Hyperfine Structure Anomalies in theP122State ofTl205andTl203

The atomic beam magnetic resonance method has been used to make a precision measurement of the hyperfine structure separation of the ground state of ${\mathrm{Tl}}^{205}$ and ${\mathrm{Tl}}^{203}$. The experimentally determined ratio $\frac{\Delta {\nu }^{205}}{\Delta {\nu }^{203}}=1.00974\mathrm{}\pm 0.00003$ is to be compared with the ratio of the magnetic moments, determined by nuclear induction techniques, of $\frac{{g_I}^{205}}{{g_I}^{203}}=1.00986\mathrm{}\pm 0.00005$. It is shown that the difference between the two ratios can be accounted for by consideration of effects predicted by Bohr and Weisskopf and by Crawford and Schawlow. In particular, the agreement between theory and experiment can be construed as evidence of the reality of the effects postulated by Crawford and Schawlow, which have not been heretofore directly observed.