Table 5: The compared clocks, the value of $\frac{\dot{\alpha}}{\alpha_0}$, its corresponding error in units of $10^{-15}~{\rm yr}^{-1}$, the time interval for which the variation was measured, and the reference.
Frequencies $\frac{\dot \alpha}{\alpha_0} \pm \sigma \left[ 10^{-15}~\rm {yr}^{-1}\right]$ $\Delta t [\rm {yr}]$ Reference
Hg+ and H maser $0.0 \pm 37.0$ 0.38 (1)
Cs and Rb $4.2 \pm 6.9 $ 2 (2)
Cs and Rb $-0.04 \pm 1.60 $ 5 (3)
Hg and Cs $ 0.0 \pm 1.2 $ 2 (4)
H and Cs $1.14 \pm 2.25 $ 5 (5)
Yb and Cs $-0.58  \pm 2.1 $ 2.8 (6)
References: (1) Prestage et al. (1995); (2) Sortais et al. (2000); (3) Marion et al. (2003);
(4) Bize et al. (2003); (5) Fischer et al. (2004); (6) Peik et al. (2004).

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