.7/spl times/10/sup -14//yr. Future laser cooled clocks (Be/sup +/, Rb, Cs, Hg/sup +/, etc.), when compared, will yield the most sensitive of all tests for /spl alpha//spl dot///spl alpha/.