New tests for variations of the fine structure constant

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